BY W, F. PETTERD, C.M.Z.3. 181 



Variety — Fieihergite. 



Analysis of a pure sample from the Hercules Mine Mt 

 Read : — 



per cent. = 3201 -.32 ozs. per ton 

 = 13 dwts. 



329. Thomsonite (Hyd rated Silicate of Aluminium aiid 



Calcium). 



A zeolitic mineral occurring in basalt rock, Sheffield, in 

 small vughs, but the identification is somewhat doubtful, 

 as the samples are small and indistinct ; occurs in clusters 

 of microscopic crystals, which are of a yellow^ colour, coat- 

 ing the clefts of lode material. 



Hampshire Silver Mine, Hampshire Hills. In bunches 

 of white capillary fibres coating vughs in the nephelinite 

 of the Shannon Tier. 



330. Topaz (Fluosilicate of Aluminium). 



This mineral crystallises in the orthorhombic system. 

 The crystals have a distinct and characteristic habit, the 

 prisms being always striated longitudinally, with a per- 

 fect basal cleavage. They affect very pale tints, ranging 

 from water-clear colourless gems to pale-blue, and thence 

 to a sea-green shade. A cursory inspection might give 

 the impression of quartz, but the form of crystal, cleavage, 

 and striation is very distinct and characteristic. Topaz is 

 also harder than quartz, coming next in the scale of hard- 

 ness to sapphire. It is an accessory mineral in the gran- 

 itic rocks, and may in some cases be an original constituent. 

 In the majority of occurrences it is of secondary origin, 

 resulting usually from the alteration of felspar ; it is there- 

 fore presumed to be of pneumatolytic origin. It has been 

 shown by Penfield and Minor (" On the Chemical Composi- 

 tion and Related Physical Properties of Topaz," American 

 Journ. of Science, Vol. XLIL, p. 387, 1894) that the fluor- 

 ine constituent in the mineral may be partially replaced 

 by hydroxyl. Such a medium as hydrofluoric acid may be 

 able, under favourable conditions, to effect the conversion 



