Growth mechanisms, structural defects and composition of synthetic tremolite: what are the effects on macroscopic properties?
Growth mechanisms, structural defects and composition of synthetic tremolite: what are the effects on macroscopic properties?
复制标题
合成透闪石的生长机制、结构缺陷和成分：对宏观性能有何影响？
DOI：
10.1007/bf00306649
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发表时间：
1994
影响因子：
3.5
通讯作者：
W. Schreyer
中科院分区：
文献类型：
--
作者：
W. Maresch;M. Czank;W. Schreyer
在1-22kbar和600-875℃的压力/温度范围内，用常规的水热方法将块体成分为Ca2Mg5Si8O23的无定形凝胶和氧化物与含透闪闪石的相组合反应，并用X射线衍射、光学显微镜和高分辨电子显微镜对产物进行了表征。角闪石微观结构的性质和共生生物沸石的类型随着合成温度和压力的变化而明显不同，反映了不同的角闪石生长机制。在P(H2O)<；10kbar，在Fale的稳定区内或非常接近Fale的稳定区，这种亚稳结晶的中间相通过拓扑定向反应形成角闪石；m≥3的链多重数缺陷(CMF)很多；无序角闪石晶体中的整体成分移动在运行产物中被共存的透辉石补偿。在P(H2O)≥10kbar，但仍在滑石稳定区，角闪石通过溶解/再生长过程在介稳透辉石上局部生长并优先成核；两相形成细小(≈100？)的片层互生生长，几乎没有与m≥3的Cmf，且由装甲透辉石残留物引起的成分移动被离散的滑石补偿。在滑石稳定场以上的温度和所研究的所有压力下，观察到相同的片状角闪石/透辉石微结构，但成分漂移被顽火辉石+石英补偿。改变实验参数(运行时间；水含量)不会对上述观察结果产生显著影响。对于所研究的广泛的压力和温度范围，本研究的合成角闪石似乎与端元透闪石非常接近。观察到的角闪石的产物组合和表观成分位移，其钙镁比相当于高达11摩尔%的镁铝榴石组分，可以通过在该相中掺入CMF来解释。我们认为，CMF引起的钙/镁比的移动对目前文献中类似实验中通常假设的“10mol%镁铝榴石组分”也起着重要作用。真正的镁砂固溶体组分可能远低于10摩尔%。晶体端部择优腐蚀结构的经验高分辨电子显微镜观察表明，从第一个近似值来看，结构无序的透闪石表现出机械混合物的反应行为，细至晶胞尺度。因此，合成透闪石的热力学性质，即使是无序晶体中相互生长的小片层，也应该非常接近于离散的、理想的透闪石单晶的热力学性质，这与最近对天然和合成透闪石相关系的对比实验研究所显示的趋同一致。
Amorphous gels and oxides corresponding to Ca2Mg5Si8O23 in bulk composition have been reacted to phase assemblages containing tremolitic amphibole using routine hydrothermal methods in the pressure/temperature range from 1–22 kbar and 600–875° C. The products have been characterized by X-ray diffraction, optical microscopy and high-resolution electron microscopy. The nature of the amphibole microstructure and the types of intergrown biopyriboles vary markedly as a function of synthesis temperature and pressure, reflecting different amphibole growth mechanisms. At P(H2O)<10 kbar, within or very near the stability field of fale, amphibole forms by topotactic reaction from this metastably crystallized, intermediate phase; chain multiplicity faults (CMFs) with m≥3 are numerous; the bulk compositional shift in the disordered amphibole crystals is compensated in the run product by coexisting diopside. At P(H2O)≥10 kbar, but still in the talc stability field, amphibole grows topotactically from and also nucleates preferentially on metastable diopside by a dissolution/regrowth process; the two phases form fine (≈ 100 Å) lamellar intergrowths with almost no CMFs with m≥3, and the compositional shift induced by armoured diopside relics is compensated by discrete talc. The same lamellar amphibole/diopside microstructure is observed at temperatures above the talc stability field and at all pressures investigated, but the compositional shift is compensated by enstatite+quartz. Varying the experimental parameters (run duration; H2O content) does not significantly affect the above observations. For the wide range of pressures and temperatures investigated, the synthetic amphiboles of the present study appear to correspond very closely to end-member tremolite. The observed product assemblages and apparent compositional shifts of the amphibole, equivalent in Ca/Mg ratio to as much as 11 mol% magnesiocummingtonite component, can be explained by the incorporation of CMFs in this phase. We argue that CMF-induced shifts in Ca/Mg ratios also play a significant role for the “10 mol% magnesiocummingtonite component” commonly assumed in analogous experiments in the present literature. The genuine magnesiocummingtonite solid-solution component may be much less than 10 mol%. Empirical high-resolution transmission electron microscopy observations of preferential corrosion structures at crystal terminations suggest that, to a first approximation, structurally disordered tremolites exhibit the reaction behaviour of mechanical mixtures down to the unit-cell scale. The thermodynamic properties of synthetic tremolite, even in small intergrown lamellae within disordered crystals, should therefore closely approach those of discrete, ideal tremolite single crystals, in accord with the converging agreement shown by recent comparative experimental studies on the phase relationships of natural and synthetic tremolite.