PLAGIOCLASE FELDSPAR
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.