132 schaller: plancheite and shattuckite 



the original one, but which does agree with the analyses of shat- 

 tuckite. Explanations of his results are suggested at the close 

 of this paper. 



The nonidentity of shattuckite with plancheite, notwithstand- 

 ing their very close resemblance in properties and in chemical 

 composition, was definitely determined before the name shat- 

 tuckite was proposed. The direct comparison of the two min- 

 erals was readily made, as Prof. Lacroix had kindly presented 

 to the writer in Paris in 191 2 a typical specimen of plancheite. 

 The available specimen could not yield a sample of plancheite 

 of the requisite purity for chemical analysis. Plancheite is in- 

 timately mixed with other copper silicates, the most abundant 

 of which in the single specimen examined, is what is ordinarily 

 called chrysocolla. A set of three thin sections of parts of the 

 plancheite specimen shows that probably several other copper 

 silicates are also present, although the two named are predom- 

 inant. The fibers and spherulites of plancheite are imbedded, in 

 places, in the massive pale green chrysocolla and the other copper 

 silicates. The thin sections also show that although small 

 fairly pure masses of plancheite spherulites occur in the rock, 

 these masses are bordered by a layer of some other copper min- 

 eral. Judging only from the single specimen, it would be most 

 difficult, if not impossible, to prepare even a very small sample 

 of nearly pure plancheite for chemical analysis. 



Abundant shattuckite was available, from which samples 

 were prepared that after careful selection contained only small 

 amounts of included tenorite. The analyses of three different 

 samples of shattuckite establish its formula as 2CuO.2SiO2.H2O; 

 whereas the formula of plancheite, as revised, is given as 6CuO.- 

 5Si02.2H20 (the original formula proposed is i5Cu0.i2Si02.- 

 5H2O). 



If the only quantitative basis for determining the question of 

 the supposed identity of shattuckite with plancheite were the 

 chemical analyses, then the two minerals would readily be con- 

 sidered as identical. But there is a simple and absolutely con- 

 clusive method by which the question as to the identity of the 

 two minerals can be answered. This is by a comparison of their 



