DESCLOIZITE FROM SOUTH-WEST AFRICA. 145 



limestone and reddish dolomite encased by crusts of dark green 

 or brown descloizite crystals, the cementing medium being 

 coarsely crystallised white calcite. The latter forms a network 

 of irregular veins up to 2 centimetres, across which are mter- 

 spersed ^^'ith vughs lined with crystals of descloizite and colour- 

 less calcite. At least four varieties of descloizite are present, 

 namelj' : — 



(1) A blackish green variety occurring in well-formed 

 pyramidal crystals somewhat elongated in. the direction of 

 the macrodiagonal. The prism faces are only very poorly 

 developed, and the habit of th-e crystals is not unlike that 

 of the descloizite from Lake Valley, New Mexico, shown in 

 Fig. 3, p. 788, of Dana's Mineralogy. With the aid of a 

 contact goniometer the following forms were fovmd to be 

 present : — 



= 111 



j(t = 110 

 u = 021 

 (d)? = 012 

 On some crystals the faces of the unit pi-ism are bevelled 

 by those of another prism, probably / = (130). On others the 



edge (111) : (HI) is terraced owing to an oscillatory com- 

 bination of the two clinodomal faces. 



(2) A bottle-green variety occurring in smaller crystals 

 of practically the same habit, the prism being, however, even 

 more poorly developed. One peculiar feature of these 

 crystals is that the clinodomal faces often have a peculiar 

 greenish-yellow coating. Whether or not this is due to the 

 alteration of the descloizite is not clear. 



(3) A dull greenish-brown variety in which the habit of 

 the crystals is tabular owing apparently to the abnormal 

 development of the macropinacoid, the other forms present 

 being the unit pyramid and the unit prism. 



(4) A lustrous chrome-brown variety occurring mostly 

 in peculiar parallel-growth aggregates, up to I'o centimetres 

 in length, imbedded in white calcite. The aggregates are 

 due to the regular conjunction of small crystals of pyramidal 

 habit and are terminated by the unit pyramid. Some of them 

 are doubly terminated. 



Chemically the several varieties enumerated appear to differ 

 mainly as regards their copper content. The bright chrome- 

 brown variety is practically free from that element, whereas the 

 very dark variety contains fair quantities of it. No analysis of 

 any particular variety is available, but a sample of the concen- 

 trate obtained by jigging the crushed ore, in which all of them 

 are probablv represented, showed: — 

 PbO = 51-81 per cent. 

 ZnO = 18-06 

 CuO- 1-10 

 V,0, = 20-60 

 P,0,= 0-08 

 The writer's thanks are due to Dr. F. F. Weight and 

 Professor Charles Palache for assistance in his investigations. 



