Sedimentary Rock Classification

(Last update: March 8, 2015


Start Here			Rock Type
Grains Not Visible â	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 â	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