Saturday, November 23, 1996
About 150 oxides are known, but very few are easily collected and even fewer are found as beautiful specimens. Oxides are formed in a variety of environments, many being formed in igneous or metamorphic rocks, some in ore veins and a great number are found in pegmatites. Others form near the surface under low pressure and temperature. The rust on your car is one example of this kind of deposit! Because of the strength of the bonds between oxygen and metals, oxides are resistant to further chemical attack and generally form strong, hard minerals.
Cuprite
- Copper oxide
- Brown to purplish red colour
- Cubic crystals
- Hardness 3.5 - 4.0
- Usually found as small crystals with native copper
Hematite
- Iron oxide
- Black or steel grey, sometimes as small red inclusions in other minerals
- Hexagonal crystals
- Hardness 6.5 (crystals)
- Colour, hardness and red streak are distinctive.
Brucite
- Magnesium hydroxide
- Transparent to translucent white grey, pale blue
- Hexagonal crystals - rarely found as crystals
- Hardness 2.5
- Pearly lustre and harder than talc
Spinel
- Magnesium aluminum oxide
- Wide variety of colours
- Glassy lustre
- Isometric crystals
- Hardness 8
- Conchoidal fracture which is usually very smooth
Corundum
- Aluminum oxide
- Transparent and is found in many colours
- Hexagonal crystals
- Hardness 9.0
- Hardness and crystal form are good aids to identification
Cassiterite
- Tin oxide
- Black, brown and tan colours
- Greasy to adamantine lustre
- Tetragonal crystals
- Hardness 6-7
- Very heavy brilliant black crystals
Goethite
- Hydrogen iron oxide
- Black, brownish black, brownish yellow, brown streak
- sub metallic to dull lustre
- Orthorhombic crystals (rare)
- Hardness 5 - 5.5
- Brown streak distinguishes it from hematite
Magnetite
- Ferrous-ferric iron oxide
- Opaque, iron black, grey black, sub metallic to dull lustre
- Isometric crystals
- Hardness 6
- Attracted to a magnet more than any other mineral