Rock Classification | Sanuja Senanayake

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To be honest, this is one of those “grey” areas in science. The classifications (naming) of rocks are usually based on empirical observations. Rarely a Geologist would name a rock based on chemical analysis. The qualitative observations of minerals in a sample is commonly used in place of chemical analysis. Therefore, we can categorically argue naming conventions are based on the chemical properties of the rock. Depending on the scheme, the rock name may be sightly varies. This is why I recommend recording the modal proportions of minerals when naming a rock (or should include a plotted diagram). This will avoid confusion in different classification schemes.

Identification of Igneous Rocks

Grain Size	Usual Color	Other	Composition	Rock Type
fine	dark	glassy appearance	lava glass	Obsidian
	light	many small bubbles	lava froth from sticky lava	Pumice
	dark	many large bubbles	lava froth from fluid lava	Scoria
fine or mixed	light	contains quartz	high-silica lava	Felsite
	medium	between felsite and basalt	medium-silica lava	Andesite
	dark	has no quartz	low-silica lava	Basalt
mixed	any color	large grains in fine-grained matrix	large grains of feldspar, quartz, pyroxene or olivine	Porphyry
coarse	light	wide range of color and grain size	feldspar and quartz with minor mica, amphibole or pyroxene	Granite
	light	like granite but without quartz	feldspar with minor mica, amphibole or pyroxene	Syenite
	light to medium	little or no alkali feldspar	plagioclase and quartz with dark minerals	Tonalite
	medium to dark	little or no quartz	low-calcium plagioclase and dark minerals	Diorite
	medium to dark	no quartz; may have olivine	high-calcium plagioclase and dark minerals	Gabbro
	dark	dense; always has olivine	olivine with amphibole and/or pyroxene	Peridotite
	dark	dense	mostly pyroxene with olivine and amphibole	Pyroxenite
	green	dense	at least 90% olivine	Dunite
very coarse	any color	usually in small intrusive bodies	typically granitic	Pegmatite

Identification of Sedimentary Rocks

Hardness	Grain Size	Composition	Other	Rock Type
hard	coarse	clean quartz	white to brown	Sandstone
	coarse	quartz and feldspar	usually very coarse	Arkose
	fine	very fine sand; no clay	feels gritty on teeth	Siltstone
	fine	chalcedony	no fizzing with acid	Chert
hard or soft	mixed	mixed sediment with rock grains and clay	gray or dark and “dirty”	Wacke/ Graywacke
	mixed	mixed rocks and sediment	round rocks in finer sediment matrix	Conglomerate
	mixed	mixed rocks and sediment	sharp pieces in finer sediment matrix	Breccia
soft	fine	clay minerals	splits in layers	Shale
	fine	carbon	black; burns with tarry smoke	Coal
	fine	calcite	fizzes with acid	Limestone
	coarse or fine	dolomite	no fizzing with acid unless powdered	Dolomite rock
	coarse	fossil shells	mostly pieces	Coquina
very soft	coarse	halite	salt taste	Rock Salt
very soft	coarse	gypsum	white, tan or pink	Rock Gypsum

Identification of Metamorphic Rocks

Foliation	Grain Size	Usual Color	Other	Rock Type
foliated	fine	light	very soft; greasy feel	Soapstone
	fine	dark	soft; strong cleavage	Slate
	fine	dark	shiny; crinkly foliation	Phyllite
	coarse	mixed dark and light	crushed and stretched fabric; deformed large crystals	Mylonite
	coarse	mixed dark and light	wrinkled foliation; often has large crystals	Schist
	coarse	mixed	banded	Gneiss
	coarse	mixed	distorted “melted” layers	Migmatite
	coarse	dark	mostly hornblende	Amphibolite
non foliated	fine	dark	soft; massive structure	Argillite
	fine	greenish	soft; shiny, mottled surface	Serpentinite
	fine or coarse	dark	dull and opaque colors, found near intrusions	Hornfels
	coarse	red and green	dense; garnet and pyroxene	Eclogite
	coarse	light	soft; calcite or dolomite by the acid test	Marble
	coarse	light	quartz (no fizzing with acid)	Quartzite