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Feldspar Feldspar [(K,Na,Ca)(Al,Si)408] is a common rock-forming mineral that is a major constituent of many igneous and metamorphic rocks. Feldspar has a hardness of around 6, a white streak, and two directions of cleavage that are at about a 90° angle to one another. The two major groups of feldspars are alkali (KAlSi3O8 through NaAlSi3O8) and plagioclase (NaAlSi3O8 through CaAl2Si2O8). |
The sample in the Texas Rock Kit is microcline (KAlSi3O8), a member of the alkali feldspar group, which was collected from a feldspar/milky quartz-rich pegmatite cropping out in Burnet County within the Llano Uplift area. The feldspar is Precambrian in age.