updated in 2018
Is Feldspar a mineral or a group of minerals?
Feldspars are Aluminum Silicates, (primarily) of potash, soda or lime, which includes some 20 known varieties. Most Feldspar minerals can be described by these three ending members in their chemical formula, Potassium-Feldspar (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8).
Some Mineral collectors and geologist view all the Feldspars as varieties of a single mineral species. Viewed from this perspective Feldspar should be considered the most plentiful mineral on earth.
Building Blocks of the Earth’s crust.
The important thing to remember about Feldspars is that they are one of the major rock forming minerals making up the Earth’s crust. Crystallizing out of magma it forms within both intrusive and extrusive, igneous rocks; it can also be found in many metamorphic and sedimentary rocks as well.
Orthoclase, Microcline and Sanidine KAlSi3O8
These three Feldspar minerals have the same chemical compound but present distinct crystalized forms at the molecular level (dimorphism). Sanidine (not shown) forms at higher temperatures than Orthoclase; Microcline forms at lower temperatures than Orthoclase
Amazonite is the name used for blue-green colored Microcline, the color is likely caused by elevated content of lead but some Microcline may receive their green color from divalent iron content. You will only find the bright blue-green color of Amazonite in the Microcline variety of Feldspar. Amazonite, particularly when found with Smoky Quartz is highly prized by many mineral collectors.
Next - Apophylite
Feldspar is a group of minerals but some consider the Feldspars as varieties of a single mineral species.
Chemical Class - silicates
Crystal System - Triclinic or monoclinic