67 



MINERALOGICAL NOTES. 



By D. Mawson, B.E., B.Sc. 



[Read May 1, 1906.] 



Fetid Felspai; (Necronite) and Quartz, from 

 Umberatana. 



The material described was discovered by Mr. W. How- 

 chin, F.G.S., who, noting its fetid character when emshed, 

 perceived it to be of special interest mineralogically. 



Mr. Howchin describes it as occurring about two miles 

 east of Umberatana^ on the track to Illinawortina Station. 

 The rocks in the vicinity are probably quite as old as Lower 

 Cambrian. In the immediate neighbourhood bold outcrops 

 of binary granite ai'e conspicuous, forming hills as 

 much as two hundred feet in height ; veins of felspar, quartz, 

 and graphic granite, with tourmaline and other minerals, 

 were also noted. 



The specimen examined is aplitic in appearance and 

 slightly Dorous. It is composed of quartz, present to the 

 extent of about 25 per cent., and an adularian felspar in 

 typical rectangular sections. Grainsize averages one milli- 

 metre. On fracturing the rock a distinct fetid odour 

 is produced, to be likened somewhat to that proceeding from 

 carbon bisulphide. 



Examined microscopically in thin sections, the felspars 

 are seen to be idiomorphic; whilst the quartzes are, as a 

 rule, subordinate. The felspar crystals also show the effects 

 of crushing. Albite twinning is very common; whilst twins 

 after the baveno law and the cross-hatching of microcline are 

 also to be frequently noted. The refractive index is lower 

 than that of the balsam. 



Both minerals contain very numerous inclusions; these 

 are most abundant in an outer zone around the felspars, and 

 are also crowded thickly in a felspathic cement, occupying 

 interstices between the grains. The inclusions are chiefly 

 liquid and gaseous, in addition to whicli much opaque dust 

 is sometimes present. This opaque matter is very likely 

 largely carbonaceous, as several of the more translucent 

 patches were identified as bitumen. The objectionable 

 odour of the fractured rock is evidently derived from the 

 contents of the liquid and gaseous inclusions; these are quite 



* Umberatana is located about 50 miles in a direct line west of tlie 

 northern extremity of Lake Frome. 

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