392 MINERALOGY 



B.B. Infusible; reduced with soda and borax on coal yields a 

 white coat which becomes green with cobalt solution (Zn). Dis- 

 solves in hot dilute HC1, completely when pure, with effervescence 

 yielding carbon dioxide. Highly colored green varieties contain 

 copper ; the brown, iron or manganese. 



General description. Crystals are not common, but occur in 

 small simple rhombohedrons, or scalenohedrons, as at Friedensville, 

 Pennsylvania. Smithsonite is more often massive, incrusted, 

 botryoidal, banded, or earthy, than crystalline. The pure zinc 

 carbonate is white, and the mineral owes its various colors to the 

 impurities and isomorphous carbonates which crystallize with it. 

 A very beautiful green variety containing considerable copper occurs 

 at Kelley, New Mexico ; also in Greece. The bright yellow variety, 

 known in Arkansas and southern Missouri as " turkey- fat ore," 

 is colored with the sulphide of cadmium, greenockite, which is 

 associated in small amounts with the zinc ores of this region. 



Smithsonite is connected with most zinc ore deposits as a second- 

 ary mineral, a surface oxidation product derived from sphalerite, 

 after which pseudomorphs are common. Zinc may be carried, by 

 percolating waters, in solution, either as the sulphate or bicarbonate, 

 to be precipitated as sulphide or by replacement of carbonate of 

 lime as carbonate. Zinc ores therefore fill veins, pockets, or lenses 

 in limestones, and may have been concentrated from the lime- 

 stone itself or carried to it from neighboring regions. As a vein 

 mineral smithsonite is associated with calcite, barite, siderite, 

 galena, pyrite, most of which have originated through the same 

 agencies. 



Smithsonite as a zinc ore is of minor importance in the United 

 States. It is mined in Virginia, Pennsylvania, Arkansas, and Colo- 

 rado. 



Its synthesis is effected by the same means as the other car- 

 bonates of the group. 



Sphserocobaltite, a carbonate of cobalt isomorphous with the 

 calcite group, is found in small rhombohedral crystals at Schnee- 

 berg in Saxony. Some rhodochrosites contain small quantities 

 of cobalt. 



ARAGONITE 



Aragonite. Calcium carbonate, CaC0 2 ; CaO = 56,_ C0 2 

 = 44 ; Orthorhombic ; Type, Didigonal Equatorial ; a : b : c = 



