Sue's Mineral Identification tips for Igneous Minerals
Mafic minerals (dark colored)
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.
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.
Felsic minerals (light colored)
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
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.