Crystallography and Mineralogy
Crystallography and Mineralogy covers the seven crystal systems, symmetry, Miller indices, and the physical properties used to identify minerals. The exam tests systematic mineralogy, silicate structures and Bowen's reaction series.
Crystal systems
- Seven systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal and trigonal. They are distinguished by symmetry and axial ratios and angles.
- Symmetry elements: planes, axes and centre. Cubic has four 3-fold axes; hexagonal has one 6-fold axis.
Mineral identification
- Mohs hardness: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10.
- Cleavage is planar and repeatable; fracture is irregular. Streak is the powder colour; lustre is surface appearance.
Silicate structures
- Nesosilicates (olivine), inosilicates (pyroxene, amphibole), phyllosilicates (mica, clay), tectosilicates (quartz, feldspar).
- Bowen's reaction series: olivine, pyroxene, amphibole, biotite then orthoclase, muscovite, quartz.
Worked example
A mineral is scratched by quartz but scratches orthoclase, has two cleavages at 90 degrees, and a white streak. Reasoning: hardness between 6 and 7 points to a feldspar; two right-angle cleavages and white streak indicate orthoclase. Answer: orthoclase feldspar.
Traps
Do not confuse hardness with cleavage: diamond is hardest but has perfect octahedral cleavage. Graphite and diamond are both carbon but differ in structure and hardness.