416 Transactions. — Geology. 



To2)a.z (Silico-fluoride of Alumina). — This is generally of a 

 remarkably pure white, distinct from the whiteness of the 

 purest quartz, from which it is easily distinguished by this 

 character when the eye has been educated to it. The mineral 

 occurs in a crystallo-granular form, permeating quartz or form- 

 ing irregular-shaped masses in it, and frequently enclosing 

 cassiterite and wolfram. It is infusible in the blowpipe-flame. 



Aiudysis. 



Silica .. .. .. .. .. 33-06 



Alumina . . . . . . . . . . 57-02 



IManganese . . . . . . . . Trace 



Iron-protoxiJe . . . . . . . . 0-11 



Fluorine .. .. .. ,. .. 14-82 



10501 

 The excess of 5-01 in the sum of my results is owing to a 

 part of the silicon existing in the mineral as a fluoride being 

 computed as silica. The fluorine in this mineral was not 

 certainly recognized by the blowpipe test, using a flux of 

 microcosmic salt, the well-known etching-power of phosphoric 

 acid itself standing in the way of this. However, by fusing 

 the mineral with four times its weight of carbonate of soda, 

 and treating the fused mass with sulphuric acid, very good 

 reactions of this element (fluorine) were obtained. 



Zinc-hlendc (Sulphide of Zinc). — A few very small, almost 

 microscopic crystals, reflecting from certain of their planes a 

 deep-blue colour, were brought under my notice by Mr. 

 McKay. They were considered by him as likely to be a form 

 of some inferior oxide of tungsten, an opinion which coincided 

 with my own until I tested them. This proved to be a re- 

 markable form of zinc-blende. The sulphide of iron in these 

 crystals is in sT;ch proportionately large quantity as to colour 

 the powder intenseh' black. The property that certain of 

 these planes have of reflecting blue colour in ordinary light is 

 one that to my knowledge has not been hitherto observed in 

 connection with zinc-blende. In this respect they compare 

 with the planes of some of the specular iron-ores from Elba 

 and Cumberland. 



Gahnite (Zinc-spinelj. — Occvirs as small light -green- 

 colom^ed crystals, interspersed through certain of the matrices 

 of the tin-ore of this island. These crystals belong to the 

 isometric system : many of them are perfect octahedrons. 

 They are infusible in the blowpipe-flame, but blacken therein. 

 Owing to the very intimate admixture of tliis mineral with 

 topaz, cassiterite, and quartz, I have not been able to separate 

 it quite clear of impurities ; but, as the impure mineral aiforded 

 me 18-14 per cent, of zinc-oxide and but traces of magnesia, 

 and 5-71 per cent, of silica, I have not hesitated to name it as 

 I have done — that is, gahnite, or zinc-spinel. 



