Mineral and Rock Identification Made Easy

Minerals

calcite: hardness = 3 (not very hard), 3 cleavages not 90°, fizzes in dilute HCl

quartz: hardness = 7 (hard), conchoidal fracture

micas: flake off in thin sheets

muscovite: clear or light colored mica
biotite: black (mafic) or dark colored mica

feldspars: hardness = 6 (hard), 2 cleavages at 90°

potassium feldspar (k-spar): perthite (wavy lines)
plagioclase feldspar (plag): striations (parallel grooves)

hornblende (an amphibole): a black, mafic mineral with splintery cleavage

augite (a pyroxene): another black, mafic mineral

olivine: ultramafic; masses of sand-sized olive green granules

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Igneous Rocks

1. determine texture: coarse-grained (visible crystals) = plutonic, fine-grained (microscopic crystals), or porphyritic (visible crystals in fine groundmass) = volcanic

2. determine color/composition: mostly light minerals = felsic; half light, half black (or gray in fine grained rocks) = intermediate; mostly black minerals = mafic; mostly olivine = ultramafic

3. select proper rock name from classification of igneous rocks (memorize)

volcanic

rhyolite

andesite

basalt

 

plutonic

granite

diorite

gabbro

peridotite

 

felsic

intermediate

mafic

ultramafic

other textures: glassy (obsidian), scoria, vesicular

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Sedimentary Rocks

clastic
breccia: contains angular pebbles and gravel
conglomerate: contains rounded pebble and gravel
sandstone: sand-sized grains (mostly quartz sand)
shale: microscopic (clay or silt) grain size

biogenic
limestone: made from calcite (fizzes), may have visible fossils
dolomite: made from dolomite (fizzes weakly when powdered), may have visible fossils
coal: black, low density (light weight), soft

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Metamorphic Rocks

foliated
slate: slaty cleavage,fine-grained, usually gray
schist: made from parallel aligned visible micas, has a sheen, may have accesory minerals like garnet
gneiss: dark-light = mafic-felsic bands

non-foliated
marble: compact mass of intergrown calcite crystals (fizzes)
quartzite: compact mass of intergrown quartz crystals (hard)

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