Selected Members of the Feldspar Group All contain aluminium, oxygen and silicon K - Na - Ca
.	Potassium (Alkali) Series An isomorphous series between the end members albite and orthoclase - contain potassium and sodium. (includes the polymorphs orthoclase and microcline) Orthoclase, Sandine, Microcline, Anorthoclase, Albite		Plagioclase Series An isomorphous series between end members albite and anorthite - contain sodium and calcium. Lamellar twinning is so common that it's presence is considered diagnostic; it can be seen in transparent stones using polarizers. Albite, Oligoclase, Andesine, Labradorite, Bytownite, Anorthite
Species	Orthoclase	Microcline	Oligoclase	Labradorite
Crystal System	Monoclinic	Triclinic	Triclinic	Triclinic
Variety	-Non-phenomenal (TP yellow) -Phenomenal: ++ Moonstone (adularescence -blue, grey or white) Cat's Eye Moonstone (chatoyancy and adularescence) Star Moonstone (asterism and adularescence) Star or Cat's Eye Sunstone* Moonstone (aventurescence, adularescence and asterism) Photo: E. Skalwold.	Amazonite (green or bluish-green)	-Non-phenomenal (TP pale green) -Phenomenal: Sunstone* aventurescence- iridescent, translucent, angular hematite platelets	-Non-phenomenal (TP yellow - look for twinning under the polariscope) -Phenomenal: Labradorite -"Spectrolite" original trade name for spectacular labradorite found in Finland- shows labradorescence (shiller) Sunstone* - "Oregon Sunstone**" may show aventurescence (opaque, round metallic copper platelets), cat's eye and star also. ++Rainbow "Moonstone" shows multi-colored sheen similar to adularescence

Isomorphs: same crystal structure, but different chemical composition. Polymorphs: same chemical composition, but different crystal structure. Sunstone Inclusions * "Sunstone" is a name applied to three species here which appear to be somewhat similar, but are different varieties of feldspar. **"Oregon Sunstone" is closer to labradorite than oligioclase. None of these feldspars have anything to do with the fabled "Viking Sunstone" - for more on that, follow the link below.
Photo: E. Skalwold. Sunstone: andesine/labradorite L-R ct wt: 2.79, 8.95, 1.16 2.17, 2.31, 2.03 5.38, suite: 2.63
++ Moonstone Nomenclature:
Moonstone	The  gem trade uses the terms "moonstone" and "rainbow moonstone," but the term "moonstone" should be restricted to the former which is an alkali feldspar and owes its phenomenon to the Raleigh scattering of light caused by minute lenticular inclusions of albite exsolved from host orthoclase (perthitic exsolution of potassium-rich and sodium-rich feldspar). When the width of the stuctures is less than that of the wavelengths of white light, scattering occurs, producing blue adularescence.
Labradorite	Rainbow  "moonstone" is plagioclase feldspar and its appearance is a result of light interference in lamellar stuctures similiar, but differing in size, to that causing labradorescence (peristeritic lamellar intergrowth of calcium-rich and calcium poor plagioclase, each with a different refractive index). Thinner lamellae cause peristerescence, appearing similar to orthoclase moonstone, called peristerite - that is so-called "rainbow moonstone." - while thicker layers cause iridescent labradorescence seen in labradorite. These last phenomenona are caused by miscibility gaps. The word peristerescence comes from the Greek "peristera" or pigeon, recalling the iridescent shiller seen on the bird's neck feathers (Howie, 1998).
Photo: E. Skalwold. Lamellar twinning in "rainbow moonstone" labradorite.
It is suggested that nomenclature in gem feldspars follow that of mineralogy (Krzemnicki, 2006): Sunstone oligoclase feldspar Sunstone orthoclase feldspar Moonstone orthoclase feldspar (sources: Krzemnicki, 2006; Howie, 1998; Henn, 1992; Nassau, 2001)

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