106 



THE MINERALS OF TASMANIA 



185 Magnesite (Carhonate of Magnesium). 



Occurs in serpentine, Parson's Hood Mountain ; in veins. 

 Trial Harbour ; Meredith Range ; Dundas ; Heazlewood. 



This mineral has only been detected in comparatively 

 limited quantity, although usually not by any means a 

 rare species under favourable conditions. It is always 

 massive, and often contaminated with earthy matter. The 

 pure mateiial is milk-white, and of a close, compact tex- 

 ture. When free from silica and lime it is of considerable 

 commercial importance. 



186. Margarite (Hydrated Silicate of Aluminium and 



Calcium). 

 This substance belongs to the class of basic brittle micas 

 which form a transition from the micas proper, or those 

 mineral species which are characterised by their distinctly 

 elastic laminae, and the chlorites, which latter are com- 

 monly foliated or show a granular aggregate of scales, and 

 which are, moreover, usually coloured green by ferrous 

 iron. Margarite or calcium mica crystallises in the mono- 

 clinic system, but' is rarely in distinct crystals; it usually 

 occurs in aggregates of pearly subtranslucent scales of pale 

 tints. It is an alteration product, which is not rarely a 

 common ingredient in the geologically older schistose rocks. 

 It occurs in the schists of the Lyell-Read districts, which 

 are presumed to be mainly of igneous origin. Professor 

 J. W. G-regory remarks: — ''The rocks of the Mt. Lyell 

 schists are, moreover, very poor in lime ; accordingly they 

 cannot include any considerable amount of the lime-mica, 

 margarite " (" The Mount Lyell Mining Field," Aus. In. 

 Min. En., 1905.) The indefinite substance which has been 

 termed margarodite is also referred to in the same report 

 as forming the base of the Lyell schists, and may be 

 regarded as composed of a very fine-grained intermixture 

 of paragonite (white soda mica) and one of the varieties of 

 damourite (white potash mica), with occasional inclu- 

 sions of margarite. " These minerals are not always pre- 

 sent in a crystalline form, but the crystalline patches 

 pass off imperceptibly into crypto-crystalline and granu- 

 lar material. The material has no doubt been formed 

 from the decomposition of an alkali felspar, from which 

 most or all of the alkalies have been leached away " 

 {loc. cit.). 



187. Limonite (Hydrated Veroxide of Iron). 



This is not what may be termed a good species, as it 

 never occurs in distinct crystals. The streak is always 



