Anorthite - Encyclopedia
Class : Silicates
Subclass : Tectosilicates
Crystal System : Triclinic
Chemistry : (Na,Ca)(Al,Si)4O8
Rarity : Very common
Anorthite constitutes the calcium pole of plagioclase feldspars (An90 to An100 and Ab0 to Ab10), the sodium pole corresponding to albite. It is a rare constituent of basic igneous rocks (basalts and calc-alkaline gabbros) and of certain metamorphic limestones. Its name comes from the Greek anorthos (oblique) in reference to its triclinic crystal system. In geodes, anorthite often occurs as almost perfect and very richly shaped crystals, prismatic or elongated, colorless or sometimes weakly tinted (yellowish, grayish, reddish).
Anorthite in the World
Main photo : Mount Somma anorthite, Vesuvius, Italy © Gianfranco Ciccolini
Twinning
Twins are common and commonly polysynthetic ; many other single and multiple contact twins exist.
Fakes and treatments
No fake identified for this mineral species.
Hardness : 6 to 6.5
Density : 2.75
Fracture : Irregular to conchoidal
Trace : White
TP : Translucent to transparent
RI : 1.573 to 1.590
Birefringence : 0.012 to 0.013
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None
Solubility : Concentrated hydrochloric acid
Magnetism : None
Radioactivity : None