What is Oligoclase-Anorthite?
Oligoclase-anorthite is a plagioclase compositional range used to describe sodium-calcium feldspar in igneous and metamorphic rocks.
Mineralogy and crystal structure
Reference formula: (Na,Ca)(Al,Si)4O8. This is a compositional feldspar range, not a separate fixed IMA species. The structure and properties change with sodium-calcium substitution, so the guide is intentionally labelled as a reference variety. Oligoclase-Anorthite 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. The usual tabular, granular, massive habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Oligoclase-Anorthite forms in or is associated with plutonic rocks, volcanic lavas, anorthosite and metamorphic feldspar assemblages. Important documented localities include Norway, Canada, the United States, India, Madagascar and the Alps.
Feldspar composition written as a spectrum
Plagioclase is best understood as a continuum. Oligoclase-anorthite helps explain why two pale feldspars can share cleavage while differing in chemistry and geological setting.
Collecting and care
Avoid impact and acids; clean gently and document twinning, cleavage and locality.
