CHAPTER IX 



CARBONATES 



CALCITE 



Calcite. Calcium carbonate, CaCO 3 ; CaO = 56, CO 2 = 

 11; Hexagonal ; Type, Dihexagonal Alternating ; c = .8542 ; 

 0001 A 1011 = 44 36'; r A r' = 74 55'; e_ A e' = 453'; V A V' = 

 75 22'; Common forms, r (lOTl), m (1010), e (OH2), f (0221), 

 y (3251) L v (2131), a (1120), c (0001) ; Twinning planes, 1011, 

 0001, 10T2; Cleavage, rhombohedral (r) perfect; Brittle, fracture 

 cnnchoi.lal, but difficult; H. = 3 ; G. = 2.713; Color, white and 

 pale shades of yellow, brown, red, purple, or gray according to the 

 impurities ; Streak, white ; Luster, vitreous ; Transparent to 

 opaque; = 1.487; = 1.659; <>- = .172; Optically (-). 



B.B. On heating whitens and becomes opaque, yielding a cal- 

 cium flame; after ignition reacts alkaline with turmeric paper. 

 Dissolves in cold dilute HC1 with effervescence, yielding C0 2 ; 

 when pure leaves no residue. Calcite may be distinguished from 

 aragonite by treating the fine powder in dilute cobalt solution in 

 the cold ; aragonite becomes lilac-colored, which remains on boil- 

 ing; calcite remains white in the cold solution and becomes blue 

 on boiling. If powdered calcite is heated in a solution of ferrous 

 sulphate, it precipitates a yellow ferric hydroxide, while aragonite 

 will produce a green precipitate of ferrous hydroxide. 



General description. Calcite crystals are very rich in forms ; 

 some 330 forms have been described, with about one hundred addi- 

 tional forms which are doubtful. With so many possible forms 

 it can be imagined that, while occurring very widely distributed, the 

 crystals of many localities are combinations locally peculiar. 



Four general habits may be recognized: 1. Rhombohedral ; 

 several rhombohedrons occur as simple crystals as e, f, or r. The 

 flat rhombohedron e occur as a simple crystal and in flat combina- 

 tions, in West Cumberland, England, where they are known as nail- 

 head spar, from their resemblance to the old hammer-made wrought- 



379 



