Feldspar

Feldspar is the name given to a group of minerals distinguished by the presence of alumina and silica (SiO2) in their chemistry.  This group includes aluminum silicates of soda, potassium, or lime. It is the single most abundant mineral group on Earth.  They account for an estimated 60% of exposed rocks, as well as soils, clays, and other unconsolidated sediments, and are principal components in rock classification schemes. The minerals included in this group are the orthoclase, microcline and plagioclase feldspars.
Mineral Classification: silicates
Chemical Formula:    KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
Specific Gravity:   2.55-2.76
MOHs Hardness:   6-6.5