What is Plagioclase?
Plagioclase is the sodium-calcium feldspar series that makes up a large part of Earth's crust and often shows twinning and fine colour flashes.
Mineralogy and crystal structure
Reference formula: (Na,Ca)(Al,Si)4O8. Plagioclase is a compositional series from albite to anorthite, not one fixed species. The displayed albite model is explicitly representative of the triclinic framework. Plagioclase is classified with silicates and has a triclinic crystal system. Typical hardness is 6–6.5 on the Mohs scale and specific gravity is approximately 2.6–2.8.
Colour, lustre and habit
Typical colours include white, grey, cream, pink, green. The usual tabular, granular, massive habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Plagioclase forms in or is associated with basalt, gabbro, diorite, granite, anorthosite and metamorphic rocks. Important documented localities include Worldwide, including Iceland, Norway, Canada, India, Madagascar and the Andes.
The feldspar series that builds the crust
Plagioclase is commercially and scientifically important because one family spans pale crystals, labradorite flashes and the mineral framework of much of the oceanic crust.
Collecting and care
Protect cleavage surfaces from impact and clean with mild water; use thin sections or analysis when exact composition matters.
