384 MINERALOGY 



the orthorhombic calcium carbonate. Some stalactites or stalag- 

 mites may be calcite and others aragonite according to the tempera- 

 ture at which they were formed. The large caves so common in 

 limestone regions owe their origin to the solubility of the rock in the 



FIG. 443. A Section of Calcite, Crossed Nicols, showing Twinning Bands parallel 

 to the Rhombohedron e (Olf2). 



natural ground waters containing CO 2 , and it is carried out in 

 solution to some locality where, by relief of the pressure and loss 

 of C02, the calcium carbonate is redeposited either as calcite or 

 aragonite. 



Calcite appears as pseudomorphs after a long list of minerals, 

 either formed by replacement, double decomposition, or as casts 

 filling cavities after the decomposed crystals of other species have 

 been carried away. A beautifully clear and pure calcite occurs at 

 Eskifjord, Iceland, known as Iceland spar. This form of calcite, 

 like quartz, has been a great servant of science, for its perfect cleav- 

 age pieces and strong double refraction led Bartholinus and Huy- 

 gens to the discovery of the law of double refraction, and later 

 Mai us to the discovery of polarized light. Iceland spar is used in 

 the construction of the nicol prism, in the dichroscope, and for 

 other parts of optical apparatus. Unfortunately the Iceland de- 

 posit has been exhausted and calcite suitable for optical instruments 

 is very expensive, and hard to obtain at any price. Good crystals 

 of calcite are to be obtained at numerous localities in the United 

 States ; noted localities are Rossie and Sterlingbush, New York ; 

 Joplin, Missouri; Thunder Bay, Lake Superior; Mineral Point, 

 Wisconsin. Calcite may appear massive, oolitic, pisolitic, or sta- 



