Hibonite and coexisting zoisite and clinozoisite in a calc-silicate granulite from southern Tanzania
Hibonite and coexisting zoisite and clinozoisite in a calc-silicate granulite from southern Tanzania
复制标题
坦桑尼亚南部钙硅酸盐麻粒岩中的黑铝石和共存的黝帘石和斜帘石
DOI：
10.1180/minmag.1980.043.332.07
复制
发表时间：
1980
影响因子：
2.7
通讯作者：
E. Burke
中科院分区：
文献类型：
--
作者：
P. Maaskant;J. Coolen;E. Burke
在坦桑尼亚南部富鲁阿麻粒岩杂岩的麻粒岩相岩中发现了第三个海陆产hibonite。这种矿物形成黄褐色板条状晶体，在一种含有次级榍石（斜石）黝帘石、海长石、磷灰石、钛铁矿和刚玉钛铁矿的粗钙长石中。电子探针分析得到了广义式（Ca1−x REx）[(Al,Fe3+)12−2a+x (Ti,Si)(Ti,Si) a−x (Fe2+, Mg) a]O19, RE = Ce + La + Nd, x = 0.2, a = 0.8。单个矿物分析表明，Ca + Ti + Fe3+ = RE + 2Al的阳离子取代。相对较高的稀土和铁含量是其与陨星辉长岩的主要化学差异。六角形单元胞的a = 5.61 Å， c = 22.18，与其他陆生hibonite基本一致。（斜）黝帘石有三种组成类型：1.8-3.1 wt% Fe2O3（正晶和单斜晶）、3.9-6.0 wt% Fe2O3（单斜晶）和5.8-7.9 wt% Fe2O3（平均6.3 wt% re2o3（单斜晶））。邻近岩石中共存辉石岩和石榴石的测温和测压数据表明，麻粒岩相平衡条件为750 ~ 850℃，6 ~ 11 kb。随着水分压的增加，海辉石与斜长石和石榴石发生反应，形成尖晶石、榍石和含re斜沸石。刚玉-钛铁矿相互生长可能是由于Fe-högbomite的破裂造成的。
Summary The third terrestrial occurrence of hibonite is reported from granulite-facies rocks in the Furua Granulite Complex in southern Tanzania. The mineral forms yellowish-brown lath-shaped crystals in a grossular-anorthite rock containing subordinate sphene (clino)zoisite, hercynite, apatite, ilmenite, and corundumilmenite intergrowths. Electron-microprobe analyses indicate a generalized formula (Ca1−x REx )[(Al,Fe3+)12−2a+x (Ti,Si)(Ti,Si) a−x (Fe2+, Mg) a ]O19, with RE = Ce + La + Nd, x = 0.2, and a = 0.8. Individual mineral analyses show a cation substitution of Ca + Ti + Fe3+ = RE + 2Al. Relatively high RE and Fe contents represent the main chemical differences with meteoritic hibonite. The hexagonal unit cell has a = 5.61 Å, c = 22.18, in good agreement with the other terrestrial hibonites. Three compositional types of (clino)zoisite are distinguished: 1.8–3.1 wt% Fe2O3 (orthorhombic and monoclinic), 3.9–6.0 wt% Fe2O3 (monoclinic), and 5.8–7.9 wt% Fe2O3 with an average of 6.3 wt% RE 2O3 (monoclinic). Thermometric and barometric data for coexisting pyroxenes and garnet from adjacent rocks indicate granulite-facies equilibration conditions of 750 to 850 °C and 6 to 11 kb. During retrogression with increasing partial H2O pressures, hibonite reacted with plagioclase and garnet to form spinel, sphene, and RE-bearing clinozoisite. Corundum-ilmenite inter-growths probably resulted from the breakdown of an Fe-högbomite.