( AKI'.ONATKS 



:i. Twins in which the twinning axis is normal to the face of the 

 rhombohedron r(KMl> arc not uncommon ; hen- the vertical tOSt 

 of tin- two individuals lie ;.t an angle of '.<) Hi', as shown in Fig. 442, 

 which arc twins from Cumberland, Kngland. 



1. Twins in which the twinning axis is normal to (0221) are rare. 

 In this type the angle between the c axes is 53 46'. 



In rock sections ralcite is colorless between crosx-d niml> and 

 yields a gray interference color of high order. The relief is high 

 when the ray is vibrating parallel to w, and the marked cleavage 

 cracks and surface scratches are all distinct ; by a rotation of the 

 -ret ion through an angle of 

 90 the marks are less dis- 

 tinct , as the ray is now vi- 

 brating parallel to e, which 

 is the fast ray having the 

 least index of refraction. 

 The cleavage parallel to r is 

 nearly always to be noted in 

 sections, and in many cases 

 I )oly synthetic twinning la- 

 mella 1 parallel to the rhom- 

 bohedron e appear as light 

 and dark hands when be- 

 tween crossed nicols. 



( hving to the high double 

 refraction, even thin basal 

 sections of calcite yield good 

 interference figures, with one or more colored rings. 



Occurrence. Calcium carbonate is very widely distributed ; as 

 massive rock it forms the large beds of limestone, which crystallizes 

 to marble when associated with metamorphism. As a product 

 derived from the weathering and solution of many silicates con- 

 taining calcium, calcite may occur filling the cracks, veins, and cavi- 

 ties in almost any rock, and it is therefore a very common vein 

 filler, associated with quartz, sulphates, sulphides, fluorite, oxides, 

 carbonates, and other oxidized vein minerals. Calcium carbonate is 

 soluble in water containing CO 2 as calcium bicarbonate, Ca(HCC>3)2, 

 and is the cause of the temporary hardness of natural waters; on 

 boiling such hard waters the excess CO 2 is driven off and the water 

 becomes milky by the precipitation of calcium carbonate; from 

 cold solutions calcite is formed, and from hot solutions, aragonite, 



FIG. 442. Calcite twinned on r (1011). Cum- 

 berland, England. 



