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Webster 1913 Edition

Noun.

[Cf.

(Min.)

A finegrained rock, flintlike in fracture, consisting essentially of orthoclase feldspar with occasional grains of quartz.

Noun.

[See Feldspar.] A species of compact feldspar, of an azure blue or green color, found amorphous associated with quartz and mica.

felsite ‎(plural felsites)

(mineralogy) A fine grained volcanic extrusive rock, generally light in color, composed of felsic minerals.
The buff-colored dacites of the Superstition Mountains are felsites.
(geology) Any light-colored volcanic rock whose true mineralogy is not yet determined.