ALTERATION OF LEAD AND ZINC MINERALS 193 



2. Crystallized blende, usually of garnet or ruby red color, often in 

 small, brilliant, translucent crystals; crystallized galena. 



3. Crystallized pyrite, marcasite, chalcopyrite, calcite, barite, and 

 amorphous tallow clay. 



4. Anglesite, cerussite, calamine, smithsonite, and greenockite result- 

 ing from the alteration of the ores. 



It may be noted that the primary ores are composed almost wholly 

 of simple sulphides of zinc and lead and of the gangue minerals dolo- 

 mite and cherokite (crystalline silica with spots of bitumen). Calcite 

 appears to be in all cases of secondary and late formation. Pyrite, 

 marcasite, and barite are probably of both primary and secondary depo- 

 sition, but are of subordinate importance. 



In personal conversation upon the subject Mr Jenney tells me that 

 the tests made for him of these redeposited sulphides show that the sec- 

 ondary minerals are always much purer than the primary ones ; thus 

 the blende, which is always colored by impurities in the primary ore, is 

 almost pure and translucent in the redeposited form. The galena also 

 is much purer in the secondary form. 



In the paper quoted he says :* 



" The perfect faces and sharp edges of crystals of blende and galena found lining 

 the water channels in the lower parts of the ore body show that below the zone of 

 oxidation no solution or decomposition of the metallic sulphides takes place. 

 Even where the lead and zinc become oxidized and pass into solution in the cir- 

 culating waters, contact with the organic matter contained in the rocks in the 

 presence of alkaline sulphates, which occur in all mineral waters, immediately 

 reduces and precipitates the metals as sulphides. 



PbO, C0 2 -f CaO, S0 3 + H 2 + 2C = PbS + CaO, C0 2 + 2C0 2 + H 2 0. 



" It is this protective action of the organic matter disseminated through the strata 

 that has limited the zone of oxidation to so shallow a depth in the mining regions, 

 for until all the carbon contained in the rocks is first consumed by oxidation, no 

 decomposition of the minerals can occur or any segregation of minerals take place." 



In the discussion of this paper, further notes are also given upon this 

 process : f 



" The only deposition now taking place is that resulting from oxidation of the 

 previously existing ores and their reformation due to the secondary action of ox- 

 idizing surface waters precolating downward through the upper portion of the ore 

 bodies. Stalactites of blende, galena, and marcasite, not infrequently of calamine, 

 smithsonite, and cerussite, occur in the vugs and open channels in the ore bodies, 

 but they all appear in all observed instances to have been formed by the second- 

 ary action of surface waters in the primary ore." 



* Trans. Am. Inst. Mining Engineers, 1893, p. 222. 

 t Ibid., p. 646. 



