144 



DESCLOIZITE FROM SOUTH-WEST AFRICA. 



The precise significance of the zoning is not clear. It may be 

 taken for granted, however, that the differently coloured shells 

 also differ in chemical composition, so that instead of dealing 

 with a liomogenous mineral we have to do with at least three 

 isomorphous substances. 



A chemical analysis by Dr. J. McCrae, of some selected 

 crystals is given under I, in the following table: — 



Under la are given the molecular proportions corresponding 

 with the analysis. It will be seen that the figures agree fairly 

 cIosel3' with the theoretical composition of the mineral, coitcs- 

 ponding with the formula 2 PbO. 2(ZnCu) O. V.O^. 2 H^O, 

 according to which lead oxide and zinc plus copper oxide are 

 present in equimolecular proportions, and the ratio of 

 (Pb Zn Cu) O : VoO, : : 4 : 1. The analyses under II and 

 III ai'e adduced for comparison. II is an analysis by W. F. 

 Hillebrand of descloizite from Beaverbrook, Montana, 0'03 per 

 cent, of CaO and 0"06 per cent, of MgO being present in addition 

 to the constituents named. Ill is an analysis of cuprodescloizite 

 from an unnamed locality quoted from Dana.* As the term 

 cuprodescloizite appears to be reserved for varieties containing 

 a mininuun of 6 per cent, of copper oxide, the Olifantsfontein 

 mineral, which, as we have seen, is probably a complex 

 isomorphous mixture of at least three distinct substances, is 

 best described as cupriferous descloizite. 



The associated minerals are pellucid bluish-white quartz and 

 calcite. The latter occurs in mammillary crusts up to 1 centi- 

 metre in thickness. In these the calcite is evidently paramorphic 

 after aragonite — a phenomenon also noted at other localities in 

 the Otavi Eange. | The original aragonite was clearly of later 

 formation than the descloizite, having been deposited on the 

 crystals of that mineral which project into the crusts. The 

 quartz, on the other hand, appears to be of earlier formation 

 than the descloizite. 



Abcnab. — The Abenab ore is a breccia of strikingly hand- 

 some appearance made up of angular fragments of pink surface 



* " A System of Mineralogy," p. 789. 

 t cf. Schneiderhohn, loc. cit., p. 41. 



