123 



sections, are scattered in the interstices of the topaz. The rock 

 of the Queen lode is a nriss of radiating and prismatic topaz. 

 The latter shows fine cleavage lines in sections not parallel to 

 oP. We have found this basal cleavage of great value in 

 distinguishing crystalline topaz from quartz. The paragenetic 

 association of quartz and topaz with cassiterite is occasionally 

 well displayed by prismatic crystals of the two first minerals 

 being enclosed in crystals of cassiterite. The formation of 

 all three was evidently synchronous. 



In some specimens from the Mount we have the third 

 form of topaz, in which it forms an amorphous or allotrio- 

 niorphous granular mass, apparently taking the place of the 

 ground-mass of the original rock. 



A very interesting feature is the conversion of quartz to 

 topaz, which is visible in hand specimens. A quartz-sintery 

 looking rock, composed of quartz in hexagonal prisms, shows 

 its iudividual crystals bordered with a white cloudy marginal 

 zone of pseudotnoi'phous topaz. Heated in the open tube, 

 its vapour etched glass. This topaz effervesces slightly when 

 treated with HC1, owing to the unexpected presence of lime, 

 derived possibly from the alteration of spliene and apatite. 

 We witness here a second conversion, that of topaz into 

 prosopite, a double fluoride of calcium and aluminium. 

 When this change is effected, topaz loses its transparency, 

 becomes cloudy and opaque. Its hardness diminishes, and 

 its specific gravity becomes less. Yon Groddeck describes 

 this pseudomorphosis fully in his paper " On the tin ore 

 deposits of Mount BischofT, Tasmania," 1886. Sandberger 

 quotes this rather peculiar mineral from Altenberg, Greyer, 

 and Hengsteverb in Saxony, and mentions that he has often 

 remarked pseudornorphoses of prosopite — aggregates after 

 pycnite and crystalline topazt. Vauquelin had previously 

 noticed the presence of calcium and water in pycnite from 

 Altenberg, which is explicable upon the conversion theory. 

 At Mount Bischoff this pseudomorphous alteration product 

 has been mistaken for kaolin. 



The pycnite variety of topaz is not often mentioned in 

 mineralogical works. It is cited from Schneeberg in Saxony, 

 Durango in Mexico (Bauerman), Altenberg (Collins), 

 Schlackenwoald and Ziunwald in Bohemia (Bristow), 

 Maulson in France in steatite, and Kongsberg in Norway 

 in mica slate (Phillips). We have also found it associated 

 with tourmaline and quartz in an argentiferous galena lode 

 traversing syenite at the Lidjessy mines near Kara-Hissar-i- 

 Sharki, Asia Minor, On the other hand there is dyke rock 

 at Mount Bischoff destitute of topaz. Thus a specimen 

 from the West Bischoff Tin Mining Company's ground is a 



t " Untersuchuigen iiber Erzgange." F. Sandberger, vol. ii., 18S5, p. 171. 



