Source : Webster's Revised Unabridged Dictionary (1913)

Bisilicate \Bi*sil"i*cate\, n. (Min. Chem.)
   A salt of metasilicic acid; -- so called because the ratio of
   the oxygen of the silica to the oxygen of the base is as two
   to one. The bisilicates include many of the most common and
   important minerals.