Borosilicates

Borosilicates 

a group of minerals; salts of silicic acid in whose crystalline structure boron has quaternary coordination, structurally replacing silicon in the radicals. Among the elements occurring as cations in the borosilicates are Ca, Na, Fe, Si, and Al. Approximately eight minerals of the borosilicate group, which belong to skeleton and laminated structural types, are known. The most abundant are calcium borosilicate (danburite), Ca[B₂Si₂O₈], and datolite, Ca₂[B₂Si₂O₈] (OH)₂. Calcium borosilicates are characterized by a rather high content of boron oxide (danburite, 28.32 percent; datolite, 21.78 percent B₂O₃). The borosilicates crystallize into rhombic, monoclinic, and triclinic systems. They are usually colorless or light-green; their density is 2,500–3,500 kg/m³.

Borosilicate deposits and ore formations are connected primarily with skarn-formation processes. Here, boric mineralization is superimposed on calciferous skarns, skar-noids, and, to a lesser extent, on hornstone. Borosilicates are sometimes encountered in pegmatites.