Cadmium, and Mercury. 387 



Optical and crystallographic relations between Sphalerite and 

 Wurtzite. 



The development of double refraction is the only conclusive 

 evidence we have of the change of sphalerite to wurtzite by 

 heating. Inasmuch as the transformation is slow, its progress 

 can be studied. In ferriferous blendes transformation appears 

 to be most rapid, starting usually at a single point in a grain 

 and progressing so that the final product has like orientation 

 throughout. In grains of the purest sphalerites, the wurtzite 

 usually begins developing at more than one point and in differ- 

 ent orientations. The structure produced is an intergrowth of 

 lamellae of wurtzite, each lamella having its principal axis 

 parallel to one of trigonal axes of the sphalerite grain.* It is 

 evident that the strength of the double refraction of a grain 

 thus transformed will be conditioned by the relative develop- 

 ment of the four possible sets of lamellae. J. Beckenkampf 

 has considered that lamellae of wurtzite may develop parallel 

 to the trapezohedron of sphalerite. Such lamellae would out- 

 crop on a cleavage face parallel or normal to cleavages or 

 bisectiug the acute angle between cleavage surfaces. All of 

 the outcropping planes would be oblique to cleavage planes. 

 No lamellae of this sort were seen in the large number of prep- 

 arations examined during this investigation. 



In material furnished by Mr. B. S. Butler, from Beaver Co., 

 Utah, prismatic crystals of wurtzite from a brecciated vein 

 have fragments of sphalerite as nucleii. The traces of the 

 prismatic cleavage of the wurtzite and of the cleavage of the 

 sphalerite appear to be parallel. The wurtzite cleaves parallel 

 to the second order prism. 



Beckenkamp has discussed in detail the very close crystallo- 

 graphic relations of sphalerite and wurtzite. With these are 

 now correlated the very slight changes in optical properties, 

 volume, and energy content accompanying the inversion of 

 these minerals. 



Cadmium Sulphide. 

 Amorphous. — The flocculent precipitate of cadmium sul- 

 phide is yellow while moist, but it dries to an orange powder 

 which is lumpy and nearly opaque to transmitted light. The 

 lumps can be consolidated by pressure — as by grinding forcibly 

 in a mortar or by compressing on a microscope slide under a 

 cover-glass — into transparent films, some of which become 

 crystalline during and after the compression. The progress of 

 crystallization may be slow, several hours being required for 



* Hautefeuille (loc. eit.) recognized this relation. 

 fZs. Kryst., xliv, 243, 1908. 



