Metamorphic Rock Classification

METAMORPHIC ROCK
CLASSIFICATION

		Classification

When preexisting rocks are exposed to conditions of high temperature and/or pressure they undergo solid-state changes (they "metamorphose") to become metamorphic rocks. The rock doesn't melt, but it changes state by one or both of these processes:

mineral changes - growth of new minerals that are more stable under conditions of high temperature/pressure

textural changes - recrystallization, alignment of platy minerals, usually as a result of unequal application of stress

The first thing to notice when you look at a metamorphic rock is its texture. Is the rock foliated or not? Foliation refers to flat or wavy planar features (looking like layers) caused by the alignment of platy minerals such as mica. Foliation may also look like alternating bands of light and dark minerals. In contrast, a nonfoliated rock has interlocking grains with no specific pattern. Foliated rocks (Table 1) are classified based on metamorphic grade: the lower the metamorphic grade, the smaller and finer the crystal size. Nonfoliated rocks (Table 2) are classified based on composition, and this usually depends on the type of rock it originally formed from (called the protolith).

**TABLE 1: FOLIATED (banded) ROCK CLASSIFICATION**
Metamorphic Environment	50-300慢	300-450慢	Above 450慢
Metamorphic Grade	Low	Intermediate	High
Rock Name	SLATE	SCHIST	GNEISS
Rock Description	Minerals not visible with the naked eye or with a hand lens, rock shows slaty cleavage, is usually dark-colored. A product of low-grade metamorphism of shale or mudstone.	Rock is medium to coarse grained with visible grains of mica or other metamorphic minerals. Often shiny due to reflection of mica on foliation planes. Product of intermediate grade metamorphism of shale, slate, phyllite, basalt or granite.	Rock is coarse grained and usually banded with alternating layers of light and dark minerals. Foliation bands may be folded. Product of high grade metamorphism of shale, schist, granite or many other rock types.

**TABLE 2. NONFOLIATED (not banded) ROCK CLASSIFICATION**
	MARBLE	QUARTZITE	ANTHRACITE COAL
Mineral(s) Description	calcite Coarse-grained recrystallized limestone or dolomite. Typically harder than the protolith. May have dark bands due to organic impurities.	quartz Rock has intergrown quartz grains, thus is massive and hard. Protolith is sandstone. Intermediate to high grade metamorphism.	crystalline carbon Hard, black shiny coal; product of low-grade metamorphism of bituminous coal.

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