云母分子式计算方案的选择

本文借助主成分分析证明了花岗岩类中黑云母的Fe~(3+)、Fe~(2+)和(OH+F)主要的数量关系不表现为氧羟铁云母置换,还讨论了水不足和水过剩对云母分子式计算结果的影响,进而提出了当处理一批未受明显脱羟基氧化的云母,尤其是当其中包括明显水过剩云母时,宜优先采用O=22计算分子式方案等建议。

THE CHOICE OF CALCULATING SCHEME OF MICA FORMULA

It is considered by Eugster et al. the only available way to calculate formulae of naturally occurring biotites is based on O+OH+F=24 for the dehydroxylation of OH to O in biotites.But by principal components analysis on Fe~(3+),Fe~(2+) and(OH+F) of biotites from granitoids, it is shown in this paper that the chemical composition of biotites is determined by temperature, water fugacity, oxygen fugacity and oxidation state, but the quantitative relationship between Fe~(3+), Fe~(2+)and (OH+F) of biotites is not controlled so commonly and obviously by oxygen siderophyllite substitution. Fe~(3+)-ion can be occupied the octahedral position by similar manner to the other cations without dehydroxylation of mica.The octahedral occupancy of cations will be inclined to lower side, if the calculation of formula of the water excess mica is based on O+OH+F=24. For the water deficit mica not obviously related with oxidation, the octahedral occupancy based O+OH+F=24 will be inclined to the higher side, but this occupancy based on O=22 to the lower side.For the difficuty in determining H_2O~+ and in accurately distributing of water to each structural layer of mica, it is suggested that it is better based on O=22 to calculate formulas of a batch of micas not obviously oxidated, espeaiclly including water excess micas. "