CHAPTER II 
ROCK-FORMING MINERALS—continued 
Silicates— Amphibole and Pyroxene Group; Mica Group; Olivine 
Group; Chlorite Group; Talc Group; Epidote Group; Garnet 
Group; Tourmaline Group; Titanite Group; Andalusite Group ; 
Zeolite Group ; Kaolinite Group. Haloids—Fluorite and Rock-Salt. 
Sulphides—Pyrite, Pyrrhotite, and Marcasite. Carbonates—Cal- 
cite, Aragonite, Dolomite, and Siderite. Sulphates—Anhydrite, 
Gypsum, and Barytes. Phosphates—Apatite, etc. Elements— 
Graphite. 
THE AMPHIBOLE AND PYROXENE GROUP 
THE Amphiboles described here are calcium-magnesium 
silicates; others not referred to contain soda. Some are rich 
in aluminium and iron, while others contain little or no trace 
of either. When crystallised they appear as prisms: but 
they show a marked tendency to assume fibrous and radiated 
forms. Their specific gravity ranges from 2-9 to 3-5, and 
their hardness is between 5 and 6. They are usually fusible, 
-more particularly when rich in iron. 
Amphiboles crystallise both in monoclinic and_ ortho- 
rhombic forms, but only the former are important rock- 
formers. The monoclinic non-aluminous amphiboles are usually 
lighter in colour than those rich in aluminium and _ iron. 
The most commonly occurring representatives of the non- 
aluminous class are Tremolite and Actinolite. 
Tremolite is white, grey, or light green in colour, and occurs usually 
in the form of long blade-shaped crystals, striated longitudinally : or it 
assumes the appearance of thin fibrous crystals radiating from a centre. 
The crystals have a pearly or silky lustre. This mineral is a constituent 
of some schistose rocks; it occurs not uncommonly in crystalline 
limestone (marble) and dolomite near their point of contact with plutonic 
rocks, Now and again it is met with as an alteration-product in olivine- 
rocks and serpentine. Actinolite differs from tremolite in containing a 
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