CONSTITUENT MINERALS OF ROCKS. 



63 



from white to yellowish, reddish; grayish-brown to black when 

 impure. Easily scratched with a knife. G. = 2.5-2.8. Crystalline 



Fig. 52. 



varieties readily distinguished by the angle, 105° 5 / , between the 

 cleavage-faces ; and all kinds by the low degree of hardness, ready 

 effervescence when touched with a drop of dilute muriatic acid, and 

 infusibility. When burnt, it becomes quicklime. Composition : Car- 

 bonic acid, 44.0, lime, 56.0. 



The same compound, carbonate of lime, occurs under a prismatic crystalline 

 form, and is then called Arayonite : it is easily recognized by the absence of the 

 rhombohedral cleavage, and also by means of polarized light. It is somewhat 

 harder than calcite. G. = 2.9-3.0. In shells the pearly part is Aragonite, 

 while the rest is usually calcite. 



(23.) Magnesite. — Crystals rhombohedral or hexagonal, like calcite, with 

 the angle between two planes II 107° 2$' ; but usually massive, and often look- 

 ing like porcelain biscuit. Harder than calcite. Color, white. G. = 2.8-3. 

 Infusible : no immediate effervescence when touched with dilute acid, though 

 dissolving when powdered and heated with acid. Comjwsition : Carbonic acid, 

 52.4, magnesia, 47.6. 



(24.) Dolomite. — Rhombohedral, as in calcite, with R on R = 106° 

 15'. Also massive, constituting much of the white architectural 

 marble, and some earthy limestones. G. = 2.8-3.0. Distinguished 

 from calcite by not affording effervescence when touched with 

 dilute acid, unless heated. It is a carbonate of magnesia and lime, 

 containing carbonate of lime, 54.4, carbonate of magnesia, 45.6. 



(25.) Chalybite. — A carbonate of the protoxyd of iron, having the same 

 crystallization as carbonate of lime or calcite, but higher specific gravity (G. = 

 3.7-3.9), somewhat greater hardness, and a grayish or brownish color becoming 

 deep brown on exposure. 



70. (26.) Gypsum. — Crystals as in the figures 54 and 55, an oblique 

 (monoclinic) prism, with very perfect cleavage in one direction, 

 affording large pearly plates, which bend in one direction and 



