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Updated: October 14, 2026
Feldspars when arranged according to their system of crystallisation are MONOCLINIC, having one axis obliquely inclined; or TRICLINIC, having the three axes all obliquely inclined to each other. If arranged with reference to their cleavage they are ORTHOCLASTIC, the fracture taking place always at a right angle; or PLAGIOCLASTIC, in which the cleavages are oblique to one another.
Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic. That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his analysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes.