396 Allen and Crenshaw — Sulphides of Zinc, etc. 



The other two forms change into it, either by heating alone or 

 more readily in the presence of solvents like concentrated 

 ammonium sulphide or 30 per cent suphnric acid. The absorp- 

 tion of light by cinnabar increases markedly with rising tem- 

 perature, bnt it regains its color on cooling after long heating 

 at 325°. Heated above 400° it becomes permanently black. 

 This is not an inversion as some have supposed ; the cinnabar 

 contains (inly about 1 per cent or less of a thin coating of the 

 black sulphide which perhaps is caused by condensation of the 

 vapor. 



8. Amorphous cadmium sulphide is so fluid, that during pre- 

 cipitation small particles may aggregate into globules O005 to 

 0"0i"" u in diameter which remain permanently plastic. Amor- 

 phous zinc sulphide aggregates similarly but the globules may 

 harden, either without crystallizing, or by crystallizing. In the 

 former case they simulate doubly refracting spherulites owing 

 to the development of strains in a wholly amorphous substance; 

 in the latter case double refraction is due to wurtzite. 



9. Comparing the genetic relations of the minerals sphaler- 

 ite, wurtzite, cinnabar and metacinnabar with the genetic rela- 

 tions of pyrite and marcasite, we find certain remarkable regu- 

 larities. The stable forms, sphalerite, cinnabar and pyrite are 

 always obtained by crystallization from alkaline solutions (solu- 

 tions of the alkali sulphides), while the unstable forms wurtzite, 

 metacinnabar and marcasite are obtained from acid solutions 

 only. The stable forms also may be crystallized from acid under 

 certain conditions. Of these temperature and acid concentra- 

 tion appear to be the important ones. 



Certainly with pyrite and marcasite and in all probability 

 with sphalerite and wurtzite, the higher the temperature the 

 greater is the percentage of the stable form obtained, while the 

 higher the acid concentration at any temperature the greater is 

 the percentage of the unstable form obtained. These facts 

 appear to agree remarkably well with the field evidence which 

 relates to the genesis of the natural minerals, while they give 

 new significance to the general geologic distinction between 

 deep-seated and surface waters in nature. 



10. None of the sulphides of the group zinc, cadmium, mer- 

 cury melts at atmospheric pressure. 



In conclusion the authors wish to express their thanks to Dr. 

 Geo. P. Merrill of the National Museum for placing at their 

 disposal much valuable material for study ; to Mr. C. E. Sie- 

 benthal of the U. S. Geological Survey for mineral specimens, 

 and to Mr. B. S. Butler, and especially to Mr. Waldemar Lind- 

 gren, also of the U. S. Geological Survey, for geological data 

 and valuable criticism. 



Geophysical Laboratory, Carnegie Institution of Washington, 

 Washington, D. C, July 9, 1912. 



