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The Collaborative International Dictionary
glassy feldspar

Sanidine \San"i*dine\, n. [Gr. ?. ?, a board. So called in allusion to the tabular crystals.] (Min.) A variety of orthoclase feldspar common in certain eruptive rocks, as trachyte; -- called also glassy feldspar.

Wiktionary
glassy feldspar

n. (context mineralogy English) sanidine