The mineralogy of the chrome spinels of the Bird River Sill, Manitoba

Abstract

Hermatite exsolves from specimens of the Bird River chromite on heating. Exsolution begins in iron-rich areas, such as the rims of crystals and marginal to fractures where deuteric alteration of the chromite has occurred. With prolonged heating, exsolution extends throughout the crystals; optimum heating conditions in air are 1000*C at one atmosphere for seventy two hours. X-ray fluorescence analyses and cell edge determinations were made on non-magnetic and ferromagnetic fractions of the concentrates. Two spinel phases were recognized in X-ray diffraction powder photographs of the four chromite specimens having the most pronounced rims. These two phases represent the altered, iron-rich rim and the central area of the crystals. Cell edge variation with composition is discussed. 'Stratigraphic' variation of the parameters was investigated; in the chromite mineral itself, Cr2O3 and total iron increase towards the top of the sill, Al2O3 remains constant across the section and MgO appears to decrease slightly towards the top of the sill; cell edges increase generally with increasing 'stratigraphic' order. Magnetism may depend on the proportion of ferrous to ferric iron in the chromite molecule. The evidence suggests that hematite will exsolve from any chromite which as a Cr:Fe ratio of about 1.2:1 or less.