Instructor: Arthur Reed

Common Rock-Forming Minerals

(Last update: 1/8/10)

Igneous Rock

 

Name

Formula

(variations do occur)

H

Color

Fracture/Cleavage

Other Diagnostic Properties

 

felsic

Quartz

SiO2

7

Colorless, white, gray

Concoidal fracture

Glassy, hexagonal crystal form is distinctive when present

Potassium Feldspar

(group)

orthoclase,

microcline,

sanidine,

adularia

KAlSi3O8

6

White, pink, buff gray

2 directions at about 90°

Exsolution lamellae present in some samples

Plagioclase Feldspar

(group)

albite,

oligoclase,

andesine,

labradorite,

bytownite,

anorthite

CaAl2Si2O8

To

NaAlSi3O8

6

White, pink, gray, dk gray

2 directions at about 90°

Polysynthetic (albite) twinning on best cleavage surface

Muscovite (mica)

KAl2(AlSi3)O10(OH)2

2-2.5

colorless

1 direction perfect cleavage

Color is distinctive

mafic

Biotite

(mica)

K(Mg,Fe)3(Al,Fe)Si3O10(OH)2

2-2.5

black

1 direction perfect cleavage

Color is distinctive

Amphibole (principle of group is hornblende)

Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

5-6

Dk green, black

2 good cleavages at about 120°

Elongated crystals, better cleavage than pyroxene

Pyroxene

(principle of group is augite)

Ca(Mg,Fe)Si2O6

to

(Mg,Fe)2Si2O6

[augite – (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6]

5-6

Green, gray green, white

2 poor cleavages at about 90°

Poor cleavage, stubby crystals

Olivine

(Mg,Fe)2SiO4

6.5-7

Green, yellow green

Concoidal facture

Glassy, color usually distinctive

Sedimentary Rock

 

Name

Formula

H

Color

Fracture/Cleavage

Other Diagnostic Properties

 

 

Quartz

SiO2

7

Colorless, white, gray

Concoidal fracture

Glassy, hexagonal crystal form is distinctive when present

Calcite

CaCO3

3

Colorless, white

3 directions not at 90°

Rhombic cleavage fragments, reacts (fizzes) with HCL

Dolomite

CaMg(CO3)2

3.5-4

White, gray, pink

3 directions not at 90°

Reacts (fizzes) less vigorously with HCL (compared to calcite) or only when powdered

Kaolinite

(principle member of the kaolin group)

Al4(Si4O10)(OH)8

2

white

1 direction perfect cleavage

Sticks to tongue

Hematite

Fe2O3

2-6

Red, dk red, metallic gray

Weak parting, typically not expressed

Red streak, specular hematite shows bright gray metallic luster

Limonite

Fe2O3•nH2O

3.5-4

Yellow red

n.a.

Yellow-red streak

Halite

NaCl

2.5

colorless

3 directions at 90°

Salty taste, cubic cleavage fragments

Gypsum

CaSO4•2H2O

2

Colorless, white

1 good and 2 poor cleaves

Simple twins common in crystals

Metamorphic Rocksa

 

Name

Formula

H

Color

Fracture/Cleavage

Other Diagnostic Properties

 

 

Talc

Mg3Si4O10(OH)2

1

white

1 direction perfect cleavage

Greasy feel

Chlorite

(group)

(Mg,Fe)6AlSi3O10(OH)8

2-2.5

green

1 direction perfect cleavage

Color is distinctive

Garnet

(group)

(Ca,Mg,Fe,Mn)3(Al,Fe,Mn,V,Cr)2(SiO4)3

6.5-7.5

Red, pink, green, black

Concoidal facture

Dodecahedron form common in crystals

Kyanite

(trimorphous with sillimanite & andalusite)

Al2SiO5

5.5-7

Sky blue to white, grey, green, black

1 perfect and

1 good direction cleavage

Stubby blades.  The long direction of cleavage planes can be scratched with a knife; the perpendicular cannot.

Sillimanite

(trimorphous with kyanite & andalusite)

Al2SiO5

6.5-7.5

Colorless to white

1 good cleavage

Small fibers or needles with cleavage perpendicular to long axis.

Andalusite

(trimorphous with kyanite & sillimanite)

Al2SiO5

6.5-7.5

Usually pink

1 direction good

1 direction poor

cleavage

Rod shaped with almost square cross-sections.  Chiastolite cross.

Staurolite

(Fe,Mg)2Al9Si4O23(OH)

7

brown

1 direction poor

Interpenetration twins common, elongate crystals

Magnetite*

(Fe,Mg)Fe2O4

5.5-6.5

Black, metallic

Weak parting, typically not expresses

magnetic

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a Most of the minerals above also occur in metamorphic rocks.

*common igneous mineral as well

 

 

 

Common Ore/Minerals

 

 

 

Name

Formula

H

Color

Fracture/Cleavage

Other Diagnostic Properties

 

 

Azurite

Cu3(CO3)2(OH)2

3-4

Azure blue

2, good

Effervesces in weak acid

Bauxite

Al2O3•nH2O

 

 

 

Earthy luster.  A variety of clay. Generally pea-sized reddish-brown spheres included in a fine-grained mass. Not considered a mineral.

Bornite

Cu5FeS4

3

Tarnishes to iridescent purple

Imperfect

Concoidal fracture

Chalcopyrite

CuFeS2

3.5-4

Metallic luster, brass yellow

2; usually distinct

Softer than glass

Cinnabar

HgS

2-2.5

Scarlet red

Prismatic, perfect

Bright red streak

Galena

PbS

3

Metallic luster, yellow

3 directions of cleavage at 90° (cubic)

Gray streak, high specific gravity

Gold

Au

3

Metallic luster, yellow

 

Easily deformed

Halite

NaCl

2.5

colorless

3 directions at 90° (cubic)

Salty taste, cubic cleavage fragments

Hematite

Fe2O3

2-6

Red, dk red, metallic gray

Weak parting, typically not expressed

Red streak, specular hematite shows bright gray metallic luster

Limonite

Fe2O3•nH2O

3.5-4

Yellow red

n.a.

Earthy luster, yellow-brown streak

Magnetite

(Fe,Mg)Fe2O4

5.5-6.5

Black, metallic

Weak parting, typically not expressed

Magnetic, black streak

Malachite

Cu2(CO3)(OH)2

3.5-4

Bright green

2, one perfect, one distinct

Bright green streak, effervesces in weak acid

Sphalerite

(Zn,Fe)S

3.5-4

Brown to yellow

6 directions of cleavage

Lusterlike resin; yellow or cream colored streak

Talc

Mg3Si4O10(OH)2

1

White, gray, green

1, perfect

Greasy feel