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FELDSPAR

Feldspar is a chemically complex group of minerals with a wide range of members.
The two main Feldspar groups that are of interest to gemologists are Alkali Feldspars and Plagioclase Feldspars.

The Alkali varieties include Orthoclase, a monoclinic Potassium Aluminium Silicate with varieties such as Moonstone and Sanidine. Microcline, a triclinic Potassium Aluminium Silicate which includes Amazonite amongst its members. 
The second group are the Plagioclase Feldspars, all triclinic Sodium-Calcium series Aluminium Silicates, whose members include Albite, Oligoclase (Sunstone and Aventurine)  Andesine, Labradorite and Bytownite.

The chemical and structural differences between these groups, coupled with some specimens having a combination of these properties, gives the gem enthusiast a wide and varied scope of beautiful and fascinating gem materials, from the pinprick schiller of Sunstone to the blue sheen of Moonstone.