Topaz on Cleavelandite and Muscovite (#50)
|Asia||Shigar, north of Skardu, Gilgit Division, Pakistan||2 ½" x 2" x 2"|
Several of the specimens in this display are topaz, and this is a particularly attractive crystal. Muscovite is one of the two most common members of the mica family of minerals, the other being biotite (also flat, platy crystals, but they are black).
Cleavelandite is that variety of albite occurring in platy white masses, and albite is a member of the feldspar family. The Learn More for specimen #42 contains this statement: "The most abundant mineral in the crust, taking the oceanic and continental crust together, is plagioclase feldspar, which has the chemical formula (Ca,Na) (Al,Si)4O8". Albite (including, of course,cleavelandite) is the Na plagioclase feldspar: NaAISi3O8. The common feldspars can be represented on this triangle. Nature allows any composition along the red lines. This "mixing" of two or more ideal compositions is called solid solution, and it is quite common in minerals.