Mineralogy Glossary
Most definitions are from the Gem & Mineral
Council
of the Natural
History Museum of Los Angeles County.
(Last update - July 27, 2008)
A § Acicular: Needle-like crystal habit. § Acid: An acid is a chemical compound that produces hydrogen ions when dissolved in water. Strong or weak indicate the concentrations of hydrogen ions produced. Common strong acids include hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4). A common weak acid is acetic acid (CH3COOH or HC2H3O2); vinegar is a 3% solution of acetic acid. Another weak acid important in geology is carbonic acid (H2CO3) which forms whenever atmospheric - or other - carbon dioxide (CO2) dissolves in water (H2). Stronger acids will attack many minerals and can be used to help identify them. For example, acids attack carbonate minerals, such as calcite, causing carbon dioxide bubbles to be given off in an affect known as effervescence. Strong acids are dangerous and should be handled with care, only while wearing safety glasses and with plenty of water available. § Adamantine: Hard brilliant luster; like that of a diamond. § Adularescence, Adularescent: Strong pearly-to-blue floating sheen seen in the
moonstone varieties of the feldspars Orthoclase, Albite, and Oligoclase. §
Aggregate: A combination of minerals joined in irregular fashion to form a
material similar to concrete § Albite law twin: Type of lamellar twin on the
{010} plane typical of albite and exhibited by other members of the
plagioclase series. § Aliphate: Hydrocarbon in which carbon atoms are linked into chains. §
Alloy: Homogeneous mixture of two or more metals in
variable proportions. § Alteration: Change in the chemical composition and/or atomic structure
of a mineral brought about by chemical attack or changing physical
conditions. §
Alumino-: Compound containing aluminum usually in
combination with an anionic group (e.g. aluminosilicate). §
Amalgam: Alloy of mercury with another metal. §
Amide: Organic compound containing the amino group (NH2
or NH). § Amorphous, Metamict: Amorphous materials
are non-crystalline, that is they lack long-range regularity in their atomic structure. By definition they cannot be
minerals. Nevertheless, some natural amorphous substances have been accepted
as minerals. Some minerals containing radioactive atoms have become amorphous as a result
of radiation damage to their atomic structures; they are called metamict. § Analogue: A mineral which is identical to another, usually with respect to both
chemical composition and atomic structure with the exception of a single
attribute, typically one of the elements essential to its composition. For
example, Calcite, Magnesite, and Strontianite are analogues, containing
respectively calcium, magnesium, and strontium. § Anastomosing: Network of branching and rejoining veins. § Ångstrom Å: One ten billionth of a meter (0.0000000001
m); One hundred millionth of a centimeter (0.00000001
cm); One tenth of a nanometer.
While the Ångstrom is not a metric system unit, it remains in use
because it is such a convenient unit for specifying interatomic distances and
unit cell dimensions. A
typical unit cell dimension for minerals is between 2 and 30 Å. § Anhedral: Crystal with no
well-formed external faces. § Anhydrous: Mineral with no water (H 2O) in its formula. § Anisotropic: Crystal which affects light differently when light passes through the
crystal in different directions. See index of refraction. § Arborescent: Intergrowth of
crystals in the shape of slender divergent branches like a plant. § Archeotype: General type of atomic structure; used to indicate similarities in
the atomic structures
of minerals with differing chemical
compostions. SnS-archeotype refers to the atomic structure of the mineral
herzenbergite. PbS-archeotype refers to the atomic structure of the mineral
galena. §
Arcuate: Curved or bent. § Arsenate: Compound containing the arsenate group (AsO4-3). § Arsenite: Compound containing the arsenite group (AsO3-3). § Asbestiform: Crystal intergrowth, similar
to that commonly exhibited by asbestos, consisting of fibers generally
aligned in a single direction, often at right (90º) angles to the walls of a vein. See cross-fiber. § Atom, Element, Electron,
Proton, Neutron, Nucleus, Ion, Antion, Cation: An atom is the smallest particle of matter
impervious to attack with energies short of nuclear reactions. There are only
90 chemically distinct kinds of atoms found in all of nature. Thus atoms are
the building blocks on which we base our understanding of the composition of
and the formation of minerals under geological conditions; atoms are
rearranged - but not changed - during geological processes. At the
center of an atom is the nucleus, a combination of positively-charged protons
and uncharged neutrons. The outside the nucleus are negatively-charged
electrons. The number of protons in the nucleus determines the chemical
behavior of the atom. An atom has the same number of electrons as
protons (thus; electrically neutral). Most atoms, when mixed with atoms of a
different element, lose or gain electrons to form ions. Negatively charged
ions (atoms with extra electrons) are anions; positively charged ions (atoms
with a deficit of electrons) are cations. § Atomic bonding: It is useful to classify the bonds that hold atoms to one another in chemical compounds and crystals into three types - ionic, covalent and
metallic. As the name implies, ionic bonding is due to the attractions
between positively and negatively charged ions. Some atoms
are not able to retain all their electrons in competition with other electron
greedy atoms. Under these conditions they lose a few electrons forming cations, ions with a positive charge; the electron greedy
atoms gain electrons to become anions, ions with a
negative charge. Oppositely charged ions attract one another forming ionic
bonds. The optimum arrangement is one which surrounds each charged ion with
several charged ions with the opposite charge. When the different atoms
are almost equal competitors for electrons, neither
can take an extra away from the other, and so such atoms bond together by
sharing electrons. This kind of bonding is called covalent bonding. It holds
two partners together in a very precise geometrical arrangement. The two
bonded atoms can be thought of as an ellipsoid with nuclei at the two foci.
Discreet covalently bonded atom groups are found in many minerals. These
usually have a net negative charge and are called anionic
groups or polyatomic ions. An example is the phosphate ion in which four
oxygen atoms surround a central phosphorous atom; the whole unit behaves like
an anion with a charge of -3. Many of these groups form the basis for the
common mineral classification schemes (Dana, Strunz). Atoms in metals bind
together by electron sharing, but in these compounds the electrons are shared
between large numbers of atoms and are essentially free to move from atom to
atom throughout the material. This type of bonding is weaker. Shear forces
can cause the atoms to slip with respect to one another and then re-bond in
the new position. This explains the malleability of
many metals. The relatively free flow of electrons explains the electrical
and heat conductivity of metals. § Atomic group,
Anionic group, Polyatomic ions: Certain atoms form especially tightly bonded atomic groups. In minerals, these
usually consist of an covalently bonded to three or four surrounding oxygens.
These groups usually have a net negative charge, in which case they are
called anionic groups. An example is the phosphate ion, (PO4-3),
in which four oxygen atoms surround a central phosphorous atom and the whole
unit behaves like an anion with a charge of -3. § Atomic structure: Atomic structure refers to the orderly, repetitive, 3-dimensional arrangement
of atoms in a crystalline substance such as a mineral. Nowhere is
the orderliness and perfection of nature more apparent than in atomic structure.
In an atomic structure, atoms come together into stable configurations by
forming networks of bonds between one another.
Knowledge of the atomic structure of a mineral helps us to understand all of
the mineral's properties. B § Barrel-shaped: Habit of a stout prismatic crystal in which the prism faces are bowed so
that the crystal has a greater diameter at its center than at either end. The
crystal usually also has flat terminations. § Basal pinacoid: crystal form consisting of the two
parallel faces, (001) and (00-1), which transect the c axis. § Base: Compound that when dissolved in water yields free hydroxyl (OH-)
anions. § Baveno law twin: Type of simple contact twin on the {021} plane exhibited by
orthoclase, microcline and other members of the feldspar group. § Beryllo-: Compound containing beryllium usually in combination with an anionic
group (e.g. beryllosilicate). § Birefringent: Having different indicies of refraction along different
crystal directions. §
Bladed: Habit of a crystal that is flat and long; like a
knife blade. Also an intergrowth of blades stacked
together. § Bleb: A rounded crystal grain of one mineral imbedded in another. §
Blocky: Habit in which a crystal roughly block-shaped and
nearly equidimensional. § Borate: Compound containing the borate group (BO3-3) or
linked borate groups. §
Boro-: Compound containing boron usually in combination
with an anionic group (e.g. borosilicate). § Botryoidal, Reniform: Crystalline intergrowth with smooth,
rounded surfaces. § Brazil law twin: Type of penetration twin in quartz in which right- and left-handed
crystal segments are twinned across the {11-20} plane. It is usually only
possible to detect Brazil twinning under special polarized light. §
Brittle: Breaks or powders easily. A type of tenacity. § Butterfly twin: Simple contact twin
consisting of two wedge-shaped individuals, exhibiting no prominent
re-entrant angle, and having an overall shape resembling a butterfly. C § Capillary: Very thin, hair-like crystal habit. § Carbide: Compound of a metal with carbon and nothing else. § Carbocycle, Cyclic hydrocarbon: Hydrocarbon in which carbon atoms are linked
into rings. § Carbonate: Compound containing the carbonate group (CO3-3). § Carlsbad law twin: Type of penetration twin
about the {001} axis exhibited by orthoclase, sanidine and other members of
the feldspar group. § Cathodoluminescence: Emission of visible light by a mineral when it is bombarded by electrons. §
Chalky: Having the color, luster, fracture,
or general appearance of chalk. § Chemical
formula, Chemical composition: The chemical formula the standard way of stating
the chemical composition of a mineral, that is, the relative numbers of atoms
of each element contained in that mineral. The formula is given as a series
of element symbols,
often followed by subscripts. The subscript indicates the number of atoms of
that element in the formula. The presence of the element symbol indicates the
presence of at least one atom, so subscripts of 1 are considered redundant
and are omitted. With the exception of the native
metals and carbon, minerals are usually composed of positively and negatively
charged units called ions (often uncharged water molecules
are part of the composition). Many ions have only one typical charge state,
e.g. oxygen is always -2 (O-2) in minerals, and sodium is always
+1 (Na+1). If an atom is capable of forming ions with more than
one charge state, e.g. iron which can form both a +2 or +3 ion (Fe+2or
Fe+3), a superscript may be used to indicate the charge. (Standard
chemical notation is to use a Roman numeral in parentheses for this purpose.)
Notice that only some of the ions in a formula will have their charges
indicated. The total of all the ionic charges in the formula must equal 0;
positive and negative charges must exactly balance.
Parentheses in chemical formulas are used either to indicate atomic groups, e.g. the carbonate group (CO3-2),
or to indicate a solid solution relationship
between two or more atoms, e.g. (Fe,Mg). § Chromate: Compound containing the chromate group (CrO4-2). § Cleavable: Able to be easily split into smaller fragments, especially along cleavages. The term is usually used in
reference to massive intergrowths of a mineral. § Cleavage, Parting: Cleavage and parting are ways in which certain minerals to break
along flat surfaces. The tendency to cleave is a characteristic feature of
all crystals of a particular mineral species.
Cleavage occurs parallel to planes in the atomic
structure that correspond to relatively weak bonds
between atoms. For example, a mineral is likely to cleave
along a plane which leaves equal numbers of + and - charges on the two new
surfaces, but it unlikely to cleave along a plane which leaves all + charges
on one and all - charges on the other. If the bonds in the structure are of
nearly equal strength in all directions, the mineral will fracture,
but not cleave. Parting differs from cleavage in that
parting is only developed in certain samples of a mineral in response to twinning or applied pressure. The planes along which
cleavage and parting occur are designated by Miller
indices. The perfection with which cleavage and parting are seen in a mineral
is usually described with the following terms: perfect, imperfect,
distinct, indistinct, excellent, good, fair, poor. § Coarse-grained: Consisting of an aggregate of relatively large crystals or grains. § Cockscomb: Intergrowth of long,
slightly offset crystals in semi-circular fans. § Cogwheel twin: Penetration twin
with a circular pattern of protruding crystal edges resembling the teeth of a
gear or cogwheel. § Colloform: Crystalline intergrowth
with smooth, rounded surfaces. § Colloidal: Aggregates of extremely
small grains thought to have formed from a gel. § Color, Streak: Color is one of the most obvious of mineral
properties, but as a guide to recognition it must be used with caution. Some
minerals always exhibit the same color, while others can be found in many
different colors. Trace color impurities can produce dark color when viewed through
a large thickness of a mineral, thereby masking the true color of the
mineral. By scraping the mineral against a piece of
white unglazed porcelain, the streak of the finely powdered mineral is
obtained. The color of the streak can be much more reliable characteristic of
the mineral. Light absorption related to position
changes of electrons within minerals
causes most of the colors we see in minerals. While all minerals absorb
light, only some absorb those frequencies of light which are visible to the
human eye, and thus appear to be colored. The electrons involved may be
associated with transition elements, charge transfers, or color
centers. Transition elements, such as iron, copper
and manganese, contain electrons that can absorb visible light, selectively
absorbing certain wavelengths. A mineral that contains a certain transition
element as an essential constituent will always have color imparted by that
element. The exact color is dependent on the neighbors of the transition
element (e.g. all iron-containing minerals will be colored, but not all will
be the same color). A mineral that in pure form is
colorless may exhibit any of a variety of colors if it contains small amounts
of various transition elements. A good example is beryl, which in pure form
is colorless. Various impurities impart color to form a variety of colored
gemstones. Some very intense colors in minerals are
the result of light absorption by electrons that jump between atoms in the mineral,
a charge transfer. An example is the blue color of the sapphire variety of
corundum which results from an electron jumping from Fe+2 to Ti+4.
A variety of processes including mistakes made in the rush of forming a
crystal or mistakes introduced during irradiation by natural or artificial radioactive sources of the crystal after initial
formation leave atoms out of position. These defects, called color centers,
result in the absorption of visible light. § Columnar: Subparallel
intergrowth of crystals. § Compact: Dense, close-packed crystalline texture in which individual crystals cannot
be distinguished without magnification. § Complex: Group of tightly bonded atoms behaving as unit. § Conchoidal: Manner in which certain minerals fracture along smoothly curved surfaces. § Concretion: A hard, rounded mineral mass that usually forms in sedimentary rocks
surrounding a fragment of organic material. In a general sense, any spherical
crystal aggregate. § Contact twin: Twin in which the crystals meet along a well-defined plane (composition
plane). § Cross-fiber: Consisting of parallel fibers
oriented at right (90º) angles to the walls of a vein.
See asbestiform. § Cruciform twin: Penetration twin
consisting of two prismatic crystals assuming the configuration of a cross. § Cryptocrystalline: Consisting of crystals
too small to see under an ordinary microscope. § Crystal, Crystalline: When people talk about
crystals, they usually are referring to solid pieces of matter that are
bounded by regularly arranged flat faces. Crystal faces result as the solid
grows by adding atoms in a completely
orderly, repetitive, 3-dimensional array called its atomic structure. A substance is crystalline if
it has this orderly atomic structure, even if it lacks the regular faces
(which are dependent on the growth environment and the history since formation).
A crystal formed as the result of geological processes is a mineral. §
Crystal face: The flat exterior surfaces of a crystal are
called crystal faces. Obviously an infinite variety of planes pass through a
given point, only a tiny number of these are observed to bound or define a
crystal. Once scientists were able to determine crystals' atomic structures, they found that the
few planes used were those that were oriented to include the highest density
of atoms. Because of this connection with atomic structure, specimens of
different minerals exhibit different faces. The result may be a
characteristic crystal shape which can be used for identification purposes,
even without understanding of the underlying connection with the atomic
structure of the mineral. Crystal faces (and forms)
are indexed by notations known as Miller indices.
By studying the geometrical relationships between crystal faces, one may be
able to determine the symmetry of the crystal and its crystal
class. § Crystal form: The symmetry that relates the
atoms within the crystal also relates the faces on the
crystal. Thus if the atoms within a crystal are related by a mirror plane,
there will also be pairs of external faces on the crystal related by that
same mirror plane. A crystal form is a group of crystal faces that are
equivalent to one another because they are related by the crystal's
symmetry. Observation that serveral crystal faces on
specimens of a mineral form a crystal form may aid one in identifying the
symmetry of the crystal and its crystal class. § Crystal growth: A crystal begins to form when the proper atoms/ions arrange
themselves in a pattern (the atomic structure of that mineral). Crystal
growth continues as various identical atoms/ions bond
themselves to the original cluster following the same pattern. Crystal growth
may take place from molten rock, from a water solution, or from a gas.
Depending upon natural conditions during growth, atoms/ions will deposit upon
one plane more quickly than another, causing growth to occur more rapidly in
the favored direction. The faces that we observe on crystals are the atomic
planes to which atoms/ions have been added most slowly. (If you grow fast,
you get covered) § Crystal habit: Crystal habit refers to the general shape of a crystal. The crystal
habit is determined both by the atomic structure of the crystal and by the
environment in which the crystal grows. Because of variability in the growth
environment, natural crystals rarely grow in ideal geometric shapes.
Nevertheless the angular relationships between crystal faces will always
provide evidence of the symmetrical relationships between crystal faces. Some
of the terms used to describe crystal habits are: § Crystallite: Minute incompletely crystalline particle. §
Cubic: See isometric. § Cuboidal: Similar to a cube in shape. § Cubo-octahedron: A crystal shape which combines the shape of a cube and the shape of
an octahedron. § Cuneiform: Wedge-shaped. § Cyclic twin: Repeated twin in which the crystals are related by two or more
symmetry equivalent planes or axes ideally resulting in a complete circular
array, such as a disk, ring, or star-like group. § Cyclo-: Applied to minerals containing anion groups linked into rings (e.g.
cyclosilicate) § Cyclosilicate: Mineral characterized by rings of silicate groups: 3-membered rings
(Si3O9-6), 4-membered rings (Si4O12-8),
6-membered rings (Si6O18-12). D § Dana System, Dana number: Scientists have developed various schemes for
classifying minerals in order to better visualize and understand the
relationships between them. One of the most popular schemes, known as the
Dana System, is based on chemical composition and atomic structure. This
classification was originally published by James Dwight Dana in A System
of Mineralogy (1837). The system was most recently updated in Dana's
New Mineralogy (8th edition, 1997) by Richard V. Gaines, H.
Catherine Skinner, Eugene E. Foord, Brian Mason, and Abraham Rosenzweig. The Dana number is composed of four parts.
The first three parts correspond to the class and subclass, type and group.
The 4th part of the number is an integer specific to an individual mineral
species. For example, in the Dana classification,
almandine is assigned the number 51.4.3a.2, which can be decoded as follows: 51. Silicate class and esosilicate subclass,
4. Cations in 6 or greater coordination, 3a. Garnet group, 2 Almandine. § Dauphin law twin: Type of penetration twin
in quartz in which two right-handed (or two left-handed) crystals are rotated by 60º to one
another about the [0001] axis. The twin regions are separated by irregular
internal boundaries, which can be observed on the crystal surface as
interruptions in the horizontal striations on the prism faces. § Deliquescent: Dissolves and become liquid by absorbing moisture from the air. §
Deltoid: Used to refer to a four-sided polygon with two
adjacent and equal long sides and two adjacent and equal short sides, which
consequently is somewhat similar in appearance to a triangle (the Greek
letter "delta"). § Dendritic: Intergrowth of
crystals in the shape of slender divergent branches like a plant. § Density, Specific gravity: The same volume of two different minerals will
not have the same weight. This is because the minerals contain different
atoms and/or because their atoms are packed together more or less closely. A
sophisticated collector will spot an error on labeling a Calcite specimen,
labeled "Barite", because it "feels too light for its
size". Density is a measure of
the weight of a given volume of a material, usually expressed in grams per
cubic centimeter (gm/cm3). Commonly we report the ratios of
mineral densities to the density of water which is 1 gm/cm3 at
4ºC. Such relative measurements give the specific gravity, which is the same
numerically as the density, but is a dimensionless number. One way to measure
a mineral's density is to weigh it suspended by a thread in air and then to
weigh it immersed in water. It will weigh less in water by an amount equal to
the weight of the water that its volume displaces. The volume of the mineral
is equal to the difference between the two weights because each cubic
centimeter of displaced water weighs 1 gram. Finally divide the weight of the
mineral by its volume to obtain the density. Seldom
does one have a crystal of a mineral unadlerated by matrix. None of the
measurement procedures cope with tiny specimens. Thus density/specific
gravity measurements are useful, but limited to macro specimens of pure
materials. § Devitrified: Changed from glass
to crystalline. § Dihexagonal: Literally double-hexagonal, this indicates a form in which the faces are
related by a 6-fold rotational axis combined with parallel mirror planes
and/or perpendicular 2-fold axes. § Dimorphism, Dimorphous: Two minerals that have
the same chemical
composition, but different atomic structures.
See polymorphism. § Dioctahedral: Type of layered atomic structure in which only two of three
possible octahedrally coordinated sites are occupied by cations.
An octahedrally-coordinated site is a position in the structure in which a
cation can form bonds to six anions. The anions can be thought of as
positioned at the corners of an octahedron. See
also trioctahedral. §
Diploid: A crystal form consisting of 24 nonparallel faces related by the combined symmetry
of the 2/m B3 (diploidal) crystal class. § Dipyramid, Dipyramidal: A crystal form
consisting of two identical pyramids joined base to
base. § Discredited: Term used for a once accepted mineral species that has been determined not to
meet the requirements of a mineral species. A mineral species is often
discredited by proving that it actually corresponds to another known mineral
species or that it is a mixture of two or more known mineral species.
The almost universal application of x-ray diffraction analysis has lead to
the discrediting of a number of older mineral names, but the insistence on
such analysis for new minerals has made future discrediting of newly approved
species unlikely. § Dispersion: The color of light depends upon its wavelength. Normal white light
contains a mix of all visible wavelengths and includes red, orange, yellow,
green, blue, and violet (from longer to shorter wavelength). When a beam of
light enters a transparent solid at an angle, it is refracted (the angle of
the beam is changed). Longer wavelengths of light are refracted more than
shorter wavelengths, so the material separates white light into its component
colors. This phenomenon is called dispersion. Minerals differ in their
ability to create dispersion. Diamond produces strong dispersion, which is
the reason that one is able to see distinct flashes of color in an otherwise
colorless diamond gem. § Disphenoid: A crystal form consisting of four nonparallel faces related either by three 2-fold
rotational axes or by one 4-fold rotational-inversion axis. § Disseminated: Scattered as small particles throughout a rock. § Ditetragonal: Literally double-tetragonal, this indicates a form in which the faces
are related by a 4-fold rotational axis combined with parallel mirror planes
and/or perpendicular 2-fold axes. § Ditrigonal: Literally double-trigonal, this indicates a form in which the faces
are related by a 3-fold rotational axis combined with parallel mirror planes
and/or perpendicular 2-fold axes. § Divalent: Cation having a charge (valence) of 2. § Divergent: Intergrowth in which
crystals radiate from a common center. § Dodecahedron: A three-dimensional shape bounded by 12 diamond-shaped faces. The
dodecahedron is a crystal
form in the isometric crystal
system. § Dome: A crystal form consisting of two nonparallel faces related by
mirror symmetry. §
Double refraction: When light enters or leaves a transparent crystal at an angle, it is
refracted (bent). If the crystal is of low enough symmetry, light traveling
along different axes of the unit cell is refracted to different angles. An
image viewed through such a crystal appears to come from two sources. It is
doubled, as shown in the picture below. § Doublet twin: Two crystals intergrown
in a twin relationship § Druse, Drusy: Intergrowth of small projecting crystals that line the walls of a cavity in rock.
Usually only the terminations of the crystals are visible. § Ductile: Able to be drawn into a wire without breaking (e.g. Copper). See tenacity. § Dull: Lowest mineral luster typified by no reflectance; light disperses in all directions from
rough granular surface. E § Earthy: Used to describe a dull luster or a slightly rough fracture like that of hard clay. § Effervescence, Effervescent: Giving off small bubbles.
Carbonate minerals usually effervesce by giving off carbon dioxide bubbles
when they are treated with acid. § Efflorescence, Efflorescent: Formation of a fluffy or
powdery crystalline coating on the surface of a rock or mineral that occurs
as the result of loss of water from the mineral. § Eightling twin: Group of eight crystals
intergrown in a twin relationship. § Elastic: Able to return to original shape when released after bending. See tenacity. § Element symbols: The one and
two letter abbreviations for the elenemts used in chemical formulas. The
first letter is always upper case; the second, if one is part of the
abbreviation. For example: § Environment: As it is used in The
Photo-Atlas of Minerals, environment refers to the geologic conditions or the
type of deposit in which a mineral occurs. This glossary does not include
geological terms used in the Environment data field. § Epitactic, Epitaxy: The growth of a crystal of one mineral on the
surface of a crystal of a different mineral in a definite orientation
determined by the atomic structures of the two minerals. Compare twinning. § Equant, Equidimensional: Crystal habit in which a crystal is about the same size in all directions. § Equigranular: Consisting of grains of
roughly equal size. § Euhedral: Crystal completely surrounded by well-formed faces. F § Faden: A descriptive term applied to tabular quartz crystals containing a white thread-like zone at their centers.
The thread-like zone consists of fluid and gas inclusions trapped during crystal growth as a result of repeated breaking and healing. Faden quartz usually
occurs in Alpine vein environments. § Feldspathoid: Group of minerals
chemically related to the feldspars, but containing relatively less silicon. § Felted,Felt-like: Intergrowth consisting of interwoven fibers. § Fibroid: Intergrowth resembling fibrous tissue. § Fibrolamellar: Intergrowth consisting of layers of flattened fibers. § Fibrous, Filiform: Very thin, hair-like crystal habit. Also an aggregate of fibers. § Fine-grained: Consisting of an aggregate of relatively small crystals or grains. § Fishtail twin: See swallow-tail twin. § Fiveling twin: Group of five crystals intergrown in a twin
relationship. § Flexible: Bends without breaking but does not return to its original shape. See
tenacity. § Fluorescence,
Phosphorescence: When some
minerals are subjected to invisible ultraviolet (UV) radiation, they emit
visible light, seeming to "glow-in-the-dark." This effect, called
fluorescence, was named after the mineral fluorite, which often spectacularly
displays the effect. Relatively few mineral species exhibit fluorescence, and
those that do are usually not invariably fluorescent; they often require a
certain impurity ion which serves as an activator. Some minerals may
fluoresce under short- wave (SW) ultraviolet radiation, but not under
long-wave (LW), or vice-versa. Many different fluorescent colors have been
observed in minerals, and the same mineral species may exhibit different
fluorescent colors depending on its chemical impurities. If the emission
continues after the ultraviolet radiation has been turned off, the mineral is
said to be phosphorescent. § Foliated, Lamellae, Lamellar,
Micaceous: Intergrowth consisting of thin, leaf-like layers. § Fourling: Group of four crystals intergrown in a twin relationship. § Fracture: The manner in which a mineral breaks along relatively irregular or
non-flat surfaces. The following terms are used to describe fracture: § Friable: Crumbly; easily broken or pulverized. § Front pinacoid: A crystal form consisting of the two
parallel faces, (100) and (-100), which transect the a axis. § Fusible: Capable of being melted by heating. G § Geniculated twin: Repeated contact twin consisting of prismatic crystals in a back-and- forth configuration
yielding knee-shaped forms. § Glass: An amorphous substance that is actually a highly viscous (slow- flowing) liquid. § Globular, Nodular: Crystalline intergrowth with smooth, rounded surfaces. § Granular: Consisting of grains of roughly equal size. § Greasy: Poorly reflective mineral luster; similar in appearance to the surface of grease. § Gwindel: A descriptive term applied to groups of stacked, twisted
quartz crystals, which generally occur in Alpine vein environments. § Gyroid: A crystal form consisting of 24 nonparallel faces related by the combined symmetry
of the 432 (gyroidal) crystal class. H §
Hackly: Type of mineral fracture that produces a jagged surface. § Halide: Compound of a metal with a halogen (F, Cl, Br, I) § Hardness: Hardness is the resistance of a mineral to scratching. This property
is indicative of the strength of bonds within crystals because it requires the breaking
of bonds between atoms. Hardness varies greatly from species to species and
it can, therefore, be a very useful property for
identification. The most common way of determining a
mineral's hardness is to try to make a scratch on it using another material
whose hardness is known. If you are able to make the scratch, then the
mineral has a hardness less than that of the material used to scratch it. The
test may, of course, be reversed by trying to scratch the material of known
hardness with the mineral. A German mineralogist
Friedrich Mohs (1773-1839) developed a scale of hardness based upon 10 common
minerals: (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite,
(6) orthoclase, (7) quartz, (8) topaz, (9) corundum, (10) diamond. §
Heart-shaped twin: Simple contact twin consisting of two wedge-shaped individuals, exhibiting
a prominent re-entrant angle, and having an overall shape resembling a heart
(Valentine's Day type, not real). §
Hemihedral: Exhibiting only half of the faces corresponding to the maximum symmetry possible for that crystal system. §
Hemimorphic: Lacking a center of symmetry such that identical faces are not found on directly opposite sides of
crystals. §
Hemispherical: In the shape of half a sphere; usually in reference to a radial intergrowth of crystals. § Hexagonal: The hexgonal unit cell can be defined by three axes. Two of them, labeled
a, are equal in length and at an angle of 120º to one another. The third
axis, labeled c, is perpendicular to the a axes and of a different length.
The c axis corresponds to a 3-fold or 6-fold symmetry
axis. To highlight the presence of 3-fold or 6-fold
symmetry, usual practice is to include a third a axis at 120° to the other
two, and correspondingly to use a redundant 4th integer in the Miller index.
(The extra integer is placed in the 3rd position and equals the negative of
the sum of the first two.). §
Hexahedron: A three-dimensional shape bounded by 6 equivalent faces. § Hexoctahedron: A three-dimensional shape bounded by 48
triangular faces. The trapezohedron is a crystal form in the isometric crystal system. § Holohedral: Exhibiting all of the faces corresponding to the maximum symmetry possible for that crystal system. § Hopper: Crystal habit in
which the growth of crystal faces has been quicker along their edges than at their centers, so that the
centers of the faces are depressed. § Hydrate: Compound containing water (H2O). § Hydrocarbon: Organic compound composed entirely of carbon and
hydrogen. § Hydrous: Mineral with water (H2O) in its formula. § Hydroxide: Compound containing the hydroxyl group (OH-). § Hygroscopic: Readily absorbing moisture, as from the
atmosphere. I § Idiomorphic: As individual euhedral crystals. § Igneous: Pertaining to rocks formed
by solidifaction from molten material. § Imide: Organic compound containing nitrogen with two attached carbonyl
groups (CO). § Inclusion: A foreign body within a crystal.
Inclusions usually represent material trapped during crystal growth. They commonly are earlier-formed crystals of other minerals or
remnants of the fluid from which the crystal grew. § Incrustation, Crust: A coating of minerals formed on a surface. § Index of refraction,
Double refraction, Birefringence: When a beam of light strikes the surface of a transparent material at
an angle, part will be reflected away and part will penetrate the material.
The part of the beam that enters the material will be bent or refracted by an
amount related both to the angle at which the beam strikes the material (the
angle of incidence), to the density of the material, and to the light
absorbing properties of the material. In general, the denser a material, the
more the light entering it will be bent, but because additional factors
affect the bending, this determination is not the same as a measurement of
the density. Also the amount of bending may vary with wavelength of the
light. By measuring the angles of incidence and refraction, a quantity called
the index of refraction can be determined. This index can be used as an
identifying characteristic for the material. The
atomic structures of many mineral crystals are quite different in different
directions. A light beam entering such a crystal will be split into two beams
- each bent at a different angle. This is called double refraction or
birefringence. An image viewed through a birefringent crystal will appear to
be two images. § Ino-: Applied to minerals containing anion groups
linked into chains (e.g. inosilicate) § Inosilicate: Mineral characterized by
chains of silicate groups: single chains (Si2O6-4),
double chains (Si4O11-6). § Intergrowth, Aggregate: Most natural crystals form as intergrowths or
aggregates. Where crystals come in contact with one another as they are
growing, they form irregular contacts rather than regular crystal faces. The term massive refers to intergrowths in which the mineral
crystals have grown in contact with other crystals on all sides leaving no
empty space in between. Most rocks are massive intergrowths of one or more different minerals. Many
minerals grow in distinctively shaped aggregates. Crystal faces are sometimes
present at the periphery of such an aggregate. Some of the terms used to
describe crystal aggregates are: § Intermetallic: Refers to a compound
composed of two or more metals in definite proportions. (An alloy is a
mixture of two or more metals in variable proportions.) § Interstitial: Between grains or in
pores in rock. § Iodate: Compound containing the iodate group (IO3-). § Iridescent: Exhibiting rainbow-like
colors in the interior or on the surface of a mineral. §
Iron cross twin: Twin consisting of two penetrating pyritohedra related by rotation
about the [110] axis; typical of pyrite. § Isometric, Cubic: The isometric (=cubic) unit cell is defined by
three axes of equal length, all labeled a. All angles between axes are 90°.
Because of the equivalence of all axes and angles.
The isometric system can contain combinations of many different types of
symmetry elements: 2-, 3-, and 4-fold rotation axes, 3- and 4-fold rotation-inversion
axes, mirror planes, and centers of symmetry. §
Isomorphism: Two different minerals that possess the same atomic structure are called isomorphs. The minerals may be
related chemically through the replacement of the atoms of one element for
those of another. An example is the olivine series consisting of the minerals
fayalite, Fe2SiO4, and forsterite, Mg2SiO4.
Isomorphous minerals may have nothing in common chemically. For example halite,
NaCl, and galena, PbS, have identical atomic structures. § Isostructural: Having the same atomic structure (but different chemical composition) J §
Japanese law twin: Type of contact twin on the {11-22} plane of quartz. The two crystals are at a nearly
right angle (84º33') forming a "V". Both crystals are usually
flattened parallel to their front-facing prism faces. L § Lamellar twin: Polysynthetic twin in which the individual crystals are thin
plates. § Lath-like: Habit of a crystal that is flat and long; like a knife
blade. §
Lenticular: Habit of a crystal that resembles a double-convex lens in cross-section. § Lozenge: Rhombus or diamond (as in one of the four suits
in a deck of cards). § Luminescence: A general term which describes any process in which energy is emitted
from a material at a different wavelength from that at which it is absorbed.
It is an umbrella term covering fluorescence, phosphorescence, and triboluminescence. § Luster, Sheen: Luster refers to the appearance of a mineral surface as a result of
light reflecting from it. To a trained eye luster can be one of the most
important clues for the sight-identification of minerals. Minerals with
higher indices of refraction reflect more of the light that strikes them and, therefore, have a
higher or brighter luster. The following terms are often used to describe
luster: M § Magnetism: Very few minerals are
noticeably attracted to a magnet. Certain elements, most notably iron, have
nuclei that tend to align themselves in the same direction in a magnetic
field. When the atomic structure of an iron-rich mineral allows most of its
iron atoms to align their electron fields in the same direction, it will be
attracted by a magnet. Under certain conditions, the iron nuclei can be
locked in place, thereby producing a magnet. Magnetite, an iron oxide, is
attracted to magnets and can itself be naturally magnetized. Many meteorites
are attracted by magnets because they contain native iron. § Malleable: Able to be hammered into
thin sheets without breaking. See tenacity. § Mammillary: Crystalline intergrowth with smooth, rounded surfaces. §
Manebach law twin: Type of simple contact twin on the {001} plane exhibited by orthoclase, microcline and other
members of the feldspar group. § Massive: Uniform intergrowth of crystals. § Mealy: Resembling meal in texture or consistency;
granular. § Metal: Element whose atoms have outer electron shells
which are less than half full. Metals tend to lose electrons to form
positively charged ions (cations) in the presence of electron greedy elements
or polyatomic ions. When no electron greedy species are nearby, metals form
metallic bonds by sharing with adjacent metal atoms. § Metallic: Brilliant, highly reflective luster of metals. § Metalloid: Non-metallic element that
exhibits some metallic characteristics. § Metamict: Amorphous as the result of radiation damage. § Metamorphic: Pertaining to rocks that have been significantly modified by heat and pressure, for the
most part without melting. § Microcrystalline: Made up of crystals so small that they can only be seen with a microscope. § Micron, Micrometer, µm: § Miller indices: Three integers (sometimes
four in the hexagonal crystal system) used to indicate the orientation of a
plane or direction in a crystal such as those corresponding to a crystal face
or cleavage. The three numbers are related to the three (or four) axes that
define the unit cell. The three numbers are enclosed in parentheses, as
(111), to indicate a single face or plane. They are enclosed in braces, as
{111}, to indicate a crystal form (set
of planes related by symmetry). They are enclosed in brackets, as [111], to
indicate a direction. § Mimetic: Appearing to have a higher degree of symmetry as the result of twinning. § Mineral,
Mineral species: A mineral is
a chemical element or combination of chemical elements that is normally crystalline and which has formed by natural geological
processes. Being crystalline means having an atomic structure. For the mineral to be a distinct mineral
species, it must differ from every other mineral species either in its
combination of chemical elements or in its atomic structure. § Mineral class,
Mineral group: Minerals are usually
classified according to aspects of their chemical composition and atomic structure. Most classification systems (Dana, Strunz) are based principally upon the major anions and anionic groups in minerals. This leads to the following major classes: native
elements, sulfides and sulfosalts, oxides and hydroxides, halides,
carbonates, borates, sulfates and chromates, phosphates,
vanadates and arsenates, tungstates,and molybdates, silicates. § Mineral name: Each distinct mineral
species is given a distinct name. Names are chosen in many ways. Minerals
have been named after persons, places, properties, and similarities with
other minerals. The discoverer usually proposes a name as part of the
submission for approval. Current practice is to not approve self-naming. § Modified: Term used to describe a crystal shape or form that varies from another. For example, a
cubo-octahedron is an octahedron modified by a cube. § Molybdate: Compound containing the
molybdate group (MoO4-2). § Monoclinic: The monoclinic unit cell is
defined by three axes, a, b, and c, of unequal lengths. The angles between
the a and b axes and between the c and b axes are exactly
90°. The b axis is chosen to correspond to a 2-fold
axis of rotational symmetry axis or to be perpendicular to a mirror symmetry
plane. The standard convention for assigning the other axes is c < a. The
unit cell is also chosen so that the angle beta, lying between the a and c
axes, is obtuse (between 90º and 180º). §
Monovalent: Ion having a charge (valence) of 1. § Morphology: Crystal shape. § Myrmekitic: Intergrowth consisting of blebs, drops, or vermicular grains of one mineral in another. N § Nanometer, nm: § Neso-: Applied to minerals containing individual
isolated anion groups (e.g. nesosilicate) § Nesosilicate: Mineral characterized by
single isolated silicate groups of general formula SiO4-4. § Nitrate: Compound containing the nitrate group (NO3-). § Nitride: Compound of a metal with nitrogen and nothing
else. § Nodular: In the form of or consisting of nodules (small rounded lumps). § Non-metal: Element whose atoms have their outer electron shells more than half
full. Such atoms can form only covalent bonds with one another. Non-metals
(except the noble gases) tend to take electrons from metals to form
negatively charged ions (anions). O § Ocherous: Resembling ochre, an earthy, pulverulent, red, yellow, or brown iron
oxide (hematite). § Octahedron: A three-dimensional shape bounded by eight triangular faces and
having six corners. The octahedron is a common crystal form in the isometric crystal system. § Oily: Poorly reflective mineral luster; similar in appearance to the surface of oil. § Oolitic: Consisting of many spherical bodies. § Opalescent, Opaline: Having a milky or rather pearly sheen such as that shown by some kinds of opal. § Opaque: Impervious to visible light even in thin fragments. §
Organic: Compound containing carbon, hydrogen, and
whatever else. In an early version of creationism it was believed that
compounds made by living organisms were inherently different that those made
in the laboratory and that never the twain should meet. Most naturally
occurring carbon-containing compounds were formed by some living organism, so
organic became to mean carbon containing, although early on it was recognized
that compounds containing only carbon and oxygen (e.g., carbon dioxide,
carbonates, oxalates) did not fit this pattern.
Laboratory synthesis of urea from inorganic compounds disproved the premise,
but the classification into organic and inorganic has survived as a
convenient way to subdivide the huge field of chemisty. The distinctions are
dying. § Orthorhombic: The orthorhombic unit cell is defined by three axes, a, b, and c, of
unequal lengths. The angles between all axes are exactly
90°. The axes are chosen to correspond to 2-fold axes
of rotational symmetry axis or to be perpendicular to mirror symmetry planes.
The standard convention is that c < a < b. § Ovoid: Egg-shaped §
Oxalate: Compound containing the oxalate group (C2O4-2). §
Oxide: Compound of an element with oxygen. §
Oxidize: To chemically change through contact with oxygen
or oxidizing conditions typical on or near the earth's surface. This type of alteration generally results in the
formation of minerals having compositions
with greater amounts of oxygen or with one or more cations
in a higher charge state than in the original mineral. P § Paramorph: A pseudomorph with the
same composition as the mineral it replaces. §
Parting: See cleavage. §
Pearly: Pearl-like sheen caused by tiny partly-developed
cleavages parallel to the
surface. § Pedion: A crystal form that
consists of only one face. § Penetration twin: Twin in which two or more crystals appear to interpenetrate each
other with the surface between them being irregular or poorly defined. § Pericline law twin: Type of lamellar twin about
the [010] axis exhibited by the members of the plagioclase series. §
PGE, PGM: Used as acronyms for Platinum Group Elements
and Platinum Group Minerals. The Platinum group elements
are platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os)
and ruthenium (Ru). § Phosphate: Compound containing the phosphate group (PO4-3). § Phosphide: Compound of a metal with phosphorous. § Phosphorescence: See fluorescence. §
Phyllo-: Applied to minerals containing anion groups
linked into sheets (e.g. phyllosilicate) § Phyllosilicate: Mineral characterized by sheets of silicate groups of general formula
Si4O10-4. §
Piezoelectricity, Pyroelectricity: The atomic
structures of certain minerals that lack a center of symmetry become electrically polarized when heat
or pressure is applied. When heat is involved it is called pyroelectricity.
When pressure is involved it is caused piezoelectricity. The effects are
generally reversible. For example, by applying an
alternating electrical current to opposite ends of a piece of quartz, you can
make it vibrate.The piezoelectricity of quartz has proven useful for
accurately tuning frequencies in radios and clocks. The pyroelectricity of
tourmaline has made it useful in devices that measure high pressures.
Tourmaline was used to measure the pressure of the first atomic bomb test. § Pinacoid: A crystal form that consists of two parallel faces. § Pisolitic: Composed of pea-like grains. §
Pitchy: Rather dull luster similar to that of pitchblende, a massive
variety of uraninite. § Platelet: A very small platy
crystal. §
Platy: Very thin sheet-like crystal habit. § Pleochroism, Dichroism,
Trichroism: Pleochroism is an optical property observed in the crystals of
certain minerals in which light is absorbed differently as it passes through
the crystals in different directions. Differences in the atomic structure of a crystal in
different directions account for the differential light absorption. Three
distinct colors (trichroism) or two distinct colors (dichroism) may be seen
as a crystal is held in front of a light and turned. Most pleochroic mineral
crystals exhibit only small differences in color intensity as they are
turned. §
Plumose: An aggregate of many small scales spreading into a
formation that looks like a feathery plume. § Poikiloblast: A large crystal of metamorphic origin that contains within it many
small grains of other minerals. § Polished section: A slice of rock
that has been highly polished for viewing under the microscope. Polished
sections are usually made for studying opaque minerals
using light reflected from the polished surface of the mineral grains
embedded in the rock. § Polymorphism: Two or more minerals that have the same chemical composition, but
different atomic structures are called
polymorphs. Such a polymorphic relationship exists between the minerals
andalusite, kyanite, and sillimanite which all have the chemical formula, Al2SiO5.
Typically, polymorphs form under differing conditions of temperature and/or
pressure. § Polysynthetic twin: Repeated twin in which the crystals meet along parallel planes. § Polytype: Mineral that differs from another only in the stacking of similar
structural units in its atomic structure. § Porcelaneous: Dull white luster
resembling unglazed porcelain. § Porphyroblast: Euhedral crystal formed
in a metamorphic rock. § Positive form,
Negative form: Terms used to distinguish symmetrically similar, but distinct forms
in certain crystal classes. § Prism, Prismatic: Crystal habit characterized
by greater length than width, with prism faces parallel to and completely
surrounding the long direction of the crystal. Also, a crystal
form consisting of three or more symmetry-related faces parallel to a common
axis. § Pseudo-: This prefix is used in combination with many roots: § Pseudomorph: Any given mineral will form only within a specific range of
conditions of heat, pressure and chemical composition. When the mineral's
environment changes, the mineral will often change or alter to a different
mineral. Even though the new mineral would normally form in its own
distinctive crystal shape, occasionally the new mineral not
only takes the place of the original, but also retains its external crystal
shape. The new mineral is then said to be a pseudomorph (false form) of the
original. § Pseudo-spinel law twin: Twin relationship closely resembling the Spinel
law, but relating crystals not in the 4/m B3 2/m crystal class. § Pulverulent: Easily powdered. § Pyramid, Pyramidal: A crystal form consisting of 3, 4, 6, 8, or
12 nonparallel faces meeting at a point. § Pyritohedron: A dodecahedron (three-dimensional shape bounded by 12 faces) with
five-sided faces. The pyritohedron is so named because crystals of pyrite
sometimes take this shape. It is a crystal form in the isometric crystal system. Q § Quatravalent: Ion having a charge (valence) of 4. R § Radial-fibrous: Intergrowth in which fibrous crystals radiate from a common center. § Radiate, Radial: Intergrowth in which crystals radiate from a common
center. § Radioactivity: Some kinds of atoms have nuclei that are unstable and break apart to
form atoms of other elements. In the process of breaking apart these atoms
release radiation in the form of alpha and beta particles and gamma rays. The
most common radioactive elements in minerals are uranium and thorium. Minerals
rich in these elements will be very radioactive, causing a Geiger counter to
react strongly. Minerals containing small amounts of these elements are only
slightly radioactive. While the number of protons in
an atomic nucleus determines what element it is, the number of neutrons in
the nucleus is somewhat variable for a given element. Uranium nuclei always
have 92 protons (atomic number = 92) and they most commonly have 146
neutrons, giving them an atomic weight of 92 + 146 = 238. More rarely uranium
nuclei in minerals have only 143 neutrons yielding an atomic weight of 235.
These are the different isotopes U238 and U235. They breakdown at different
rates, known as half lives, and therefore give off radiation at different
rates. Some other elements whose most common isotopes are not radioactive
also exist as in the form of rarer radioactive isotopes. §
REE: Acronym for Rare Earth Elements
used in chemical formulas as a element symbol to represent a mixture of several
different rare earth elements: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tm, Yb. § Re-entrant angle: V-shaped depression marking the junction between two twinned crystals. § Repeated twin, Multiple twin: Twin consisting of three or more crystals. § Resinous: Very reflective luster; like the luster of broken resin or shellac. § Resorbed: Dissolved. § Reticulated: Intergrowth pattern
consisting of criss-crossed slender crystals; often indicative of a twin relationship. § Rhombic: Having the shape of a rhombus (like the diamond in a deck of playing
cards). § Rhombohedron, Rhomb: A three-dimensional shape that can be thought of
as a cube stretched or compressed in the direction of two opposite corners.
The rhombohedron is a common crystal form in the
hexagonal crystal system. §
Rock: An aggregate of minerals. Most rocks fall into three
general categories depending on processes by which they formed: Igneous:
rocks formed by solidification from molten material. Sedimentary: rocks
formed by the deposition of sediments. Metamorphic: rocks that have been
significantly modified by heat and pressure, for the most part without
melting. §
Rosette: Intergrowth consisting of numerous platy crystals
overlapping like flower petals. S § Saddle-shaped: Crystal habit with
curved faces and edges so as to resemble a saddle. § Salt: Compound that results when an acid reacts with a base. Note that
"table salt" is sodium chloride (NaCl), the mineral halite. NaCl
results from the reaction between hydrochloric acid (HCl) and the base,
sodium hydroxide (NaOH); HCl + NaOH -> NaCl + H2O § Scalenohedron: A crystal form consisting of symmetry-related faces that are scalene triangles
(three unequal sides). The tetragonal scalenohedron has 8 faces related by the
combined symmetry of the B42m (tetragonal scalenohedral) crystal class. The
trigonal scalenohedron has 12 faces related by the combined symmetry of the
B32m (trigonal scalenohedral) crystal class. § Scepter: Unusual prismatic crystal habit
in which the diameter of the prism is greater near the termination
of the crystal than near its base. § Schiller: Optical effect in which flashes of bright colors appear as the
mineral is turned. Schiller is caused by the diffraction of light from
closely spaced layers. § Schistose: Layered texture of an aggregate of fibrous or platy mineral grains. §
Sectile: Can be cut into thin shavings with a knife. See tenacity. § Sedimentary: Pertaining to rocks
formed by the deposition of sediments. § Selenite: Compound containing the selenite group (SeO3-2). § Semi-metal: Element whose atoms have outer electron shells that are one half
full. They tend to form covalent bonds with one another and with non-metals.
In pure form semimetals are often semiconductors. § Semimetallic, Submetallic: Luster somewhat less than metallic. §
Sheaf: Bundled intergrowth of fibers. § Side pinacoid: A crystal form consisting of the two parallel faces, (010) and (0-10), which
transect the b axis. § Silicate: Compound containing the silicate group (SiO4-4)
or linked silicate groups. § Silicide: Compound of a metal with silicon. §
Silky: Silk-like sheen caused by reflections from a fibrous growth structure
or from parallel hair-like inclusions. § Simple twin: Twin composed of only two
crystals. §
Sixling: Group of six crystals intergrown in a twin relationship. § Slag: Residual material from the refinement of a metallic ore. § Slickensided: Having a polished and smoothly striated surface usually the result of
movement along the surface. § Solid
solution, Series, Substitution: The chemical formula given for each mineral provides
the atomic components in the pure mineral. In fact, natural minerals almost
always contain atoms of some other elements in partial substitution for those
provided in the formula. Certain ions, such as Fe+2 and Mg+2,
are similar enough that they can take each other's place in any proportion in
an atomic structure. The minerals fayalite,
Fe2SiO4, and forsterite, Mg2SiO4,
have the same atomic structure and any composition intermediate between them
is possible. This is called a solid solution series (or simply a mineral
series), and such a substitutional relationship can be indicated by providing
the formula as, (Fe,Mg)2SiO4. The name applied to the
mineral compound depends upon which element is present in greatest amount. A
separate name may be applied to the series. For example, fayalite and
forsterite belong to the olivine series. §
Soro-: Applied to minerals containing finite linkages
of anion groups (e.g. sorosilicate) § Sorosilicate: Mineral characterized by finite linkages of silicate groups: double
groups (Si2O7-6), triple groups (Si3O8-4),
etc. § Sphenoid: A crystal form consisting of two nonparallel faces related by 2-fold rotational
symmetry. § Spherulitic: As spherical intergrowth of
crystals. § Spinel law twin: Type of twinning occurring in octahedral crystals of spinel and many other minerals
of the isometric system. The octahedron can be thought
of as cut through its center along a plane parallel to a set of octahedral faces and then one of the halves of the octahedron rotated
by 60º relative to the other. § Splendent: A luster of the highest
intensity. § Stalactitic: As concentrically layered intergrowth with conical or cylindrical shapes;
similar to stalactites. § Stellate: Star-like intergrowth of crystals, often resulting from cyclic twinning. § Striated: Marked by a series of parallel straight lines (striations). On
crystals striations generally represent the oscillation between two crystal forms. § Strunz System, Strunz number: Scientists have developed various schemes for
classifying minerals in order to better visualize and understand the
relationships between them. One of the most popular modern schemes, developed
by Professor Hugo Strunz, is based upon a combination of chemical composition
and atomic structure. The Strunz number is composed
of four parts. The first three parts correspond to the class, group, and
subgroup. In the complete Strunz System, the 4th part of the number is a
three-digit integer specific to an individual mineral species; however, in
the Photo-Atlas of Minerals we have simplified this to a one-digit integer,
so that very closely related minerals often have identical Strunz
numbers. For example, in the full Strunz
classification, almandine is assigned the number 9.AC.110, which can be
decoded as follows: 9. Silicate class, A
Nesosilicate division, C. Garnet group, 110
Almandine. § Subadamantine: Luster somewhat less than adamantine. § Subconchoidal: Fracture along surfaces
that are not quite as smoothly curving as for conchoidal
fracture. § Subhedral: Crystal partially bounded
by well-formed faces. § Subparallel: Not quite parallel, usually used in reference to intergrowths of prismatic
crystals. § Subresinous: Luster somewhat less than resinous. § Subvitreous: Luster somewhat less than vitreous. § Sulfarsenites: Compounds based on AsS3-3 groups.
Sulfantimonites (SbS3-3) and sulfbismuthinites (BiS3-3)
are closely related compounds. § Sulfarsenates: Compounds based on AsS4-5 groups. §
Sulfate: Compound containing the sulfate group (SO4-2). §
Sulfide: Compound of a metal or semi-metal with sulfur. §
Sulfite: Compound containing the sulfite group (SO3-2). § Sulfosalt: Compound of both a metal and semi-metal with sulfur. § Swallow-tail
twin, Fishtail twin: Contact twin that
results in a v-shaped termination similar in appearance to the tail of a
swallow or fish. § Symmetry,
Crystal class, Crystal system: Hold up your hands side-by-side in front of your
face with your fingers spread. At least in an approximate way, your hands are
identical, except that they are flipped as though one were the reflection of
the other in a mirror. This is a type of symmetry called mirror symmetry.
Objects can exhibit rotational symmetry so that rotation around a symmetry
axis repeats the object two or more times in each full rotation. If the object
assumes the same appearance four times (every 90º) as it is rotated a full
360º, it is said to have four-fold symmetry. A center of symmetry is present
if every point on an object has a corresponding identical point on the
opposite side of the object the same distance from its
center. Symmetry is an important property of mineral crystals. The symmetry we see in the external
shape of a crystal results from the symmetry of the mineral's atomic structure. The symmetry of the crystal
may not be obvious because of irregular growth; however, the angles between
the crystal's faces will always be related by the true symmetry of the
mineral. Careful study, sometimes along with angular measurements, may be
necessary to determine the symmetry of a crystal. Recognizing the symmetry of
a crystal will be a big help in identifying the
mineral. There are ten kinds of symmetry (symmetry
elements) that can be seen in the external form of crystals: § Symplectic: Intimate intergrowth
of two different minerals. T § Tabular: Crystal habit
appearing like a tablet of paper. § Tarnish: Formation of a thin alteration film on the surface of certain minerals.
The thin film, itself called tarnish, affects the mineral's color
and luster. §
Tecto-: Applied to minerals containing anion groups
linked into three- dimensional frameworks (e.g. tectosilicate) § Tectosilicate: Mineral characterized by a three-dimensional framework of silicate
groups of general formula SiO2. Aluminum commonly takes the place
of some of the silicon. § Tellurite: Compound containing the tellurite group (TeO3-2)
or linked tellurite groups (Te2O5-2, Te3O8-2). § Tenacity: Tenacity refers to the strength rather than the hardness of a mineral.
It actually represents its resistance to breaking, crushing, bending, or
tearing. The following terms are used to describe the tenacity of minerals: § Tenebrescence: The ability of certain minerals to change color when exposed to
sunlight. § Termination: The tip of a crystal;
ideally made up of crystal faces. § Tetartoid: A crystal form consisting of nonparallel faces related by the combined symmetry of
the 23 (tetartoidal) crystal class. § Tetragonal: The tetragonal unit cell is defined by three axes. Two of them,
labeled a, are equal in length; and the c axis is of a different length. All
angles between axes are 90°. The c axis corresponds
to a symmetry axis of either 4-fold rotation or 4-fold rotation inversion.
The c axis can be either longer or shorter than the a axes. § Tetrahedron: A three-dimensional shape bounded by four triangular faces § Tetrahexahedron: A three-dimensional shape bounded by 24 faces that are isosceles
triangles (two sides equal). The tetrahexahedron is a crystal form in the isometric crystal
system. It can be visualized as a cube with each of its six square faces
split into four triangular faces. § Thermoluminescence: Emission of visible light by a material when it is heated. § Thiosulfate: Compound containing the thiosulfate group (S2O3-2). § Translucent: Capable of allowing visible light to pass through, but not clearly
enough that an object can be seen looking through the material. § Transparent: Capable of allowing visible light to pass through clearly so that an object
can be seen looking through the material. § Trapezohedron: A three-dimensional shape bounded by 24 faces. The trapezohedron is a
crystal form in the isometric crystal system. § Triboluminescence: Emission of visible light by a material when it is scratched, crushed
or rubbed. § Triclinic: The triclinic unit cell is defined by three axes, a, b, and c, of
unequal lengths. None of the angles, alpha, beta, and gamma, between these
axes are exactly 90°.The standard convention for assigning axes is c < a
< b. The angle alpha lies between the b and c axes; beta lies between a
and c, and gamma lies between a and b. The cell is usually chosen so that
alpha and beta are obtuse (between 90º and 180º). § Trigonal: A subgroup of the hexagonal crystal system characterized by one
three-fold symmetry axis. The remaining crystal
systems in the hexagonal crystal system have a six-fold symmetry axis. § Trilling: Group of three crystals intergrown in a twin relationship. § Trimorphous: Three minerals that have the same chemical composition, but
different atomic structures. See polymorphism. § Trioctahedral: Type of layered atomic structure in which all three of the
possible octahedrally coordinated sites are occupied by cations.
An octahedrally-coordinated site is a position in the structure in which a
cation can form bonds to six anions.
The anions can be thought of as positioned at the corners of an octahedron. See also dioctahedral. § Trisoctahedron: A three-dimensional shape bounded by 24 faces that are isosceles
triangles (two sides equal). The trisoctahedron is a crystal form in the isometric
crystal system. § Tristetrahedron: A three-dimensional shape bounded by 12 three-or-four-sided faces.
The tristetrahedron is a crystal form in the isometric crystal system. The trigonal
tristetrahedron can be constructed by splitting each of the four faces of a tetrahedron into three triangular faces. § Trivalent: Ion having a charge (valence) of 3. §
Tuft: A crystal aggregate in the form of clumps of fibrous
crystals. § Tungstate: Compound containing the tungstate group (WO4-2). §
Turbid: Appearing cloudy or translucent because of suspended material. § Twinning: The intergrowth of two or
more crystals of the same mineral in a definite orientation determined by the
atomic structure of the mineral. The twin relationship is indicated by
specifying the Miller indices of a plane on
which the structures meet (e.g. on {100}) and/or an axis about which one
structure is rotated relative to the other (e.g. about [310] axis). If a
plane is specified without indicating an axis, the twinning is understood to
be by reflection across that plane. U § Uneven: Fracture characterized by
more or less flat surfaces with some roughness. § Unit cell: The unit cell is the smallest group of atoms in the atomic
structure that if repeated in each of the three dimensions would
completely generate the atomic structure. The atoms in the unit cell and
their arrangement are unique to a given mineral and define its atomic
structure, chemical composition, external
symmetry, and many of its properties. §
Uranyl-: Compound containing the uranyl group (UO2-2). V § Vanadate: Compound containing the vanadate group (VO4-3). § Vanadin-: Compound containing clusters of vanadium atoms surrounded by oxygen
atoms (exclusive of the vanadate [VO4-3] group). §
Variety: Variety names have traditionally been given to
minerals that have some distinctive physical characteristic, such as color,
that sets them apart from other minerals of the same mineral species. Variety names are most
commonly used for minerals that have use as gems. Amethyst, for example, is
the purple variety of the mineral species quartz, which has many other
varieties as well. §
Vein: A thin tabular or sheet-like body usually
partially or totally filled with intergrown crystals. § Vermicular, Vermiform: Worm-like in shape. § Vicinal: Crystal face that closely
approximates a larger face in orientation. § Vitreous: Brightly reflective; similar to the luster of freshly broken glass. § Vug: Small cavity in rock, usually lined with crystals. W § Waxy: Slightly reflective mineral luster; typical of minutely granular surfaces. X § Xenoblast: A mineral grain without faces that has grown in a metamorphic rock. § Xenomorphic: Pertaining to crystals that are not bounded by faces; anhedral. |