Mineralogy: Crystal Systems, Physical Properties and Silicate Structures
What the domain covers
Mineralogy: Crystal Systems, Physical Properties and Silicate Structures examines how minerals are defined, classified by symmetry, identified by physical tests, and grouped by silicate tetrahedral arrangement. Questions often ask you to identify a mineral from diagnostic properties or to link a silicate structure with an example.
Core rules
- A mineral is naturally occurring, inorganic, crystalline and has a definite chemical composition or composition range.
- There are seven crystal systems: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic. They are distinguished by symmetry elements and axial lengths and angles.
- The law of constancy of interfacial angles states that interfacial angles are constant for a mineral species, even if crystal faces differ in size and shape.
- Mohs hardness scale runs from talc 1 to diamond 10. Cleavage is a planar break along weak bonds; fracture is an irregular break. Luster, streak, colour, specific gravity and magnetism are also diagnostic.
- Silicate structures include isolated tetrahedra (olivine), single chains (pyroxene), double chains (amphibole), sheets (mica and clay), frameworks (quartz and feldspar) and ring structures (beryl).
Worked example
A specimen has hardness 6, two cleavages meeting at about 90 degrees, and a white to pink colour. Quartz has hardness 7 and no cleavage. Calcite has hardness 3 and rhombohedral cleavage. Mica has one perfect cleavage. The specimen is therefore feldspar: hardness 6, two cleavages near 90 degrees, and common pale colours fit alkali feldspar or plagioclase.
Traps
Do not use crystal form as a synonym for cleavage; a crystal may show good faces but no cleavage. Do not treat hardness as absolute; it is a relative scratch scale. Do not place quartz in the sheet-silicate group; it is a framework silicate.