SUBSILICATES. 287 
reddish. Streak white. Lustre vitreous. Transparent to 
subtranslucent. Pyro-electric. IH. G=3:4-3°65. 
Composition. AlLO,Si, with a part bt the oxygen replaced 
by fluorine = Silica 16 ‘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. 
es 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 athin columnar struc- 
ture and forming masses imbedded in quartz. The Physalite 
On lyr ophysalite of Hisinger is a coarse, nearly opaque va- 
riety, found in yellowish-white crystals of considerable di- 
‘mensions. ‘his variety intumesces when heated, and hence 
its name from phusao, 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, Zimuwald, 
and Kirenfriedersdorf 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 
Mines of Stockholm, weighs eighty pounds. Altenberg in 
