388 



SCIEVCE 



[N. S. Vol. XLII. No. 1081 



bornites 18 doubtless due to admixed chalcocite 

 just as the iron content of chalcocite is usually 

 due to admixed bornite. 



There are four possible explanations of the 

 variability in chemical composition of bornite, 

 viz : (1) Mechanical mixture, (2) isomorphous 

 mixture, (3) morphotropic series (Kraus and 

 Goldsberry), (4) solid solution. 



1. While it is certain that mechanical mix- 

 tures explain part of the variability, some 

 other factor is involved, as we know from the 

 analytical results of Eraus and Goldsberry. 



2. The isomorphism of bornite and chalco- 

 cite does not seem at all probable for there is 

 not much similarity in chemical composition; 

 moreover, one is isometric, the other ortho- 

 rhombic. 



3. The term morphotropy was introduced by 

 Groth^ to indicate the change in crystalline 

 form brought about by substituting in a chem- 

 ical compound an atom or group of atoms for 

 a similar atom or group. The best examples 

 of morphotropy are furnished by organic com- 

 pounds. The only clear case of morphotropy 

 (in its original restricted sense as distinct 

 from isomorphism) among minerals is the 

 humite group. Kraus and Goldsberry contend 

 that there are various bornites each with a 

 definite formula. As far as known bornite is 

 isometric and, if so, it is difficult to conceive 

 of any crystallographie change that can be 

 brought about by the addition of the Cu,S 

 molecule. Complex formulae like Cuj^Fe^S,^, 

 Cu,(,Fe„S23 and Cu^Fe^S^j are very improbable 

 and it is probably a coincidence that the per- 

 centage compositions of the crystallized bor- 

 nite from Bristol are so close to the theoretical 

 values of Cu^Fe.S, and Cu,Je,S„. 



4. In order to explain the variable chemical 

 composition of bornite, the hypothesis of solid 

 solution is advanced by the writer. As the 

 lower limit of bornite seems to be Cu3reS3, 

 the variation in composition can he explained 

 by assuming that horniie is a solid solution of 

 Gu„8 in Cu^FeS, which may be indicated thus : 

 Cu3FeS3(Cu„S)j. This makes an indefinite 

 upper limit for the copper content. There is 

 proof that it is as high as CUjJe.S^, and it 



^^Pogg. Ann., 141, 31, 1870. 



probably goes still higher. The composition 

 of bornites listed on page 547 of Kraus and 

 Goldsberry's paper is as easily explained by 

 the solid solution hypothesis as by any other 

 and from a chemical standpoint it seems far 

 more reasonable. The solid solution hypothe- 

 sis also helps to explain the fact that chalco- 

 cite rarely occurs as an original hypogene® 

 mineral for it seems probable that cuprous 

 sulfoferrite (Cu^FeSj) can take up or dissolve 

 appreciable amounts of cuprous sulfid, and 

 hence chalcocite is not formed until a later 

 stage when a change of conditions is brought 

 about by decrease of temperature. It may also 

 explain the readiness with which bornite alters 

 to chalcocite. There is very little microscopic 

 evidence to show that bornite and chalcocite 

 are formed simultaneously except perhaps lo- 

 cally in the so-called intergrowths. The origin 

 of these graphic intergrowths will be discussed 

 by the writer in a forthcoming paper. 



The long series of sulfo-salt minerals given 

 by Kraus and Goldsberry fails to convince me 

 of the general application of morphotropy in 

 this group. Minerals represented by some of 

 the formulse in the series are doubtless exam- 

 ples of solid solutions, and some of these min- 

 erals are undoubtedly mechanical mixtures. 

 Imagine what a bewildering lot of transparent 

 minerals might have been recognized if min- 

 eralogists had had no microscopic check on 

 their chemical work. A revision of the opaque 

 sulfo-salt minerals seems necessary. In such 

 work the metallographio microscope will be of 

 great assistance. 



Austin F. Rogers 



Stanfom) University, Calif. 



STUDIES in the MEASUREMENT OF THE ELEC- 

 TRICAL CONDUCTIVITY OF SOLUTIONS'- 



It is a striking fact that very few investi- 

 gators of conductivity have striven to make 

 the measurements with an accuracy approach- 



1 This work was made possible by a grant from 

 the Carnegie Institution of Washington to Pro- 

 fessor S. r. Acree. New Orleans meeting of the 

 American Chemical Society, April, 1915. 



6 This useful term is used by Ransome for min- 

 erals or ores formed by ascending solutions. 



