ON CORUNDUM -SYENITE FROM MONTANA 749 



The feldspar crystals are sometimes arranged in rough augen. The 

 corundum occurs in grayish-blue crystals with an average size of 5 

 mm. and a maximum size of about 2 cm. The corundum crystals are 

 tabular or prismatic in habit with the common forms: c {0001 }, 

 a |ii2of, r jionf, n 12243!, and 6 ^8*8-16-3 K The most fre- 

 quent combination is acrn. 



The corundum is often surrounded by a zone of feldspar, which 

 is nearly free from biotite. A fibrous mineral occasionally observed 

 proves to be sillimanite as tested in fragments. Muscovite is often 

 observed in thin, cleavable flakes. It does not appear to be an 

 alteration of the corundum. Thin sections show a very small 

 amount of zircon in minute prismatic crystals. The baddeleyite 

 is a black, submetallic mineral which is usually found between the 

 corundum and the feldspar. It occurs in rounded blebs and in 

 prismatic crystals not over 3 mm. in size and usually only about 

 1 mm. in greatest dimension. The baddeleyite will be described 

 by the author in a forthcoming number of the American Journal 

 of Science. 



Baddeleyite rather than zircon forms in this type of rock prob- 

 ably on account of the low silica percentage. 



A rock sample weighing 243.6 grams was crushed, and after 

 sizing, the constituents were separated by means of Thoulet 

 solution. It was found that good separations could be made by 

 panning with the Thoulet solution. The following shows the 

 amounts of the various minerals and also the percentages by weight, 

 assuming the loss to be equally distributed among the minerals : 



Grams Percentage 



Feldspar 136.6 62.7 



Corundum 67 . 3 30. 9 



Biotite 12.6 5.8 



Baddeleyite 1.1 0.5 



Loss 26.0 



Total 243 .6 99 . 9 



We may assume the feldspar to be a eutectic of albite and 

 orthoclase. Vogt gives the eutectic ratio for these two minerals 

 as ab= 58 per cent, 0^=42 per cent. We then have 36.4 per cent 

 albite and 26 . 3 per cent orthoclase. The biotite is the only mineral 



