Microscopy and Microanalysis of Magmatic and Metamorphic Minerals—Part 2: Feldspar

Feldspar is the name for a mineral group consisting of nearly 20 triclinic or monoclinic mineral species, of which only nine are well documented and widely distributed in the Earth's crust. Single members of the feldspar group belong to the so-called framework—or tectosilicates, where SiO4- and AlO4-tetrahedra share all their oxygen ions, resulting in an infinite three-dimensional network . Feldspar crystals include the major elements K, Na, Ca, Al, as well as Si and thus may be attributed to one of the following three endmembers: potassium-feldspar (K-feldspar) KAlSi3O8, albite NaAlSi3O8, or anorthite CaAl2Si2O8 (Table 1). Solid solutions occur between K-feldspar and albite, forming what is commonly known as alkali feldspar, and between albite and anorthite, forming plagioclase. The alkali feldspar group consists of orthoclase, microcline, sanidine, and anorthoclase, whereas the plagioclase group includes albite, oligoclase, andesine, labradorite, bytownite, and anorthite .