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
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
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
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)