宇宙尖晶石与地球尖晶石的比较

A COMPARISON BETWEEN COSMIC SPINEL AND TERRESTRIAL SPINEL

本文就宇宙尖晶石的形态，化学组成及产状特征作了概要性论述，对它的成因机制作了介绍；并和地球尖晶石作比较，表明它们之间的成因和形成条件的差异，突出了宇宙尖晶石的特点，即宇宙尖晶石为陨石物质在大气氧化过程中结晶而成，无例外的都有一个较高的氧化态和高镍含量。在地质纪录中宇宙尖晶石的发现具有双重意义：它既是地外物质的标志。

Magnetic spinels are reported in all extraterrestrial objects which have experienced melting in the earth atmosphere,such as in the fusion crusts of meteorites and micrometeorites,in the chondritic extraterrestrial particles extracted from deep sea sediments,and in polar ice,and in glassy microspherules from a late Pliocene impact.Based on above mentioned reports as well as our studies of cosmic dusts from Suzhou granite,an introduction to the physical and chemical characteristics of “cosmic” spinels,such as the section appearances,chemical compositions,occurrence features and their origins,as well as a comparison with terrestrial spinels,is given in this review. Cosmic spinels usually display dendritic,skeleton,cruciform and octahedral morphologies,which are characteristic of rapid crystallization from a high temperature melt.They contain variable amounts of MgO,Al 2O 3,and Cr 2O 3,with high NiO and relatively low TiO 2.The average bulk composition of cosmic spinels is expressed as a solid solution of spinel end members,showing that the two dominant components are magnetite and magnesioferrite.All cosmic spinels have a trevorite content.Cosmic spinels always display a relatively high iron oxidation state,usually being characterized by a Fe 3+ /Fe total ratio greater than 70%,and even higher,nearly reaching 100 atom% in late Pliocene microspherules and Suzhou granite cosmic dusts. But for the terrestrial spinels,they contain no more than 1 mol% trevorite.Magnesioferrite is also rarely present.All terrestrial spinels generally have a Fe 3+ /Fe total ratio less than 70%,and have not been reported to be with a dendritic,skeleton or octrahedral morphologies in their sections. Cosmic spinels are characterized by a high nickel concentration due to the relatively high abundance of this element in extraterrestrial and a high iron oxidation state resulting in crystallization under a high oxygen fugacities.So it is suggested that the cosmic spinels in the geological record is of two implication:they are a marker of cosmic material and they contain information about the mode of accretion of this material.