SUBSILICATES. 



287 



reddish. Streak white. Lustre vitreous. Transparent to 

 subtranslucent. Pyro-electric. H. = 8. G = 3*4-3*65. 



Composition. Al 5 Si, with a part of the oxygen replaced 

 by fluorine = Silica 10-2, silicon fluorid 28*1, alumina 55*7 



= 100. An analysis of one specimen afforded, Silica 34*24, 

 alumina 57*45, fluorine 14*99. B.B. infusible. Some kinds 

 become yellow or of a pink tint when heated. Moistened 

 with cobalt nitrate and ignited assumes a fine blue color. 

 Insoluble in acids. 



Diff. Topaz is readily distinguished from tourmaline and 

 other minerals it resembles by its brilliant and easy basal 

 cleavage. 



Obs. Pycnite is a variety presenting a thin columnar struc- 

 ture and forming masses imbedded in quartz. The Physalite 

 or Pyrophysalite of Hisinger is a coarse, nearly opaque va- 

 riety, found in yellowish-white crystals of considerable di- 

 mensions. This variety intumesces when heated, and hence 

 its name from pkusao, to blow, and pur, fire. 



Topaz is confined to metamorphic rocks or to veins inter- 

 secting them, and is often associated with tourmaline, beryl, 

 and occasionally with apatite, fluorite, and tin ore. 



Fine topazes are brought from the Uralian and Altai 

 mountains, Siberia, and from Kamschatka, where they occur 

 of green and blue colors. In Brazil they are found of a deep 

 yellow color, either in veins or nests in lithomarge, or in 

 loose crystals or pebbles. Magnificent crystals of a sky-blue 

 color have been obtained in the district of Cairngorm, in 

 Aberdeenshire. The tin mines of Schlackenwald, Zinnwald, 

 and Ehrenfriedersdorf in Bohemia, St. Michael's Mount in 

 Cornwall, etc., afford smaller crystals. The physalite va- 

 riety occurs in crystals of immense size at Finbo, Sweden, 

 in a granite quarry, and at Broddbo. A well-defined crys- 

 tal from this locality, in the possession of the College of 

 Mine3 of Stockholm, weighs eighty pounds. Altenberg in 



