What is Oligoclase?
Oligoclase is a sodium-rich plagioclase feldspar valued in granite, pegmatite and ornamental stone, sometimes showing attractive aventurescence.
Mineralogy and crystal structure
Reference formula: (Na,Ca)(Al,Si)4O8. Oligoclase is a compositional plagioclase range rather than a perfectly fixed end member. The triclinic COD model represents a natural sodium-rich composition and should not be read as the formula of every sample. Oligoclase 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 2.6–2.7.
Colour, lustre and habit
Typical colours include white, grey, green, pink, colourless. 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 forms in or is associated with granites, granodiorites, metamorphic gneisses, pegmatites and feldspar-rich igneous rocks. Important documented localities include Norway, Canada, the United States, Madagascar, India and Scandinavia.
A feldspar with a subtle silvery flash
Oligoclase rewards close inspection: a quiet grey or salmon surface can reveal cleavage, twinning and aventurescent inclusions that make feldspar feel unexpectedly gem-like.
Collecting and care
Avoid strong impact and harsh acids; clean with lukewarm water and a soft brush.
