69 



only 0'02 cm. thick. As atacaniite is brittle, tliese speci- 

 mens are, therefore, very delicate, the least rough handling 

 causing them to crumble into fine sand, the (iri<tniU(i of the 

 Chilians. 



The lateral development of tlie lamellae was expected 

 to be in tlie direction of the h face, though such has not 

 been borne out by observation on cleavage directions evi- 

 denced on fractured edges. This cleavage angle approxi- 

 mates to 66'' 30', and would appear to be referable to the 

 angle vnn'", indicating a laminar development parallel to 

 the r face. As original crystal edges are not available for 

 measurement, the point remains unsettled. 



Viewed under the microscope, the delicate plates are 

 noticeably ridged in a direction normal to the obtuse angle 

 of the dominant cleavage ; perpendicular are fine striations 

 of a subordinate nature. Its orthorhombic character is evi- 

 denced by straight extinction along the two latter directions 

 and biaxial character of the interference figure. The colour 

 of the mineral is dark emerald green. 



A complete qualitative analysis was supplemented by 

 quantitative determinations of the chlorine and copper: — 



Chlorine 16-78% 



Copper 59-34% 



Insoluble ... ... ... ... trace 



Iron ... ... ... ... trace 



Nickel and Cobalt ... ... nil 



Corresponding to the following composition : — 



CuCL 31-83 



CuO 55-43 



H./) (by diff.) 12-74 



100 



The other two varieties of atacamite from this locality 

 are closely associated in a reddish clayey matrix. One of 

 these is in the form of small tablets, black by reflected light, 

 but sufficiently translucent to show the characteristic green 

 by fairly strong transmitted light. The tables, which have 

 the following average dimensions, 4*7 mms. by 3 mms. by 1 

 mm., are developed parallel to the h face, bounded by 



jllOJ, JOIOJ, (Ollj, and exhibit the usual cleavage. The 



chlorine was determined as 17 03 per cent. 



The third modification consists in grass green granular 

 aggregates, apparently deposited subsequent to the tabular 

 variety, often enclosing the latter and always subordinate to 



