Feldspar

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Feldspar
Chemical composition Potassium, sodium and calcium-aluminum silicates

I. K-feldspar (KALSi308)
II . Plagioclase feldspar (isomorphous)

Crystal system Monoclinic - triclinic
Habit Prismatic, often twinned
Cleavage Good to perfect
Fracture Conchoidal
Hardness 6
Optic nature Biaxial ±
Refractive index 1.52 - 1.53 (K-feldspar)

1.528 - 1.588 (plagioclase)

Birefringence 0.006 - 0.007 (K-feldspar)

0.008 - 0.011 (plagioclase)

Specific gravity 2.55 - 2,58 (K-feldspar)

2.60 - 2.80 (plagioclase)

Lustre Vitreous
Oligoclase feldspar from Tanzania.
Photo courtesy of Scott Davies, americanthai.com


Feldspar is a group of minerals that are very important in rock formation, accounting for over half of Earth's crust! There are a number of varieties that are used in jewelry. The most important are andesine, sunstone, amazonite, moonstone, and labradorite, the latter two known for their phenomenal adularescence and iridescence . Amazonite was used by the ancient Egyptians for carving images of deities, the stone considered a catalyst between the living and the gods. Moonstone was thought to drive away sleeplessness.

Feldspars are divided into two types:

  • K-feldspars (potassium feldspars)
  • Plagioclase feldspars (an isomorphous series between albite and anorthite)

K-feldspar

K-feldspars grow in monoclinic crystals (except for microcline) and have a chemical composition of KAlSi3O8.

Varieties

  • Orthoclase
  • Orthoclase moonstone
  • Microcline (amazonite)
  • Sanidine

Plagioclase feldspar

Plagioclase feldspars grow in triclinic crystals. Its varieties belong to an isomorphous series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).

Varieties

  • Albite (100-90% albite, 0-10% anorthite)
  • Oligoclase (90-70% albite, 10-30% anorthite)
  • Andesine (70-50% albite, 30-50% anorthite)
  • Labradorite (50-30% albite, 50-70% anorthite)
  • Bytownite (30-10% albite, 70-90% anorthite)
  • Anorthite (10-0% albite, 90-100% anorthite)
Timeline image indicating the percentages of the albite-anorthite series