524 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



Fig. 2. Intergrowth of bornite and chalcocite. X 40. 266 feet level, Blue Wing mine. 

 Dark areas = bornite. 

 Lighter areas =chalcocite. 

 There does not appear to be any marked crystallographic relationship 

 between the two minerals in this section as in those shown in Plate 68. 

 The appearance is that of a spongy mass of bornite merged and inter- 

 grown with another similar mass of chalcocite. 



Plate 68. 



Fig. 1. Intergrowth of bornite and chalcocite. X 40. Wall mine. 

 Dark areas=bornite. 

 Lighter areas=chalcocite. 

 A typical example of crystallographic intergrowth of bornite and chalcocite. 

 Such intergrowths are believed to form only at the eutectic point of a 

 solution and are strong evidence that the two minerals were deposited 

 contemporaneously . 

 2. Crystallographic intergrowth of bornite and chalcocite. X 220. Wall mine. 

 Dark areas=bornite. 

 Lighter areas=chalcocite. 

 This is an area of the finely intergrown portion of the section shown in fig. 1, 

 highly magnified, and shows that the relationship of the two minerals is 

 a typical crystallographic intergrowth. 



Plate 69. 



Fig. 1. Intergrowth of bornite and chalcocite. X 220. Wall mine. 

 Stippled areas=bornite. 

 White areas = chalcocite. 

 This is a deeply etched section of the two minerals and shows clearly by the 

 etch figure, the lines at right angles to each other in the chalcocite, that 

 the portion of this mineral here seen is a part of a single individual crys- 

 tal, and also brings out the crystallographic relations of these two minerals. 

 2. Intergrowth of bornite and chalcocite. X 40. High Hill mine. 

 Stippled areas=bornite. 

 WMte areas=chalcocite. 

 The peculiar lines in the chalcocite areas are the etch figures and show the 

 coarsely granular condition of this mineral. 



