24 W. E. Ford — Remarkable Twins of Atacamite. 



Fig. 12. 



Brochantite. — Brochaiitite was the most common mineral 

 observed on tlie specimens. It occurs generally in slender 

 prismatic crystals that intei-laee and cross each other to form 

 granular crystalline masses. Occasionally a crys- 

 tal with good faces was observed that could be 

 measured. The following forms were identified : 

 &(010), ^(HO), r(120)/and ^^ClOl), and their 

 chai-acteristic development is shown in figure 12. 

 In addition to this type of occurrence, brochant- 

 ite was observed in very slender, almost capillary, 

 prisms arranged in radiating tufts on the surface 

 of the gangue material. With its transparent, 

 bright green color, this variety makes very strik- 

 ing and beautiful specimens. A quantibative 

 analysis of these slender crystals Avas carried far 

 enough to establish their identity with brochant- 

 ite. An analysis of the type of material first 

 described was carried to completion. The ma- 

 teria,! analyzed was of ideal purity, and the 

 results agree very closely with the theoretical 

 values. The results of the analysis follow : 



CuO 



SO3 



H,0--. - 



I 



70-41 



17-51 



11-91 



II 

 70-16 

 17-59 

 12-01 



Average 

 70-29 

 17-54 

 11-96 



Theory 

 70-37 

 17-69 

 11-94 











99-79 



100-00 



Olivenite occurs on the specimens as slender to acicular pris- 

 matic crystals of light to dark olive-green color. The mineral 

 gave the appropriate tests for olivenite, and one crystal was 

 found which had terminal planes sufficiently good to permit of 

 decisive measurements being made. The prism zone of the 

 crystals was always deeply striated, and terminal planes were 

 only rarely observed. 



Clinoclasite was observed on only a few specimens, and then 

 in microscopic crystals. It was identified by its blue-black 

 color and the blowpipe tests that it gave. Conichalcite was 

 foixnd very sparingly in the characteristic emerald-green glob- 

 ular form. Other copper minerals in small amount were 

 observed, some of which could not be positively identified with 

 any known species but which occurred in too small an amount 

 to permit of investigation. 



Mineralogical Laboratory of the Sheffield Scientific School 

 of Yale University, New Haven, Conn. , 

 February, 1910. 



