Geo 101 CRITIQUE FOR rOCKS
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1. Igneous Grain visibility Texture/composition Name Properties
Grains are Obvious Quartz Present Granite
Large grained; peachy-‐pink grains of orthoclase and the transparent grains of quartz
Quartz Absent Diorite Similar to granite, but not orthoclase or quartz Glassy Obsidian Black; glassy texture
Vesicular (porous) Scoria Dark in color; porous
Grains aren't Obvious Pumice Light in color; porous
Fine grains Basalt Dark gray-‐black; only very small crystals visible Rhyolite Peachy-‐pink; only very small crystals visible
2. Sedimentary Grain visibility Grain size Grain type Name Properties
Grains are not Shells Breccia Large angular grains
Grains are Large Conglomerate Large rounded grains
Grains are Shells Fossiliferous Limestone Fossils imbedded in considerable matrix
Grains are Visible Coquina Fossils imbedded in very little matrix
Grains are Not Large Sandstone Does not react to acid Oolitic Limestone Reacts to acid
Shale Thinly bedded Grains are not Visible Chert Very smooth and angular
Petrified Wood Characteristics of wood preserved
3. Metamorphic Grain visibility Banding Name Properties
Banding Present Gneiss Grains align in bands
Grains are Visible Marble Granular; will effervesce in acid Banding Absent Phyllite Fine, but visible grains
Garnet schist Large, 12-‐sided garnets imbedded in matrix Grains are not Visible Slate Dark colored; flat on top and bottom