Tables to Help Identify Rocks

CLASSIFICATION OF IGNEOUS ROCKS Composition Light-Colored Intermediate-Color Dark-Colored Very Dark Color 10-20% quartz No quartz No quartz 100% K-feldspar > plagioclase > plagioclase ≈ 50% ferromagnesian plagioclase K-feldspar 50% ferromagnesian minerals ≈10% 25–40% minerals ferromagnesian ferromagnesian minerals minerals Pegmatitic (very coarse- Pegmatite grained) Porphyritic (mixed crystal Porphyry sizes) Phaneritic Granite Diorite Gabbro e (coarse-grained) Granodiorite r u t

x Peridotite e Aphanitic Rhyolite

T Andesite Basalt (fine-grained) Dunite Glassy Obsidian Porous Pumice Scoria (glassy, frothy) Pyroclastic Volcanic tuff (fragments < 4mm) (fragmental) Volcanic breccia (fragments > 4mm)

CLASSIFICATION OF CLASTIC SEDIMENTARY ROCKS Particle size Rock Name Characteristics Conglomerate Rounded grain fragments Coarse (> 2mm) Breccia Angular grain fragments

Quartz grains Quartz (often well rounded, well sorted)

Quartz and feldspar grains Medium (1/16–2mm) Sandstone Arkose (often reddish color)

Quartz grains, Small rock fragments, Graywacke and clay minerals (often grayish color)

Silt sized particles, Fine (1/256–1/16 mm) Siltstone surface is slightly gritty Clay sized particles, Fine (< 1/256 mm) Shale surface has smooth feel, no grit

CLASSIFICATION OF METAMORPHIC ROCKS Foliated Metamorphic Rock Crystal Size Rock Name Characteristics Very fine, crystals not visible Slate Excellent rock cleavage Fine grains, crystals not visible Phyllite Well developed foliation; glossy sheen Muscovite schist Coarse texture Chlorite schist Crystals visible with unaided eye Biotite schist t Mineral content reflects s i

Micaceous minerals h Garnet schist increasing metamorphism c Often contains large crystals S Staurolite schist from top to bottom Kyanite schist Sillimanite schist Coarse Gneiss Banding of light and dark minerals