Art. XIV. — Account of the Separation and Identi- 

 fication of a Kaolin Incrustation on Pyrolusite 

 from Broken Hill. 



By G. S. WALPOLE, B.Sc. 



(Commixnicated by Professor Orme Masson). 



(With Plate XXIV.). 



[Eead 13th October, 1904]. 



The specimen investigated was obtained by Professor Orme 

 Masson from Broken Hill. It is bluish white externally, and 

 consists, for the most part, of a lacework of rods and films. 

 Parts of it are more solid and rather botryoidal in appearance. 

 Any part that has been broken shows that the colour is due to 

 a thin white film. Inside this the mineral appears to consist 

 of black pyrolusite, enclosing occasional rounded grains of glassy 

 quartz. No other mineral can be obserTcd raacroscopically. 



The Main Pkoblkm was to Identify the Incrustation 



OR Film. 



1. Several sections of the specimens were therefore prepared for 

 microscopic examination. These were cut across the more solid 

 parts of the mineral, and showed that the external layer is 

 transparent and crystalline. No definite crystal shapes can 

 be seen, but the mineral shows weak double refraction. The 

 refractive index is very low. In the sections prepared, this 

 mineral is always in direct contact with the pyrolusite, and 

 even when it occurs very close to the other minerals is always 

 separated from them by a thin film of pyrolusite. From a 

 number of measurements made : — 



Greatest thickness - - .083 mm. 



Least ,, - - .024 ,, 



Mean ,, - - .054 ,, 



