100 THE MINERALS OF TASMANIA. 



165. JoHNSTONOTiTE (Manganese Garnet). 



This presumably new manganese garnet was named and 

 described by Messrs, Macleod and White in the proceed- 

 ings of the Royal Society of Tasmania, 1898-99. It occurs 

 abundantly distributed in the mica-solvsbergite of Port 

 Cygnet. It is of a brownish-yellow tint, in crystals, some- 

 times over a quarter of an inch in diameter, and shows 

 well-developed trapezoidal faces. 



The cavities containing the garnet are often lined with 

 a coating of purple fluorite, and a small quantity of 

 arsenopyrite is usually present in addition. Pseudo- 

 morphs of marcasite after johnstonotite are occasionally 

 to be found in the rock. 



Under the microscope zonal banding is observable, and 

 in many cases aegirine-augite is enclosed. 



The following is the analysis, viz. : — 



Per cent. 



166. 



167. Kaolinite (Hydrated Silicate of Aluminium). 



The ordinary porcelain clay or kaolin which, when pure, 

 does not contain any alkaline substance, and should not 

 fuse on extreme heating. The major portion of the numer- 

 ous clays termed kaolin are more or less fusible, and there- 

 fore impure, if not misnamed. The mineral is the result 

 of the alteration of felspars which have undergone extreme 

 change through the probable action of hydrofluoric emana- 

 tions from profound depths. It has been noted that there 

 is a constant and conspicuous presence of fluorine-bearjng 

 minerals, notably fluorite, throughout the China clay dis- 

 tricts of Cornwall, England. The true kaolinite has super- 

 ficially extremely local distribution, but considerable down- 

 ward extension, and it is supposed that it may represent 

 pipes up which fluoric and boracic vapours escaped (" Sum- 

 mary of Progress of Geological Survey for 1901-1902," p. 

 26). The superficial alteration or kaolinisation of felspar 



