8 



PROCEEDINGS OF THE NATION"AL MUSEUM 



VOL. 74 



two cleavages, presumably parallel to the prism (110) and the pina- 

 coid (010). The acute bisectrix is perpendicular to the latter cleav- 

 age and the axial plane is across the elongation, so that the long 

 direction of the fibers is Y. The dispersion, r<v is strong and the 

 mean index of refraction, ,8, is 1,455. These are essentially identical 

 with the properties given by Larsen ^^ for epsomite, MgSO.j.TH.O 

 as determined on the freshly crystallized salt. Almost immediately, 

 however, the material began to dehydrate and become chalky and 

 opaque-looking on the outside and this dehydration proceeded until 

 when analyzed after having been exposed for a year to the air of 

 the museum the whole lot was dead and lusterless and exceedingly 

 fragile and friable. Since several examples of epsomite which had 

 naturally dehydrated in the open air have recently been shown to 

 consist of the material called hexahydrite ^- some interest attached 

 to the analysis of the altered Utah material. A sample was accord- 

 ingly analyzed with the following results : 



Analysis of air dried epsomite 

 (Earl v. Shannon, analyst) 



These results show that the material has been dehydrated 

 to a degree where the composition approximates the formula 

 MgSO^.SHoO. Under the microscope this analyzed powder was 

 found to be ver}^ finely crystaline granular with the gi*ains were too 

 small to yield a definite interference figure although very hazy and 

 questionable figures appear to indicate a biaxial negative mineral 

 with 2V medium. The refractive indices are variable, but the mean 

 index is about 1.490. This powder, when exposed to dry air in a 

 desiccator over calcium chloride for 40 hours did not change in 

 weight. Heated for six hours at 110° C. the powder lost 22.90 per 

 cent and the sample thus dehydrated regained 1.10 per cent on 

 standing three weeks over calcium chloride. 



A second specimen of the lot was unlike the fibrous epsomite in 

 appearance and was granular, translucent, and white in color. It 

 was readily soluble in water and by qualitative tests was proven to 



" Esper S. Larsen, Microscopic determination of the nonopaque minerals. U. S. Geol. 

 Survey Bull. 679. 



" T. L. Walker and A. L. Parsons. Hexahydrite from Oroville, Wash., U. S. A., Univ. 

 Toronto Geol. Series No. 24. Contributions to Canadian mineralogy, 1927, p. 21. 



