The sodium- and calcium-feldspars are called plagioclase ("plag" for short).  Six different mineral names are available for the plagioclase feldspars: albite, oligoclase, andesine, labradorite, bytownite, and anorthite.  Albite is ~pure sodium feldspar (NaAlSi3O8) and anorthite is ~pure calcium feldspar (CaAl2Si2O8).  The other mentioned minerals are plagioclase feldspars having a mix of sodium and calcium.  The pure end-members are whitish-colored.  The plagioclase feldspars having a mix of sodium and calcium tend to be light gray to dark gray to mottled gray.  Some have a spectacular play of color (labradorescence).


Oligoclase (sodium-rich plagioclase).



Plagioclase feldspar (probably labradorite).



Labradorite plagioclase feldspar (1.8 cm across at its widest) showing its famous play of colors (labradorescence), especially electric blue.



Spectrolite (field of view ~3.4 cm across) - polished surface of gem-quality labradorite plagioclase feldspar displaying intensely colored labradorescence.  This comes from a coarsely crystalline plagioclasite (= metamorphosed anorthosite) in the Ankafotia Anorthosite Massif (~mid-Neoproterozoic, ~660 Ma).  Ankafotia spectrolite is commercially mined from the Benonoky area in southern Toliary Province, southern Madagascar.



Photo gallery of albite

Photo gallery of oligoclase

Photo gallery of andesine

Photo gallery of labradorite

Photo gallery of bytownite



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