394 



Heretofore all non-crystalline deposits of calcium carbonate 

 were termed aragonite, but calcite is deposited in a massive state 



at low temperatures 

 also, and the iden- 

 tity of such forma- 

 tions should be 

 determined by ac- 

 tual tests (see 

 calcite) . Aragonite 

 passes over to cal- 

 cite, with which it 

 is dimorphous, by a 

 change of its physi- 

 cal properties, form- 

 ing pseudomorphs, 

 the chemical com- 

 position remaining 

 unchanged. 



Good aragonite 

 crystals are not 

 common ; those of 

 Herrengrund are 



FIG. 450. Chisel-shaped Crystals of Aragonite from P robabl y tne best. 

 Cumberland, England. Pseudohexagonal 



forms occur at 



Aragon, Spain, where the mineral was first discovered. At 

 Girgenti, Sicily, it occurs associated with celestite, sulphur, and 

 gypsum. Often associated with zeolites, and in clays with gypsum. 

 Artificial. When a solution of calcium bicarbonate is heated 

 above 30 C., aragonite separates ; below 30, calcite separates. 



WITHERITE 



Witherite. Barium carbonate, BaCO 3 ; Ba = 77.7, CO 2 = 

 22.3 ; Orthorhombic ; Type, Didigonal Equatorial ; S : b : c = 

 .6032: 1:.7302; 100 A 110 = 31 6'; 001 A 101 = 50 26' 

 30"; 110 A 001 = 36 8' 15"; Common forms, c (001), m(110), 

 b (010) ; Twinning plane parallel to 110; Cleavage, b distinct, m 

 imperfect ; Brittle ; Fracture, uneven ; H. = 3-3.75 ; G. = 4.29- 

 4.35 ; Luster, vitreous ; Color, white, gray, or yellow ; Streak, white ; 



