Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas
Sillimanite and Ilmenite from High-grade Metamorphic Rocks of Antarctica and Other Areas
复制标题
南极等地区高级变质岩中的硅线石和钛铁矿
DOI：
10.1093/petrology/21.1.39
复制
发表时间：
1980
影响因子：
3.9
通讯作者：
E. Grew
中科院分区：
文献类型：
--
作者：
E. Grew
各种高级变质岩中的硅线石含Fe2O30.13-1.82%，TiO20.1%以下。铁是三价的，只能替代铝。与硅线石伴生的钛铁矿含有不超过0.4%的Al_2O_3、SiO_2、CaO和MnO，而MgO不超过1.6%。对10个所分析的硅线石的晶胞数据进行最小二乘拟合，得到无铁硅线石的晶胞尺寸为：a=7.4830，b=7.6708，c=5.7694×ANDV=1.66%。在所分析的21个样品中，有15个样品中的硅线石的Fe2O3含量随着伴生钛铁矿中Fe2O3含量的增加而有规律地增加，表明15个样品中的硅线石和钛铁矿是平衡结晶的。作者分析的10个硅线石-钛铁矿对的组成符合以下经验曲线：Sol；(XFe_2O_3)Il=1.110×10~(−)~3。这一Fe_2O_3含量的规律性增加符合硅线石中两个独立位置上的Fe~(3+)替代Al和钛铁矿中两个位置上的Fe~(2+)与Ti的耦合替代模型。硅线石和钛铁矿在硅线石和钛铁矿中的组成范围分别为0.013<；xFe_2SiO_3<；0.15和0<；xFe_2O_3<；0.15，是理想的溶液。例如，含1.5摩尔分数Fe2SiO_3的硅线石与无钛赤铁矿共存时，压力每增加5kb，其Fe2O_3含量就会降低11%。含赤铁矿组合中硅线石的Fe2O含量随温度的增加速率大于含钛铁矿组合。
Sillimanite from a variety of high-grade metamorphic rocks contains from 0.13 to 1.82 weight per cent Fe2O3and less than 0.1 weight per cent TiO2. The iron is trivalent and substitutes for Al only. Ilmenite associated with the sillimanite contains no more than 0.4 weight per cent Al2O3, SiO2, CaO, and MnO; and MgO does not exceed 1.6 weight per cent. It ranges in composition from Ilm99Hem1to Ilm85Hem15.A least squares fit of precision unit cell data on 10 analyzed sillimanites gives the following cell dimensions for iron-free sillimanite:a= 7.4830 Á,b= 7.6708 Á,c= 5.7694 Á andV= 331.15 Á3. The projected increase in cell volume with substitution of 10 mole per cent Fe2SiO3is 1.66 per cent.A regular increase in the Fe2O3content of sillimanite with increasing Fe2O3content of associated ilmenite in 15 of 21 samples analyzed suggests that sillimanite and ilmenite crystallized in equilibrium in the 15 samples. The compositions of the ten sillimanite-ilmenite pairs analyzed by the author fit the following empirical curve (sol;(XFe2O3)Il = 1.110 x 10−3. This regular increase in Fe2O3contents fits a model of Fe3+substitution for Al on two independent sites in sillimanite and a coupled substitution of for Fe2+Ti on two sites in ilmenite. Sillimanite and ilmenite are behaving as ideal solutions over the compositional range 0 < XFe2SIO3< 0.013 in sillimanite and 0 < XFe2O3< 0.15 in ilmenite.Equations have been derived for expressing the variation in Fe2O3content of sillimanite associated with quartz and ilmenite or hematite as a function of pressure, temperature, and Fe2O3content of the oxide minerals. For example, the Fe2O3content of a sillimanite with 1.5 mole per cent Fe2SiO3coexisting with Ti-free hematite is calculated to decrease 11 per cent with a 5 kb increase in pressure. The rate of increase with temperature of the Fe2O3content of sillimanite is greater in hematite-bearing assemblages than in ilmenite-bearing assemblages.