OCCURRENCE OF GADOLINITE IN WEST AUSTRALIA. 287 



carrying cassiterite, gadolinite, monazite, etc. occurred in 

 the gneiss." 



Physical Characteristics. — Colour black in mass, like 

 coarse bottle glass. In splinters transparent and grass- 

 green. Weakly doubly refracting. Pleochroism distinct 

 but not strong, from grass-green to slightly bronze-green. 

 Streak light greenish grey. Lustre vitreous and somewhat 

 greasy. Fracture conchoidal to subconchoidal. Hardness 

 7, brittle. Specific gravity, as determined by Mr. B. F. 

 Davis 4*14. 



The mineral occurs in large masses as much as 10 cm. 

 in diameter, approximately rhombic in cross section, though 

 not sufficiently definite in shape for crystalline form to be 

 determined. The masses appear to be roughly prismatic 

 and are interrupted by an imperfect transverse parting, 

 the division planes being covered with a film of yellowish- 

 green decomposition products. 



The mineral is intergrown with white felspar to a certain 

 extent. The central portions of the gadolinite individuals 

 are free from felspar, which occurs zonally about the 

 periphery roughly defining the crystals of the host. This 

 fact indicates that the crystallisation of the gadolinite on 

 the whole preceded that of the felspar, though the two 

 periods overlapped somewhat. The portions of the gado- 

 linite exposed to the weather have undergone considerable 

 decomposition, the resulting product being reddish earthy 

 limonite with a concentric structure and a thickness up to 

 3 or 4 millimeters. 



The following is the analysis by Mr. Davis : — 



Silica 



Ferrous Oxide (FeO) 



Glucina (BeO) 



Cerium Oxide (Oe 2 3 ) 



Lanthanum and Didymium (La 2 3 and Di 2 3 ) 



Yttrium group calculated as (Y 2 3 ) 



Magnesium Oxide (MgO) 



Loss on ignition 



101*20 



23-33 





10-38 





12*28 





2*50^ 





« 18-30 } 



54*16 



33*40 J 





•69 





•32 





