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Classification of Metamorphic Rocks

TEXTURES Textures of metamorphic rocks fall into two broad groups, FOLIATED and NON-FOLIATED. Foliation is produced in a rock by the parallel alignment of platy minerals (e.g., muscovite, biotite, chlorite), needle-like minerals (e.g., hornblende), or tabular minerals (e.g., feldspars). This parallel alignment causes the rock to split easily into thin layers or sheets. Foliation is common in aphanitic as well as phaneritic metamorphic rocks. Some foliated rocks are also banded. Banding means that the rock consists of alternating, thin layers (typically 1 mm to 1 cm) of two different mineral compositions. Normally, the two types of layers have the same kinds of minerals, but in different proportions, giving the rock a striped appearance. Banding, by itself, defines a foliation.

In order of increasing grain size, foliated textures are referred to as SLATY (aphanitic, very fine-grained), PHYLLITIC (aphanitic, fine-grained), SCHISTOSE (phaneritic). The corresponding rock types are called SLATE, PHYLLITE, and SCHIST. These rocks are not normally banded. The composition of the rock (as expressed by it's minerals) is uniform throughout the volume of the rock. The banded, foliated texture is referred to as GNEISSOSE. The corresponding rock type is GNEISS. Gneiss is normally phaneritic, but in some cases the layers are aphanitic.

As the term implies, NON-FOLIATED rocks lack foliation or banding. Such rocks are most commonly composed of minerals that are neither platy nor needle-like, but rather more equidimensional (more or less the same dimension in all directions). Quartz, calcite and dolomite are the most common such minerals. In phaneritic rocks the texture is referred to as GRANOBLASTIC. Common granoblastic rocks included QUARTZITE (quartz), and MARBLE (calcite or dolomite).

The textures of some metamorphic rocks do not fit neatly into any of these categories. In such cases, where the protolith is obvious because the texture of the protolith is well preserved, the prefix "META" is simply used in front of the protolith name, e.g., METAGRANITE, METABASALT, METAWACKE, METACONGLOMERATE, etc.

MINERALOGY Most of the minerals in igneous rocks and many minerals in sedimentary rock can occur in metamorphic rocks. However, depending on the protolith, many minerals form only during the course of metamorphism. In this sense, minerals such as chlorite, garnet, epidote, staurolite, kyanite, sillimanite, and several others are generally thought of as "metamorphic." A shale protolith (initially clay minerals with lesser amounts of quartz and feldspar) undergoes a number of mineralogical changes when exposed to progressively higher pressures and temperatures during the course of metamorphism. These mineralogical changes are marked by the appearance and growth of metamorphic index minerals at the expense of original minerals (such as clay in shale) and previously former metamorphic minerals. Starting with shale, the order of appearance of index minerals with increasing grade of metamorphism (from slate to phyllite to schist to gneiss) is typically chlorite, biotite, garnet, staurolite (Fe,Mg)₂Al₉Si₄O₂₂(O,OH)₂, kyanite (high pressure Al₂SiO₅) or andalusite (low pressure Al₂SiO₅), sillimanite (high temperature Al₂SiO₅), K-feldspar. Normally, by the time kyanite or andalusite appears, previously formed metamorphic chlorite has long since become unstable and replaced by higher grade minerals. Thus, the kinds of metamorphic minerals in a rock reflect the changing conditions of pressure and temperature that the rock experienced. This kind of information is important for understanding aspects of plate tectonics and the formation of mountain ranges.

CLASSIFICATION CHART

Metamorphic Rocks					MINERAL CONTENT - Examples of minerals that you might find	ROCK NAME	PROTOLITH (Original Rock)

---- T E X T U R E ----	---- F O L I A T E D ----	NOT BANDED	APHANITIC	Very fine-grained, SLATY texture, produces flat fragments	MINERALS TOO SMALL TO SEE	SLATE	Shale Siltstone
				Fine-grained, PHYLLITIC texture, surfaces have satiny sheen, with crenulations (small wrinkles)		PHYLLITE
				May appear grainy		GREENSTONE (Metabasalt)	Basalt
			PHANERITIC	Medium- to coarse-grained, SCHISTOSE texture	MUSCOVITE BIOTITE CHLORITE HORNBLENDE Quartz, Feldspar Garnet, Epidote Staurolite kyanite, etc.	SCHIST	Shale Siltstone Sandstone Basalt
		MAY BE BANDED		Coarse-grained, with pebbles preserved from original sedimentary rock	Quartz, Feldspar Muscovite Chlorite	META- CONGLOMERATE	Conglomerate
				Medium- to coarse-grained with large crystals of some minerals (e.g., garnet, staurolite)	Quartz, Feldspar Muscovite Biotite Chlorite	PORPHYROBLASTIC SCHIST	Sandstone Granite or other Igneous Rock
		BANDED		Medium- to coarse-grained, GNEISSOSE texture	Quartz, Feldspar Biotite Hornblende Muscovite Chlorite	GNEISS

	N O T F O L I A T E D		PHANERITIC	Coarse-grained, with pebbles preserved from original sedimentary rock	Quartz, Feldspar Muscovite Chlorite	META- CONGLOMERATE	Conglomerate
			APHANITIC OR PHANERITIC	Fine- to coarse-grained, GRANOBLASTIC texture	QUARTZ	QUARTZITE	Quartz Arenite
					CALCITE DOLOMITE	MARBLE	Limestone Dolostone
			APHINITIC	May appear grainy	MINERALS TOO SMALL TO SEE	GREENSTONE (Metabasalt)	Basalt

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