Facts about Feldspar
Feldspar is derived from the German Feld (field) and Spat (a rock that does not contain ore).
Solid solutions between K-feldspar and albite are called alkali feldspar.
Feldspar is the name of a group of rock-forming minerals that make up as much as 60 percent of the Earth's crust.
Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock.
Diffusion, however, is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes.
Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition.
The perthitic textures in the alkali feldspars of many granites are coarse enough to be visible to the naked eye.
Sanidine (monoclinic), orthoclase, and microcline (triclinic) refer to polymorphs of K-feldspar.
The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
The immiscibility gaps in the plagioclase solid solution are complex, compared to the gap in the alkali feldspars.
Intermediate compositions of plagioclase feldspar also may produce two feldspars of contrasting composition during cooling.
