116 THE MINERALS OF TASMANIA. 



mines of India, Russia, Canada, and even in Central Aus- 

 tralia. Several of the species are of considerable com- 

 mercial importance, being employed as electrical insulators 

 and for other uses. Lepidolite is the main source of lith- 

 ium used in the pharmacopoeia. In chemical composition 

 they are difficult to classify, as they have a tendency to 

 merge one into the other. They may be conveniently 

 divided into — 



(1) Ferro-magnesium mica = biotite. 



(2) Alkali mica with potash = muscovite. 



„ ,, soda — paragonite. 



,, „ lithia --: lepidolite. 



That rich in both iron and lithium is termed zinnwald- 

 ite, and that containing high magnesia and little iron is 

 termed phlogopite. Muscovite is an essential in many 

 granites and porphyries, but is not contained in effusive 

 lavas. It also occurs in rocks subject to regional meta- 

 morphosis. Lepidolite occurs in pegmatite granites usually 

 associated with toiirmaline and minerals containing fluor- 

 ine. Zinnwaldite is invariably present, if not masked by 

 alteration, in the acid tin-bearing rocks. Biotite is found 

 in some granites, and is common to many rocks of volcanic 

 origin, and also to the crystalline schists. Many minor 

 varieties have been named owing to local peculiarities or 

 variation in constituent composition, such as fuchsite 

 (chrome-mica), roscoelite (vanadium mica), alurgite (man- 

 ganese mica), and many others. 



See Biotite, Lepidolite, Muscovite, Phlogopite, 

 Stricite, Zinnwaldite. 



205. Microcline (Polymicate of Potassium and 



Alum.uiium). 



A triclinic potash felspar occurring abundantly in the 

 hypersthene granite of St. Mary's Pass. 



206. MiLoscHiNiTE (Hydrous Salient e of Aluminium and 



Chromic Acid). 



This is also known as chromic ochre. It is a soft clay- 

 like pulverulent or earthy mixture, which is coloured 

 various shades of green by chromic acid. It occurs plenti- 

 fully at the Blue Tier, near Beaconsfield ; at Dundas, near 

 Mt. Claude; and at Zeehan. 



