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METAMORPHIC ROCKS CLASSIFICATION KEY FOLIATED ROCKS “Protolith” = Parent Rock ROCK NAME GRAIN SIZE SLATE Fine-grained (too small to see with the unaided eye) Foliation: thin, parallel layers; flat, nearly smooth surfaces; may look as if it can easily be split into thin sheets. Dull surface appearance Typically has uniform color; comes in many different colors including black, dark gray, green, brown, red Fine-grained sedimentary rocks (e.g. shale) Fine-grained (grains too small to see with the unaided eye) Foliation: thin, sometimes wavy layers; flat, nearly smooth surfaces; may look as if it can easily be split into thin sheets. Surface has a glossy sheen; colors similar to those of slate. Shale; fine-grained sandstone; slate SCHIST Medium-grained prominent mica minerals, often contains visible crystals Foliation: layers highlighted by large, platy mica minerals (muscovite, biotite, chlorite) in thin, parallel or wavy bands. Surface is very shiny; may contain isolated crystals, commonly garnet. Sandstone; volcanic igneous rock; any finegrained metamorphic rock GNEISS Coarse-grained crystals Foliation: distinct crystals arranged in subparallel to parallel bands of alternating light and dark colored minerals; appears striped. Crystals may appear flattened or elongated parallel to foliation. Plutonic igneous rock (e.g. granite, diorite); coarsegrained clastic sedimentary rocks; any fine-grained metamorphic rock PHYLLITE CHARACTERISTICS POSSIBLE PROTOLITH NONFOLIATED ROCKS ROCK NAME GRAIN SIZE MARBLE Fine to coarse crystals Composed of calcite or dolomite; reacts to HCl Often has shiny or glassy luster; soft (may or may not scratch penny, does not scratch glass); many colors, e.g. white, tan, pink, green. Any type of limestone QUARTZITE Fine to coarse crystals Composed mostly of quartz. Glassy luster; may have crystalline or sugary appearance; hard (can scratch glass); many colors, e.g. white, tan, pink, green. Sandstone Fine (not visible) Black, shiny (almost glassy) appearance; looks similar to obsidian but not as heavy or as dense Coal ANTHRACITE COMPOSITION/CHARACTERISTICS POSSIBLE PROTOLITH