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NEW YORK STATE MUSEUM 



A steep rhomboliedron also occurs modified in many 

 ways, also crystals of prismatic habit (pi. 25J. Twins are of 

 common occurrence and are of several forms one of which is 

 shown in fig. 199. Calcite also occurs massive with easy rhombo- 



Fig. 194 



Fig. 195 



Fig. 196 



Fig. 197 



Fig. 199 



hedral cleavage, fibrous (satin spar), coarse and fine granular 

 (crystalline limestone and marble), pulverulent (chalk), stalac- 

 titic, etc. 



The luster of calcite ranges from vitreous in the crystallized 

 varieties to dull in the limestones and chalk. It is normally 

 colorless or white but often red, green, blue, violet, yellow, brown 

 or black from impurities. 



Calcite is readily distinguished by its characteristic rhombic 

 cleavage in three directions as well as by the fact that it is 

 easily scratched by a knife (H. 3) and that a drop of dilute hydro- 

 chloric acid will cause it to effervesce violently. 



