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Feldspar and mica groups of minerals

Short answer

Feldspars are aluminosilicates of K, Na, Ca with hardness 6, two good cleavages near 90°, and weather to clay, reducing rock strength. Micas are sheet silicates with perfect basal cleavage, hardness 2-3, and occur as muscovite (light) and biotite (dark); their flaky nature makes rocks weak, slippery, and unsuitable for foundations and aggregate.

Describe the properties of the feldspar and mica groups of minerals.

Feldspar group

The most abundant minerals of the crust (about 50-60%). They are aluminosilicates of potassium, sodium and calcium.

  • Types: potassium feldspars (orthoclase, microcline) and plagioclase (albite to anorthite, a series of sodium-calcium feldspars).
  • Crystal system: monoclinic (orthoclase), triclinic (microcline, plagioclase).
  • Hardness: 6. Specific gravity: 2.55-2.76.
  • Cleavage: two good cleavages at about 90° (or near 86° in plagioclase); plagioclase shows fine striations.
  • Colour: white, pink, cream or grey; vitreous to pearly lustre; white streak.
  • Occurrence: granite, gneiss, syenite, basalt.
  • Engineering: durable when fresh, but weathers to clay, lowering rock strength.

Mica group

Sheet silicates with very perfect cleavage in one direction, splitting into thin, flexible, elastic sheets.

  • Types: muscovite (white mica, light) and biotite (black mica, iron-magnesium rich).
  • Crystal system: monoclinic.
  • Hardness: 2-3. Specific gravity: 2.7-3.1.
  • Lustre: pearly to vitreous; colourless to brown or black.
  • Occurrence: granite, schist, gneiss, phyllite, sandstone.
  • Engineering: mica-rich rocks split easily along planes, are weak and slippery, so unsuitable as foundation, aggregate or for stable slopes.

✓ Answer checked by Claude Opus 5.5 (high effort)

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