Classification of Igneous Rocks

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TEXTURES

			DESCRIPTION	INTERPRETATION
PHANERITIC	1/16 mm to 3 cm		Average size of crystal-grains is more or less uniform and the average grain size is in the range, 1/16 mm to 3 cm.  For instance, in a granite typically 85% or more of the crystals might be close to 1 cm in size.  There may be some smaller grains and some larger grains, but not many of each.  The average crystal size would be about 1 cm.	Plutonic.  Slow rate of cooling.
	>3 cm		PEGMATITIC texture, if most of the volume of the rock is made up of mineral grains larger than 3 cm. Again, there may be some smaller grains, but they don't account for much of the volume.	Plutonic.  Pegmatitic rocks are generally believed to crystallize from H2O-dominated fluid, sort of like a brine.  The fluid is not a conventional silicate magma, but is generally associated with the evolution of conventional granitic magmas.
APHANITIC	< 1/16 mm		Essentially all of the grains are less than 1/16 mm in size.  There may be rare larger crystal, but if the larger crystals don't account for more than about 5% or 10 % of the volume of the rock, the texture may still be considered aphanitic.	Volcanic.  Rapid cooling.
PORPHYRITIC			Bimodal distribution of crystal-grain sizes.  This is a very common texture in volcanic rocks.  "Bimodal" grain-size distribution means two distinct grain-size populations.  In a porphyritic andesite, for instance, 75% of the volume of the rock may be aphanitic (<1/16 mm) and the remaining 25% of the volume may be represented by a population of larger (~8 mm) crystals (e.g., hornblende, plagioclase).	Volcanic or plutonic.  In volcanic rocks, the small grain-size part of the rock (matrix) is aphanitic, and large grain-size crystals are generally phaneritic.  In plutonic rocks, the small grain-size part of the rock (matrix) is phaneritic, and the large grain-size crystals just larger. The texture usually implies a change in the rate of cooling from slow to rapid.
GLASSY			Natural glass (not crystalline).  Smooth, vitreous, usually black, but translucent.  Breaks along conchoidal (smooth, curved) surfaces.	Volcanic, very rapid cooling.
VESICULAR			A high proportion of the volume of the rock (say 40% or more) consists of spherical or elliptical empty spaces.  The texture may be likened to an ordinary kitchen sponge, soap suds, or foam rubber.  Because a significant portion of the rock is air-space, the rocks may be very light.	Volcanic.  Gas bubbles, formed as lava cools on the Earth's surface.

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MINERALOGY Compositions of igneous rocks are expressed by a limited number of combinations of the minerals identified at the top of the CLASSIFICATION CHART below.  The combinations involving OLIVINE, AUGITE (PYROXENE), and Ca-rich PLAGIOCLASE crystallize from magmas that originated in the ASTHENOSPHERE.  The resulting plutonic rocks are GABBRO, PERIDOTITE, DUNITE, and the resulting volcanic rock is BASALT.  Except for the olivine in a dunite, these combinations of minerals produce very dark-colored rocks.  The combinations K-FELDSPAR+QUARTZ, and Na-rich PLAGIOCLASE+HORNBLENDE crystallize from magmas that originated by melting near the base of continental crust or oceanic island arc crust adjacent to convergent plate boundaries.  The resulting plutonic rocks are GRANITE and DIORITE, and the corresponding volcanic rocks are RHYOLITE and ANDESITE.  In GRANITE the dominant minerals (quartz, K-feldspar) are both light in color, so the rock as a whole is light in color.  In DIORITE, one of the dominant minerals (Na-rich plagioclase) is light in color (usually white) while the other dominant mineral (hornblende) is nearly black, so the rock has a coarse salt-and-pepper look.  RHYOLITE is light in color or red. ANDESITE is intermediate in color.

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CLASSIFICATION CHART

Igneous Rocks			--	MINERAL COMPOSITION
				K-FELDSPAR QUARTZ +/- plagioclase biotite muscovite	Na-rich PLAGIOCLASE HORNBLENDE +/- biotite quartz augite	Ca-rich PLAGIOCLASE AUGITE +/- olivine hornblende	PYROXENE(S) OLIVINE	OLIVINE
				ROCK NAME
---- T E X T U R E -----	Phaneritic (Plutonic)	grains  < 3 cm		GRANITE	DIORITE	GABBRO	PERIDOTITE	---DUNITE---
		grains  > 3cm		PEGMATITIC GRANITE	rare or unknown	rare or unknown	rare or unknown	rare or unknown
	Aphanitic (volcanic)			RHYOLITE	ANDESITE	BASALT	rare or unknown	rare or unknown
	Porphyritic (volcanic)			PORPHYRITIC RHYOLITE	PORPHYRITIC ANDESITE	PORPHYRITIC BASALT	rare or unknown	rare or unknown
	Glassy (volcanic)			OBSIDIAN	rare or unknown	rare or unknown	rare or unknown	rare or unknown
	Vesicular (volcanic)			PUMICE (small vesicles)	rare or unknown	VESICULAR BASALT (Scoria)	rare or unknown	rare or unknown
COLOR				Light Shades or red,  except obsidian which is usually black	Intermediate Shades	Dark-shades of green, to nearly black		Light to  Intermediate green

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