ON CORUNDUM-SYENITE (URALOSE) FROM MONTANA 



AUSTIN F. ROGERS 



Leland Stanford Junior University 



Specimens of a corundum-bearing rock from the property of the 

 Bozeman Corundum Company, fourteen miles southwest of Boze- 

 man, Gallatin County, Mont., were obtained for Stanford Uni- 

 versity by Mr. R. M. Wilke of Palo Alto, Cal. No information 

 concerning the country rock could be obtained except the state- 

 ments of Pratt in his monograph on corundum: 1 "The corundum 

 seams vary from a few inches to three feet in thickness Boze- 

 man: Fourteen miles southwest of this town corundum is found 

 in syenite." From this it would seem that the country rock as a 

 whole is a syenite with bands or seams of the corundum rock. 

 These bands, the writer will show, are corundum-syenite. The 

 rarity of this type of igneous rock accounts for the present paper. 



Corundum-syenites have been described only from the Urals, 2 

 from eastern Ontario, 3 and from the Coimbatore district, India. 4 



The corundum-syenite is a medium to coarse-grained, gray- 

 mottled, more or less banded rock, the banding due principally to 

 the fact that the biotite flakes are mostly in parallel position, though 

 the other minerals are occasionally in rough, parallel position. 

 The gneissoid corundum-syenite, as it may be characterized, is 

 composed of microcline-perthite, biotite, and corundum with sub- 

 ordinate sillimanite, muscovite, zircon, and baddeleyite. The 

 feldspar is for the most part a perthitic intergrowth of microcline 

 and albite, though one slide shows plagioclase, orthoclase, and 

 microcline without any perthite. On a section of the microperthite 

 parallel to jooif the microcline has an extinction angle of n|°, 

 and the albite, one of 4§°. In this section the albite shows only 

 very faint albite twinning. On a section parallel to joio£ the 

 microcline has an extinction of — 3 and the albite, one of +20 . 



1 Bull. No. 269, U.S.G.S., 133, 144 (1906). 



2 Morozewicz, Min. u. petr. Mitth., XVIII, 217 (1898). 



3 Miller, Rept. Bureau of Mines, Toronto, Canada, VIII, Part 8, 210 (1899). 

 1 Holland, Mem. Geol. Surv. of India, XXX, Part 3, 169 (1901). 



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