10 ir. E. Ford — Remarkable Twins of Atacamite. 



Art. II. — On some Remarkable Twins of Atacamite and 

 on some other Copper Minerals from CoUahurasi, Tara- 

 paca, Chili; by W. E. Ford. 



Some time ago a suite of copper minerals from Collaliurasi, 

 Chili, was presented to the Brusli Collection by Mr. Ernest 

 Schernikow of New York City. They included the following 

 rare species : atacamite, brochautite, olivenite, clinoclasite, 

 and conichalcite. Tlie atacamite and the brochautite occurred 

 in crystals and in considerable quantity, so that they offered 

 excellent opportunities for investigation. 



Atacamite. — This mineral was found in crystalline masses 

 made up for the most part of quite small crystals, but at times 



Fig. 1. 



Fig. 2. 



Fig. 3. 



showing individuals four or five millimetei's in length. These 

 crystals on examination proved to be unusually interesting. 

 The forms present were few, being m (110), x (140), b (010), 

 ^(011), /'(111) and n (121). The majority of the crystals 

 showed only the brachypinacoid, unit prism, and dome, illus- 

 ti'ated in figure 1. The pyramids r and n and the prism x 

 were only observed on a few iudividuals and then only as small 

 truncations, as shown in figure 2. The interest in the crystals 

 lay, however, in the constant and unusual twinning that they 

 showed. The law of twinning is new to atacamite and in 

 some ways proved very puzzling and difficult to explain, but 



