484 ransome: wurtzite at goldfield 



In thin section under the microscope a considerable part of 

 the material, particularly that forming the inner portion of the 

 crust, may be seen to consist chiefly of radial aggregates of dis- 

 tinctly birefringent prisms with very indefinite individual out- 

 lines. These fibers, which are clearly wurtzite, show a tendency 

 to diverge from points on the inner surface of the crust, becoming 

 less distinct toward their free ends where the birefringent aggre- 

 gate grades into material that is chiefly sphalerite, containing 

 here and there little flecks of wurtzite. In some aggregates the 

 prisms have grown radially outward from small dark nuclei 

 which appear to consist chiefly of ferruginous zinc sulphide, in 

 part at least, wurtzite. As a rule the aggregates of wurtzite in 

 addition to their radially fibrous structure show a noticeable 

 although discontinuous concentric parting. In ordinary light 

 the wurtzite and sphalerite are practically indistinguishable ex- 

 cept in a few parts of the section where sphalerite may be recog- 

 nized by its dodecahedral cleavage; but generally both sphalerite 

 and wurtzite are traversed by a multitude of irregular cracks 

 which mask any cleavage that might otherwise be visible. 



Wurtzite in very intimate association with sphalerite and visu- 

 ally distinguishable from that mineral only by its birefringence, 

 has been described from the Horn Silver mine, Utah, by B. S. 

 Butler.'^ Mr. Butler's conclusion, which is apparently well sup- 

 ported, is that the wurtzite is younger than the sphalerite and 

 was formed by the action of descending acid solutions, although, 

 as he states, there are some features of the association of the two 

 minerals that are difficult to account for through this explanation. 



The Goldfield occurrence offers little in support of the view 

 that the wurtzite has resulted from an external attack on the 

 sphalerite by acid solutions. If both minerals originated by 

 deposition from solution, present relations would indicate the 

 sphalerite to be younger than at least a part of the wurtzite. 

 If, however, one mineral has undergone enantiotropic change to 

 the other, it is difficult, if not impossible, to determine which 

 was the primary form, as such a change would not necessarily 

 begin at the surface of a crystal grain. 



5 Op. cit., pp. 149-150. 1913. 



