GUIDE TO THE MI X ERALOGIC COLLECTIONS 



S3 



Rhodochrosite MnCO., 



Rhodochrosite is a manganese carbonate containing 38.3^ 

 carbon dioxid and 61.7$ manganese protoxid. 



It occurs occasionally in rbombobedral crystals similar in 

 shape to those of dolomite but more frequently in vitreous or 

 pearly masses of pink to brown color with a marked rhom- 

 bohedral cleavage; less frequently in globular and botryoidal 

 forms with columnar structure or incrusting; granular or com- 

 pact masses are common. 



Rhodochrosite is often found associated with gold and silver 

 ores notably at Butte Mont., in Nevada, Colorado and elsewhere. 

 As yet it has no commercial value. 



Smithsonite (dry bone ore) ZnCO., 



Smithsonite is a carbonate of zinc containing 35.2$ carbon 

 dioxid and 64.8$ zinc protoxid. Small amounts of copper, 

 cadmium, etc. frequently produce marked differences in the 

 color. 



Distinct crystals of smithsonite of rhombohedral form are of 

 quite rare occurrence. It is commonly found in reniform, 

 botryoidal or stalactitic masses, often with a drusy surface. 

 It occurs also in spongy, granular and earthy forms. The luster 

 is vitreous to dull and the color normally white or light in shade 

 but often highly colored by impurities. The common variety of 

 smithsonite resembles calcined bones, as indicated by the name 

 given to it by miners. 



Smithsonite is essentially a secondary product formed from 

 other zinc ores by the action of carbonated waters. It is found 

 in veins and beds associated with other ores of zinc as well as 

 those of lead, copper and iron. It is found abundantly in this 

 country in the zinc regions of Missouri, Virginia and Wisconsin. 



As an ore of zinc smithsonite is highly valued on account of 

 the ease with which it is reduced. The deposits are now nearly 

 exhausted. 



Aragonite CaC0. 5 



Aragonite, which is a calcium carbonate, has the same com- 

 position as calcite but differs from the latter in crystallization. 

 The crystals of aragonite are orthorhombic, sometimes pris- 



