VI) 



NEW YORK STATE MUSEUM 



A steep rhombohedron 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. 197 Fig. 198 Fig. 199 



Calcite 



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. 



