A RARE COPPER MINERAL. 287 



colouring- matter of this new mineral was Copper, whereas the 

 Emerald owed its colour to Chromium. In those days the mineral 

 was very rare and Vauquelin had only 3^^ grains of substance at 

 his disposal. In his " Traite de Mineralogie,"* Haiiy, the father 

 of Crystallography, points out the distinguishing characters of 

 Emerald and the new mineral to which he gave the name 

 " Dioptase. " 



Haiiy states his reasons for giving the name "Dioptase" in 

 the following words : — 



Lorsqu'on fait mouvoir une dioptase dodecaedre a la lumiere, on apergoit 

 a I'interieur des reflets assez vifs. situes sur des plans sensiblement paralleles 

 aux aretes terminales ; de sorteque les joints naturels sont indiques d'avance 

 par ces reflets, en pergant, pour ainsi dire, a travers le cristal. C'est ce 

 qu'exprime le nom de "dioptase." 



This observation can be made with the fine crystals from the 

 Tsumeb Mine. 



At the Altyn-Tiibe Mount the Dioptase is found in Limestone, 

 and for a long time, till 1874, that locality was the only place 

 where this interesting mineral was found. But already at the time 

 of Haiiy Dioptase was also observed associated with Mala- 

 chite, as appears from the following passage : — 



" J'ai appris de M. Inguersen, savant mineralogiste danois, que la dioptase 

 se trouvoit en Siberie, sur une gangue recouverte de malachite." t 



This is exactly the same condition under which the Dioptase is 

 found in the Tsumeb Mine with Dioptase crystals embedded in 

 fibrous Malachite. 



In 1878 small crystals of Dioptase were found at Rezbanya, in 

 Hungary, in clay with Wulfenite and Smithsonite ; then in 1880 

 in Chili at Copiapo in cupriferous quartz, and in 1874 in the Congo 

 Free State on the mine Mindauli, between Philippeville and 

 Brazzaville. 



The general characteristic features of this rare mineral, as 

 discribed in the works of Miers, Naumann-Zirkel and Hintze, can 

 be well observed in the specimens from the Tsumeb Mine. 



The two chemical analyses which I made of the Tsumeb 

 Dioptase gave the following results : — 



I. 2. 



Cupric Oxide 50.08 50.10 



Silica 37.15 37.13 



Water 11.38 11-36 



Oxide of Iron 1.38 1.39 



These results agree with the formula : SiO +CuO + OH.2 



It is very remarkable that the water is only expelled at a tem- 

 perature between 40o°C and 5oo°C. It is thus evident that the 

 water contained in this mineral is not water of crystallization but 

 water of constitution, and that, chemically speaking, Dioptase is 

 not a hydrated cupric metasilicate, but an acid cupric orthosilicate. 



I am greatly indebted to Dr. Haymann, General Manager of 

 the Damaraland Copper Mines, who kindly placed the specimens 

 of this beautiful mineral at my disposal. 



• Tome III. p. 139- 

 t Traite de Mineralogie, Tome III, p. 1 38, 



