Feldspar is a group of minerals which comprises a number of species with varying degrees of isomorphism. Feldspars are subdivided into two sub group,
potassium feldspars (orthoclase, microcline) and plagioclase feldspar.
Of gem interest are orthoclase, microcline and plagioclase (albite, labradorite and oligoclase).
Aluminum silicate of potassium, sodium and calcium. KAlSi
. Isomorphous series made up of Albite (Na), Oligoclase, Andesine, Labradorite, Bytownite and Anorthite (Ca) where the two end member are NaAlSi
Colors / Varieties:
Orthoclase: Moonstone (all colors); Transparent: yellow, pink and white; Phenomena: sheen, chatoyancy, asterism, adularescence (blue sheen)
Microcline: Amazonite (mottled green blue, grayish white, brownish red); Phenomena: sheen, chatoyancy, star
Albite: Moonstone (orthoclase-albite)
Labradorite: Transparent (white, grey, yellow); Phenomena: chatoyancy, asterism, play of color i.e., labradorescence); Spectrolite: dark background with play of color. Oligoclase:
Sunstone: Phenomena: aventurescent, chatoyancy
Crystal System / Forms:
Well formed crystals as well as massive or tabular habits. Commonly exhibit lamellar twinning.
2.55 - 2.72
Cleavage / Fracture:
Easy 2 directional / Uneven.
Cause of Color:
In all species chatoyancy and asterism is due to the presence of parallel, oriented inclusions.
Igneous rocks, pegmatite.