Instructor: Arthur Reed

Sedimentary Rock Classification

(Last update: March 8, 2015

 

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Rock Type

 

Grains Not Visible

Fizzes in dilute HCL

White, soft, and powdery

Chalk

Gray, black, brown or tan; compact, dense. Very fine grained (clay-sized)

Micrite or calcilutite (a fine grained matrix limestone)

Fossils in lime mud matrix

Fossiliferous limestone

Coarse crystalline mosaic, brown and white color bands, may be cylindrical (stalactite)

Travertine

Fine to coarse crystalline mosaic; compact, dense, massive

Crystalline limestone

Fizzes in dilute HCL when scratched and powdered

Gray or black, weathers yellowish gray to brown; compact, dense, massive; dolomite

Dolostone

Does not fizz in dilute HCL

Fissile (breaks into thin layers); may be softer than fingernail; clay-sized texture; commonly gray, black, brown, or red

Shale

Feels gritty to the finger; commonly gray, black, brown, or red

Siltstone

Salty taste, may feel slippery; often clear and transparent; cleavage

Rock salt

Softer than fingernail; white, pink, clear; may be fibrous, fine-grained, or crystalline

Rock gypsum

Hard scratches glass; opaque; color variable; smooth feel; may have conchoidal fracture

Chert

White; looks like chalk but does not fizz in acid; very low density (may float); porous

Diatomite

White; looks like chalk but does not fizz in acid; dense (does not float); may stick to moistened finger

Kaolinite (clay)

Black, may leave sooty marks on fingers or paper, may have layers

Bituminous coal

Brown to black, crumbly, very soft; porous

Lignite

Brown, porous, soft; resembles peat moss

Peat

Dark-colored, heavy, amorphous chemical residues (limonite) or microcrystalline nodules (e.g., hematite, goethite)

Ironstone

 

 

 

 

Rock Type

 

Grains Visible

Clasts or allochems larger than 2 mm

Grains are all shell fragments; no mud; fizzes in dilute HCL

Coquina

Clasts and matrix fine grained; clasts are limestone and may be flat and laminated; fizzes in acid

Intraclastic limestone

Matrix color variable; multiple clasts lithologies; clasts differ from matrix in color or composition

Breccia (angular clasts)

Conglomerate (rounded clasts)

Clasts or allochems smaller than 2 mm

White or colorless grains, mostly quartz

Quartz sandstone

Contains pink, gray, or white feldspar (look for cleavage): feldspar grains may be weathered to white kaolinite

Arkose sandstone

Contains rock fragment grains, mostly dark or gray grains (such as basalt or shale fragments)

Litharenite or lithic sandstone or graywacke

Round grains with concentric laminations; fizzes in acid

Oolitic limestone

Dark red to brown, red-brown streak, may contain replaced oolites or fossils, may fizz in acid; may be dense and heavy

Oolitic hematite or oolitic ironstone or fossiliferous ironstone

 

 

 

 

 


 

 

 

 

 

Based on information from the site of Pamela Gore, Department of Geology, Georgia Perimeter College