From The Gemology Project
Revision as of 07:42, 19 December 2006 by Doos (talk | contribs)
Jump to: navigation, search
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

Feldspar is a group of minerals that are very important in rock formation, accounting for over half the 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 known for their phenomenal iridescence and adularescence. Amazonite was used by the ancient Egyptians for the carving of deities. The stone was 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-feldspars grow in monoclinic crystals and have a chemical composition of KAlSi3O8.


  • Orthoclase
  • Orthoclase moonstone
  • Microcline
  • Sanidine

Plagioclase feldspar

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


  • 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