Mineral Identification tips

Sue's Mineral Identification tips for Igneous Minerals

Olivine - bright green, no cleavage (conchoidal fracture), often sub to anhedral. Often serpentinized, so that it appears black to brown
Clinopyroxene - bright green (diopside) to black (augite), 2 cleavages at 90 degrees, stubby prismatic crystals. Typically subhedral to euhedral. Distingish diopside from olivine by presence of cleavage, distinguish diopside and augite from orthopyroxene by color.
Orthopyroxene (enstatite, hypersthene) - brown to greenish brown to brownish green, stubby, prismatic crystals. Shreddy cleavage. Typically subhedral. Can distinguish from clinopyroxene by color and by distinctive cleavage.
Hornblende - black, typically elongate prismatic crystals with distinctive 60/120 cleavage. Distingish from augite by cleavage, elongate habit, and in fresh volcanic rocks a more "shiny" cleavage face. Typically subhedral. Distinguish from biotite by lack of flakes, duller cleavage.
Biotite - black to blackish brown. Thin flakes that can be flaked off with finger or other instrument. Very shiny, very reflective. Euhedral to anhedral. Distinguish from hornblende by the brownish tint and by the flaky cleavage.

Quartz - dull grey to white, greasy luster, equant grains with no cleavage. Conchoidal fracture. Typically anhedral in plutonic rocks and euhedral to anhedral in volcanic rocks.
K-feldspar (sanidine, orthoclase, microcline) - sub to euhedral, white to pink, stubby grains with 2 cleavages at 90 degrees. No striations visible on cleavage surfaces. Distinguish from quartz by presence of cleavage, whiter appearance. Distinguish from plagioclase by lack of striations, and sometimes pink color.
Plagioclase - sub to euhedral, white, elongate to stubby grains with 2 cleavages at 90 degrees. Distinguish from K-feldspar by presence of striations (caused by albite twinning) on cleavage surfaces.
Muscovite - euhedral to anhedral, clear to white flakes with shiny, very reflective surfaces. Appears in same habit as biotite, but are white rather than black. Muscovite often appears as tiny reflective specks in plutonic rocks. It is uncommon in volcanic rocks.