DESCLOIZIT?: FROM SOUTH-WEST AFRICA. 143 



In the material sent to the writer, the crystals range in 

 length from a millimetre and less to 1"7 cm. Actually the largest 

 had a length, measured along the c axis, of 1'72 cm, and a breadth 

 measured along the b axis of 0"8 cm. They are made up of 

 the unit pyramid and the vmit prism, the faces of the latter being 

 generally horizontally furrowed owing to an oscillatory combina- 

 tion of the two forms. More rarely the pyramidal faces are 

 teiTaced owing to the same cause. The habit of the crystals is 

 sometimes i:)rismatic, the grooving of the prism faces being then 

 especially pronounced. More usually the pyramid and prism 

 faces are about equally developed. Among the smaller 

 individuals, however, crystals of pyramidal habit are fairly com- 

 mon. Single crystals, as a matter of fact, are rare. Even what 

 appear at first sight to be simple forms are generally found to be 

 made up of sub-parallel intergrowths of several crystals, and 

 complex spear-headed and branching groups inade up of a number 

 of such crv'stals sometimes symmetrically arranged about a 

 central dominant stem crystal are very common. The crystal 

 faces give very poor signals and do not lend themselves to accurate 

 measurement, this applying particularly to those of the unit 

 prism. The only angles which could be determined by means 

 of a Fuess reflecting goniometer with even a fair degree of 

 accuracy were as follow: — 



00' = (111) : (Til) = 89° 24' 

 00'" = (111) : (ill) = 520 30' 

 the corresponding values given bv Dana* being 89° C and 

 530 4'. 



The crystals are of a very dark olive green colour and their 

 lustre is brilliant. On fractured surfaces the colour is orange 

 brown, the fracture being uneven. The streak is pale canary- 

 yellow. In the process of grinding down the crystals to prepare 

 thin sections an imperfect cleavage parallel to (100) is brought 

 out. 



Under the microscope the crv^stals are seen to have a well- 

 defined zonal structure, being built up of alternations of thick 

 greenish and thinner brown or yellow layers aiTanged parallel with 

 the faces. These layers are markedly pleochroic, the greenish 

 showing : — 



Z = a=pale yellowish green, 



Y = b = pale apple-green, 



X = e = very pale yellowish-green, 

 and the brown or yellow : 



Z = a = reddish -brown, 



Y = b = greenish yellow , 

 X = c = pale yellow. 



The brownish layers have a much stronger absorption than the 

 greenish. It should be stated that the colours given refer to 

 a fairly thick section. In addition to the brown and greenish 

 zones, a practically colourless zone is generally present in the 

 peripheral portion of the crystals. In one of the sections 

 examined this showed : 



Z = pale yellow, 



Y = colourless. 



* " A System of Mineralogy," p. 787. 



