OCCURRENCE OF CALCITE. 237 



CALOITE GROUP. 

 CALCITE, DOLOMITE, ANKERITE, PARAXKERITE, MAGXESITE, AND SIDEKITE. 



Calcite: 



CaC0 3 . 



Rhombohedral. 



Sp. gr. 2.713-2.714. 

 Dolomite: 



CaMgC 2 6 . 



Rhombohedral. 



Sp. gr. 2.8-2.9. 

 Ankerite: 



CaFeC 2 O 6 .CaMg0 2 O 6 ;(CaMgC 2 O 6 :CaFeC 2 O 6 : :}:1 to 2:1). 



Rhombohedral. 



Sp. gr. 2.95-3.1. 

 Parankerite: 



CaFeC 2 6 .2CaMgC 2 6 ;(CaMgC 2 6 :CaFeC 2 6 : :2:1 to 10:1). 



Rhombohedral. 



Sp. gr. 2.95-3.1. 



MgC0 3 . 



Rhombohedral. 

 Sp. gr. 3.00-3.12. 

 Siderite: 

 FeC0 3 . 



Rhombohedral. 

 Sp. gr. 3.83-3.88. 



-The chief sources of calcite are (1) organic precipitates, (2) 

 chemical precipitates, (3) by alteration of aragonite, and (4) by carbonation 

 of silicates. 



The chief direct source of calcite is organic. Corals and innumerable 

 other kinds of shell animals, especially in the sea, abstract calcium carbonate 

 from the water and build it into their external or internal structures. Calcite 

 as a chemical precipitate may be deposited from the waters of the sea, 

 especially in inclosed lagoons; by the waters of inland lakes, especially 

 those having no outlet; by springs and streams, especially hot springs and 

 desert streams; and by underground waters in the openings of rocks, such 

 as the interstices between grains, the cavities of porous ig-neous rocks, 

 especially amygdules, and in cave, fault, joint, and fissility openings. The 

 deposited calcite may replace a considerable number of other minerals. As 

 a deposit in the openings of rocks calcite is second in abundance only to 

 quartz. Calcite is an alteration product of a large number of minerals, of 

 which the following are the more common: Actinolite, ankerite, antho- 



