OCCURRENCE OF CORUNDUM. 223 



HEMATITE GROUP. 

 CORUNDUM, HEMATITE, AND ILMENITE. 



Corundum: 



A1 2 3 . 



Ehombohedral. 



Sp. gr. 3 95-4.10. 

 Hematite : 



Fe 2 3 .- 



Ehombohedral. 



Sp. gr. 5.20-5.25. . 

 Ilmenite: 



FeTiO s ; varies to mFeTi0 3 .nFe 2 3 . 



Ehombohedral. 



Sp. gr. 4.50-5.02. 



CORUNDUM. 



occurrence. — In Canada at one locality corundum occurs as an original 

 constituent of a syenite." Also, corundum as an accessory mineral has been 

 noted in granite, andesite, and other rocks. Corundum is, therefore, an 

 original pyrogenic constituent of igneous rocks. Corundum occurs along 

 the contact of intrusive basic rocks rich in alumina, especially those con- 

 taining more than 30 per cent, such as peridotites and pyroxenites. The 

 intruded rocks may be either igneous rocks or gneisses and schists. But 

 where corundum occurs in veins along contacts it is in many cases an 

 aqueo-igneous product (see pp. 720-728) or an aqueous deposit. Corundum 

 is a widespread accessory constituent in various micaceous, chloritic, and 

 hornblendic schists and gneisses, and in marble. Corundum, as a meta- 

 morphic mineral, is associated with chlorite and corundophilite. It is often 

 associated with other heavy metamorphic minerals, such as andalusite, 

 sillimanite, cyanite, spinel, rutile, etc. As a metamorphic mineral it is 

 derived from andalusite, cyanite, diaspore, gibbsite, sillimanite, staurolite, 

 and topaz. 



Alterations. — Corundum alters into diaspore (orthorhombic ; sp. gr. 3.40), 

 gibbsite (monoclinic; sp. gr. 2.35), spinel (isometric; sp. gr. 3 8), sillimanite 

 (orthorhombic; sp. gr. 3.235), cyanite (triclinic; sp. gr. 3.615), muscovite 

 (damourite), (monoclinic; sp. gr. 2.88), margarite (monoclinic; sp. gr. 3.035), 

 and zoisite (orthorhombic; sp.gr. 3.31). The reactions for the formation of 

 diaspore and gibbsite are simple reactions of hydration. The reactions for 



"Miller, W. G., Economic geology of eastern Ontario; corundum and other, minerals: Seventh 

 Eept. Ontario Bureau of Mines, 1897, Toronto, 1898, p. 213. 



