1887,] *^^ [Genth. 



3. Mimed/e and Pscudomorphs of Mimetite after Anglesite ? 



The greater portion of the ores Nos. 1 and' 2 from the Mina del Diablo 

 consists of crystals and crystalline aggregations of a white or yellowish- 

 ■white mineral, rough to the toucli, somewhat earthy in appearance, with a 

 Blight resinous lustre on a fresh fracture. These crystals have been picked 

 out from a great mass of clay and are a great rarit}', and even the crystal- 

 line aggregations, which occurred in somewhat larger quantities, are com- 

 pletely exiiausted. At tlie ]\Iina de San Antonio, a few but far less per- 

 fect crystals have been observed. 



a. Mimetite. — Small fragments of finely granular, frequently cavernous 

 concretions, the surf ice of which is coated with microscopic coloiless crys- 

 tals, have been found among the above mentioned ores Nos. 1 and 3. Simi- 

 lar crystalline coatings are sometimes observed upon the crystals, especially 

 when in groups, which were better protected against corroding influences, 

 and it is most likely that the whole mass of the pseudomorphous crystals 

 consists of an accumulation of such microscopic crystals, so small, how- 

 ever, that a fracture would show only a compact mineral with a slightly 

 waxy lustre. Cavities of the pseudomorphous crystals are also someiimes 

 lined with colorless microscopic crystals. It was very difficult to ol)serve 

 any distinct forms ; when magnified 60 diameters many hexagonal planes 

 were seen, but only one crystal was observed which showed a short hex- 

 agonal prism, with a seccjud hexagonal prism, a pyramid and basal plane. 

 Several others were found of the same form without the second hexagonal 

 prism. These crystals have a vitreous lustre and aie undoubtedly mime- 

 tite. 



b. Pieudomorphs of Mimetite after Anrjlcsite? — The great bulk of the 

 ore, however, is entirely diflerent and consists of pseudomorphs which 

 have appirently a rhoml)ic form while their composition is that of mime- 

 tite. Tiiey are associated with stalacliticor bolryoidal red cnssilerite often 

 in isolated crystals with all planes fully developed, upon it, or in crys- 

 talline groups or incrusting the same. Tlie whole occurrence shows the 

 more recent oricin of the mimetite pseudomorphs. 



Isolated crystals are rare, they are often cavernous and generally ar- 

 ranged in reticulated and skeleton like groups. On a most excellent 

 phototype plate by Mr. Frederick Gutekunst, I have given a few of the 

 forms magnified two diameters which show best this peculiar arrange- 

 ment : ta, 2a, and oa are the reverse of 1, 2, and 3. 



Measurements of the best isolated crystals gave angles which suggested 

 the idea that the original mineral was anglesite. 



I have submitted these crystals and aggregations to Prof G. vom Rath, 

 who had the kindness to make the following measurements, which, 

 together with his conclusions, he communicated, 1. c. On account of 

 roughness and imperfection of the surfaces of the crystals only api')rox- 

 imate measurements were possible, which were made with the help of 

 attached glass plates. 



PROC. AMER. PHILOS. SOC. XXIT. 125. E. PRINTED APRIL 28, 1887. 



