Common Opaque Minerals
Haematite - deep red in thin flakes
Magnetite - equant grains
Chromite - equant grains
Ilmenite - elongate crystals
| Shape (ideal) | Cleavage | Relief | Colour (natural) | Interference Colour | Twinning | Other features | ||
|---|---|---|---|---|---|---|---|---|
| Olivine | |
One (poor) | high | colourless | intermediate | - | Irregular fractures, often associated with alteration to serpentine (+ opaque Fe oxides) | |
| Orthopyroxene | |
Two (90° ) | high | colourless or v. pale grey | low | - | Most sections show 1 cleavage and straight extinction. May exsolve augite | |
| Augite | high | pale grey-brown | intermediate | occasional (simple or multiple) | Most sections show 1 cleavage and oblique extinction | |||
| Hornblende | |
Two (60/120°) | high | green / brown pleochroic | intermediate | occasional (simple or multiple) | Most sections show 1 cleavage and oblique extinction. May be altered to chlorite | |
| Biotite | |
One | mod | brown pleochroic | intermediate | - | Sections showing cleavage have straight extinction. Speckled appearance close to extinction. Biotite alters to chlorite | |
| Muscovite | mod | colourless | intermediate | - | ||||
| Quartz | |
- | low | colourless | low | - | Always unaltered. Deformation may cause patchy extinction | |
| Feldspar | |
Two (~90°) | low | colourless | low | common | Typically some degree of alteration to clay minerals (fine particles) and small flakes of secondary muscovite (sericite) | |
| (Plagioclase) | " | " | " | " | multiple (mainly 1 direction) ± simple (same direction) | |
Alteration as above but may also produce small grains of epidote | |
| (K-Feldspar - | Sanidine) | " | " | " | " | simple | |
Often unaltered and easily confused with quartz. Look for cleavage and twinning. |
| Orthoclase) | " | " | " | " | simple | |
Often shows exsolution of Na-rich feldspar = perthite | |
| Microcline) | " | " | " | " | multiple in 2 directions (~90°) on fine scale | |
||
| Garnet | |
- | high | colourless (grey) or pale brown | isotropic | - | Common Opaque Minerals Haematite - deep red in thin flakes Magnetite - equant grains Chromite - equant grains Ilmenite - elongate crystals |
|
| Calcite | |
Three (not 90°) | low | colourless | high | multiple | ||
| Apatite | |
- | mod | colourless | low | - | ||
| Zircon | |
- | v. high | colourless (grey) | intermediate | - | ||
| Sphene | |
Two | v. high | pale brown | high | simple | ||