铁尖晶橄榄石的穆斯堡尔谱研究

MOSSBAUER SPECTRAL STUDY OF RINGWOODITE

本文使用穆斯堡尔效应研究了在高温高压下合成的铁橄榄石和铁尖晶橄榄石。298K温度下铁橄榄石的穆斯堡尔谱由一组双峰组成，它被指派给Fe2＋（M1，M2）离子。298K温度下铁尖晶橄榄石的穆斯堡尔谱由三组双峰组成，其中两组双峰指派给八面体位置B上的Fe2＋，另一组双峰归结为四面体位置A上的Fe3＋，两组Fe2＋双峰的同质异能位移近似相等而四极分裂不同，Si原子在八面体位置上的占有率为6％～10％。铁尖晶微榄石的结构化学式可写为，其中x＝0．12～0．20。随着合成样品压力的增加，x值略有增大，但x值随合成样品温度的升高基本保持不变。此外，根据Fe2＋同质异能位移与平均键长之间的关系，得到铁橄榄石和铁尖晶三中的平均键长Fe－O和（Fe，Si）－O分别为d＝0．2143um和d＝0．2071～0.2085nm。

Fayalite and ringwoodite synthesized were examined by Mossbauer spectrometry. The M6ssbauer spectrum of fayalite at 298 K consists of one douhlet, which is assigned to Fe2+ions at M1 and M2 sites. all the spectra of ringwoodites at 28 K were fitted to three doublets. According to the values of isomer shift and quadruple splitting, two doublets were assigned to Fe2+ions at oc-tahedral site B, while one doublet was attributed to Fe2+ions at tetrahedral site A in the ringwood-ite with spinel structure. The two doublets assigned to Fe2+ (B) have nearly equal isomer shift and different quadrupole splitting and were caused by next nearest neighbor effect. The occupancy rates of Si atoms at the octahedral sites are in the range 6%-10% based on the Mossbauer mea-surements, and the structrural formula of the ringwoodite can be written as O4 (x= O. 1-0. 20). x increases with increasing pressure while x remains umchanged with increasing temperature. In addition, the bond length Fe-O in fayalite and the bond lengths (Fe,Si)-O in ringwoodite are 0. 214 3 nm and 0. 207 1-0. 208 5 nm , respectively, on the basis of the correlation of Fe2+ isomer shift with the average bond length.