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A Noteworthy Occurrence of Biotite Mica. 



By Evan R. Stanley, F.G.S., Government Geologist, Papua. 



[Read August 10, 1916.] 



This mica occurs as a constituent of the radio-active> 

 ilmenite-bearing lode in the Pre-Cambrian formation at 

 Radium Hill, situated about twenty miles east-south-east of 

 the Olary railway station. There is a considerable amount 

 of quartz and radio-active minerals associated with it, the 

 most obvious of the latter being carnotite. This carnotite 

 appears to have some connection with the black mica, and so, 

 on the suggestion of Dr. IX Mawson, a separation, analysis, 

 and optical determination have been made. Close to the 

 lode, but not directly associated with it, is a coarse mica vein, 

 dark greyish-green in colour, apparently different from the 

 black mica in the lode itself. Both these micas, when exposed 

 to atmospheric conditions for a period of a few months, 

 become coated with carnotite, and therein lies the special 

 interest which they possess. 



Microscopic Characters. 



The mica is jet-black in colour and possesses the usual 

 perfect cleavage parallel to the (001). In thin sections it is 

 light greenish-brown in colour and strongly pleochroic, the 

 pleochroic scheiue being as follows: — 



8 = very dark-greenish sepia. 

 h = preenish-vellow. 

 C = yellowish -brown . 

 The extinction is practically straight, being parallel to the 

 cleavage traces. 



In convergent polarized light a small optical axial angle 

 is seen. This angle was determined roughly by comparing it 

 with the optical axial angles of known minerals on a prepared 

 scale. It was found to be approximately 30 c , approaching 

 that of aragonite. It is optically negative and /><v- 



The mica contains numerous light-brownish-coloured 

 needles, arranged in a symmetrical manner, making angles 

 of 60° with one another. They have a straight extinction 

 and a strong double refraction, which suggests that they are 

 probably rutile. This conclusion is supported by the analyses 

 which follow. These characters, taken in conjunction with 

 the results of analyses set out below, indicate this mica to be 

 a biotite. The plates are large, and the occurrence Is a 

 notable one. 



