1865.] 



of the Brochantite Group. 



or, as occasionally, a quartzose vein-stone, usually presents on its surface a 

 very thin glaze of a greyish-white colour, and endowed with a remarkable 

 metallic lustre. On this a thin layer, sometimes but -^th of an inch in thick- 

 ness, of the blue crystals is met with, and on that a thicker agglutinated 

 mass of the same mineral of rather a paler blue colour. Sometimes this paler 

 variety exhibits a very fine sky-blue colour, and assumes the form of folia- 

 tions with the appearance of small and extremely thin crystals, which are, 

 in fact, an aggregate of crystals generally twinned, and in the form of 

 laminae. 



Above the whole is occasionally seen a coating, varying in thickness from 

 an eighth to half an inch, of a faintly bluish- or greenish-white mineral with 

 a fibrous, and sometimes a slightly foliated, structure. 



The place of the blue mineral is often taken by one of a fine green colour, 

 varying from a dark emerald to verdigris-green, and often crystalline. Oc- 

 casionally also crystals of Brochantite may be seen, sometimes in clusters, 

 and occasionally also mixed with this green mineral. 



I. On Langite. 



The first of these minerals that I propose to describe is that which occurs 

 in crystals and crystalline masses, whether of the deeper or lighter lines of 

 blue. I propose to call it Langite, in honour of my valued friend and late 

 colleague Dr. Viktor von Lang, now Professor of Physics at Gratz. 



Langite crystallizes in very minute generally dark and somewhat greenish 

 blue crystals belonging to the prismatic system, the ratios of the parameters 

 being a : b : c= 1 : O'o347 : 0'6346. The forms observed are (1 0), (0 1), 

 (1 1 0), (2 1), and (010). The inclinations found between normals to thin 

 planes being 



10000 1=90 

 11011 0=56 16 

 001 201 = 51 46 

 11020 1 = 68 8 



10011 0=61 46 61 52' calc. 



10001 = 90 



The crystals are for the most part twinned similarly to those not unfre- 

 quent in cerussite : twin axes (1 10). 



T 1 (1 1 0) I 1 0=112 32 

 1^00 (1 10) 100=123 44 

 1 1 (1 1 0) 1 "I 0= 67 28 



