Morphology and crystal structures of solar and presolar Al2O3 in unequilibrated ordinary chondrites
Morphology and crystal structures of solar and presolar Al2O3 in unequilibrated ordinary chondrites
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DOI：
10.1016/j.gca.2013.09.013
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发表时间：
2014-01-01
影响因子：
5
通讯作者：
Nagahara, Hiroko
中科院分区：
文献类型：
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作者：
Takigawa, Aki;Tachibana, Shogo;Nagahara, Hiroko
刚玉是氧化铝(Al_2O_3)的热力学稳定相，是最难凝结在演化恒星周围的尘埃之一。原始球粒陨石中的太阳前氧化铝在星周环境、星际介质(ISM)、太阳的母体分子云和原太阳盘中经受住了各种加工。日前氧化铝颗粒的形态和晶体结构可能反映了它们的形成和演化过程，但这两种过程的相对重要性却鲜为人知。在这项研究中，我们对从非平衡普通球粒陨石(Semarkona、Bishunpur和RC075)中提取的185个氧化铝颗粒进行了详细的形态观察。我们还进行了122个颗粒的电子背散射衍射分析和107个颗粒的氧同位素分析。通过刚玉和过渡氧化铝相的溶解实验，研究了球粒陨石的化学分离过程改变氧化铝颗粒表面结构的可能性，氧化铝颗粒的平均尺寸为1微米，没有观察到晶须和非常平坦的颗粒。约三分之一的颗粒表面光滑，而约60%的颗粒表面粗糙，具有10-100 nm的精细结构。粗糙表面颗粒具有不同的形貌和结晶度，表明粗糙表面结构是二次成因。95%的氧化铝颗粒与α-Al_2O_3(刚玉)匹配，75%以上的氧化铝颗粒为刚玉单晶。在107个氧化铝颗粒中发现了9个氧同位素组成异常的极前氧化铝颗粒，其中大部分具有粗糙的表面结构。虽然太阳光氧化铝颗粒以刚玉为主，但其无定形或低结晶度颗粒的相对丰度高于太阳光氧化铝颗粒。溶解实验表明，在球粒陨石的酸处理过程中，除刚玉外，其余物相均溶解。这表明刚玉颗粒的光滑表面结构最初是在空间中形成的，在ISM或原溶胶盘中被高能粒子辐照破坏的氧化铝在化学分离过程中失去了原始表面，形成了粗糙的表面结构，球粒陨石中的无定形或低结晶度的氧化铝颗粒具有不同于溶胶-凝胶合成的无定形氧化铝的耐酸结构。目前的结果还表明，球粒陨石中可能存在化学分离未发现的酸溶氧化铝相。(C)2013爱思唯尔有限公司。保留所有权利。
Corundum, the thermodynamically stable phase of alumina (Al2O3), is one of the most refractory dust species to condense around evolved stars. Presolar alumina in primitive chondrites has survived various kinds of processing in circumstellar environments, the interstellar medium (ISM), the Sun's parent molecular cloud, and the protosolar disk. The morphology and crystal structure of presolar alumina grains may reflect their formation and evolution processes, but the relative importance of these two types of processes is poorly understood. In this study, we performed detailed morphological observations of 185 alumina grains extracted from unequilibrated ordinary chondrites (Semarkona, Bishunpur, and RC075). We also performed electron back-scattered diffraction analyses of 122 grains and oxygen isotopic analyses of 107 grains. Dissolution experiments on corundum and transition alumina phases were carried out to examine the possibility of the alteration of surface structures of alumina grains by the chemical separation procedures of chondrites.The average size of the alumina grains was 1 mu m, and neither whiskers nor extremely flat grains were observed. About one-third of the grains had smooth surfaces, while similar to 60% of the grains had rough surfaces with 10-100 nm-sized fine structures. The rough-surface grains have varieties of morphology and crystallinity, suggesting that the rough surface structures are secondary in origin. Electron back-scattered diffraction patterns from 95% of alumina grains matched with alpha-Al2O3 (corundum), and more than 75% of the alumina grains are single crystals of corundum. Nine presolar alumina grains with anomalous oxygen isotopic compositions were found among 107 alumina grains, and most of them were characterized by rough surface structures. While most of the presolar alumina grains were corundum, the relative abundance of amorphous or low-crystallinity grains is higher in presolar alumina grains than in solar alumina grains. The dissolution experiments showed that all phases except for corundum dissolved during the acid treatments of chondrites. This suggests that smooth surface structures of corundum grains were originally formed in space, and that original surfaces of alumina that had been damaged by energetic particle irradiation in the ISM or the protosolar disk were lost during chemical separations to form the rough surface structures, and that amorphous or low-crystallinity alumina grains in chondrites have acid-resistant structures different from sol-gel-synthesized amorphous alumina. The present results also imply the possible presence of acid-soluble alumina phases, undiscovered by chemical separations, in chondrites. (C) 2013 Elsevier Ltd. All rights reserved.