This diagram shows how feldspar minerals are classified on the basis of their chemical composition. For example, it is common to find sodic plagioclase (oligoclase) in granite, more calcium-rich varieties (labradorite) in mafic rocks like gabbro, and intermediate andesine in intermediate igneous rocks like andesite.įig.1: Feldspar classification. Their use is justified because plagioclase is very common mineral and occurs in a wide variety of rocks and the composition of plagioclase is rather predictable. These boundaries have no structural significance. The plagioclase series is arbitrarily divided into six minerals or compositional ranges: albite (Ab 90 - Ab 100), oligoclase (Ab 70 - Ab 90), andesine (Ab 50 - Ab 70), labradorite (Ab 30 - Ab 50), bytownite (Ab 10 - Ab 30), and anorthite (Ab 0 - Ab 10). The amount of potassium that may enter the lattice is limited because of large difference in ionic radii. The charge balance in maintained by substituting Al 3+ for Si 4+. Plagioclase feldspars form a continuous solid solution between Ab and An endmembers at high temperature, but the replacement of ions needs to be coupled because of the charge difference between Na + and Ca 2+. Plagioclase feldspars are the most common feldspar minerals because calcium is somewhat more common in the crust than potassium (3.6 and 2.8 percent of the crust, respectively).
