Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet: Structural dynamic and thermodynamic properties
Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet: Structural dynamic and thermodynamic properties
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DOI：
10.2138/am-1997-7-811
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发表时间：
1997-08
影响因子：
3.1
通讯作者：
C. Geiger;T. Armbruster
中科院分区：
文献类型：
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作者：
C. Geiger;T. Armbruster
用单晶X射线衍射方法在100 K、293 K和500-550 K下对合成的锰铝硅榴石和钙铝硅榴石的结构进行了研究。二价的X-网站的阳离子，位于大的十二面体网站，显示可测量的各向异性的动态无序相反的刚性振动行为的SiO 4四面体和AlO 6八面体。锰铝榴石中Mn ~（2+）的振动幅度与铁铝榴石中Fe ~（2+）的振动幅度相似，在较长的X-O（4）键平面上，两者都是钙铝榴石中Ca ~（2+）振动幅度的两倍，尽管钙铝榴石的质量较轻。在300和1000 K之间测量的合成多晶spessartine和两个自然的近端员spessartine晶体的热容量是类似的铁铝榴石。此外，spessartine在低频下的红外活性模式是非常相似的铁铝榴石，这表明它们的热容也是相似的，在较低的温度。镁铝榴石和钙铝榴石的低能声子谱可能与这两种含过渡金属石榴石的低能声子谱有很大的不同，这可能是由于它们的低频红外激活模式不同，反映了镁和钙在石榴石中的不同成键性质。相对于其他铝硅酸盐石榴石，spessartine的大的压力-温度稳定性场似乎不是由于任何形式的内在熵稳定。
Abstract The structures of synthetic Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet have been refined using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measurable anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO4 tetrahedra and AlO6 octahedra. The amplitudes of vibration of Mn2+ in spessartine are similar to those of Fe2+ of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of Ca2+ in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic poly crystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low frequency IR active modes, reflecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability field of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization.