CHEMISTRY AND MINERALOGY OF SILICATES AND METAL OF KODAIKANAL METEORITE

A chemical and mineralogical study of the silicate and metal phases of Kodaikanal has been made by optical, X-ray, and electron microprobe techniques. The silicate inclusions are mineralogically inhomogeneous and consist of either compositionally homogeneous alkali feldspathic glass or crystalline alkali feldspar and clinopyroxene (En52Fs10Wo38) in varying proportions with orthopyroxene (En81Fs17Wo02) as a minor constituent. The glass and, to a lesser extent, the crystalline feldspar have excess SiO2 with respect to an ideal feldspar formula. The alkali feldspar is a crypto-antiperthite whose composition varies from Ab90 to Ab60 on a 20-50 μ scale. The subcalcic nature of the calcium-clinopyroxene, the partitioning of Fe and Mg between orthopyroxene and clinopyroxene, and the high temperature form of the sodic feldspar are evidence that the silicate was cooled rapidly from a relatively high temperature. The low Na and Al contents of the pyroxene suggest that the silicates equilibrated under low pressures. Textures of the metal and metal-silicate interfaces show evidence of an intense deformation-possibly shock. Moreover, examination of the Widmanstätten pattern in relatively undeformed areas gives a cooling rate of 102-3°C/m.y. which is considerably faster than for most other iron meteorites. The restrictions placed on possible evolutionary histories by both the present and previous data are discussed. © 1969.