194 ALUMINA. 



5. Combination of a SiVcate and Fluorid, 



TOPAZ. 



Trimetric. In right rhombic prisms, usually differently 

 modified at the two extremities. Pyro-electric. 

 M : M=124° 19'. Cleavage perfect, parallel to 

 the base. 



Color pale yellow ; sometimes greenish, blu- 

 ish, or reddish. Streak white. Luster vitreous. 

 Transparent to subtranslucent. Fracture sub- 

 conchoidal, uneven. 



Composition : silica 34*2, alumina 575, fluorine 15*0. 

 Infusible alone on charcoal before the blowpipe. Some 

 varieties are changed by heat to a wine yellow or pink tinge. 



Dif. Topaz is readily distinguished from tourmaline 

 and other minerals it resembles by its brilliant transverse 

 cleavage. 



Obs. Pycnite has been separated from this species. It 

 differs from topaz mainly in the state of aggregation of the 

 particles, it presenting a thin columnar structure and forming 

 masses imbedded in quartz. The physalite or pyrophysalite 

 of Hisinger, is a coarse, nearly opaque variety, found in 

 yellowish- white crystals of considerable dimensions This 

 variety intumesces when heated, and hence its name from 

 phusao, to blow. 



Topaz is confined to granitic regions, and commonly occurs 

 in granite, associated with tourmaline, beryl, occasionally 

 with apatite, fluor spar and tin. With quartz, tourmaline 

 and lithomarge, it forms the mixture called topaz rock by 

 Werner. 



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 Cairngorum, in 

 Aberdeenshire. The tin mines of Schlackenwald, Zinnwald, 

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



What are the forms and cleavage of topaz crystals 1 What are their 

 colore ? their luster and hardness 1 their composition ? How is topaz 

 distinguished from tourmaline and other minerals 1 How does topai 

 occur ? 



