Geoscience Reference
In-Depth Information
Table A7.2
Typical properties, visible in hand specimen, of common minerals that may be found in
igneous rocks.
Mineral
category
Mineral
Features (note - other colours may be possible)
Felsic
Quartz
Colourless or white (but may be tinted by impurities), no cleavages,
hardness 7. May look milky in veins
Feldspar:
orthoclase
microcline
plagioclase
Two poorly developed cleavages, at roughly 90°
Pink, simple twinning sometimes visible, hardness 6-8
White, hardness 6-7
White (but transparent grey if unaltered), twinning rarely visible,
hardness 7
Nepheline
White to pale grey, two poor cleavages, hardness 5.5-6
Muscovite
Silvery-grey to white, one excellent cleavage so that it occurs as tabular
(fl akey) crystals, shiny cleavage surfaces, hardness 2-3
Mafi c
Amphibole
Dark green to black, 60° cleavages (hard to see), crystals sometimes
elongated, hardness 5-6
Pyroxene
Dark green to black, 90° cleavages (hard to see), hardness 5-6
Olivine
Grass green or darker, equidimensional crystals, no cleavage, hardness
6-7
Chlorite
Pale green, hardness 2-3
Biotite
Dark brown to black, one excellent cleavage so that it occurs as tabular
(fl akey) crystals, shiny cleavage surfaces, hardness 2-3
Tourmaline
Usually black, but may be blue, red or green. Long thin crystals,
sometimes with longitudinal striations. Hardness 7
Accessory
Garnet
Red or green, equidimensional crystals
Apatite
Pale green-yellowish. Very poor cleavage. May look hexagonal,
sometimes fi brous. Hardness 5
Sphene (titanite)
Colourless to yellow, green or brown. One good cleavage. Often as
euhedral rhombic crystals. Hardness 5
Leucite
White or grey, no cleavage. Often euhedral trapezohedral crystals in
alkaline lavas. Hardness 5.5-6
Opaque minerals
(mostly metal
oxides or
sulfi des)
Metallic lustre (some oxides are shiny black). Various colours
Volcanic glass
(not a mineral!)
Brown to black, amorphous, often breaks with conchoidal fractures. May
devitrify to develop fi ne, feathery internal structure
