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首页> 外文期刊>Contributions to Mineralogy and Petrology >Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)
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Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia)

机译：来自Mwinilunga（赞比亚）定向金红石/赤铁矿共生过程的全轴反应

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摘要

Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the < 010 >(R){101}(R)parallel to < 001 >(H){110}(H) orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact < 010 >(R){401}(R)parallel to < 001 >(H){170}(H). The intergrowth is dictated by the formation of {170}(H)vertical bar{401}(R) equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile-rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60 degrees and 120 degrees, (2) {101} twins with incidence angles of 114.44 degrees and their complementaries at 65.56 degrees, (3) {301} twins at 54.44 degrees with complementaries at 125.56 degrees and (4) low-angle tilt boundaries at 174.44 degrees and 5.56 degrees. Except for non-crystallographic contacts, all other rutile-rutile interfaces were confirmed in Mwinilunga samples. Using a HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian-ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier-Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form {301}(R)vertical bar{030}(H) equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian-ilmenite crystals was close to Ilm(67)Hem(33), typical for Fe-Tirich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at similar to 900 degrees C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian-ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO(2).
机译：通过电子显微镜方法研究了来自赞比亚姆维尼伦加的定向金红石/赤铁矿共生体，以解决垂直轴反应的复杂序列。标本由外延生长的网状金红石网络覆盖的高达几厘米大的正方赤铁矿晶体组成。按照自上而下的分析方法，从样品的宏观晶体学特征到亚纳米级相组成和发生界面的分析，对样品进行了研究。简单的形态分析已经表明，在平行于<001>（H）{110}（H）取向关系的<010>（R）{101}（R）附近遇到了金红石和赤铁矿。但是，使用电子衍射和高分辨率透射电子显微镜（HRTEM）对金红石/赤铁矿界面的更详细的结构分析表明，金红石与主体赤铁矿之间的实际关系实际上是<010>（R）{401}（ R）平行于<001>（H）{170}（H）。共生是由{170}（H）垂直棒{401}（R）平衡界面的形成决定的，该平衡界面导致赤铁矿基质中金红石析出的12个可能方向以及共生金红石晶体之间的144种不同入射。分析潜在的金红石-金红石界面，可以将其分为四类：（1）60度和120度的非晶体接触;（2）入射角为114.44度的{101}孪晶及其互补角为65.56度; （3）{301}以54.44度成对孪生，互补性为125.56度，（4）低角度倾斜边界为174.44度和5.56度。除了非晶体学接触，在Mwinilunga样品中还确认了所有其他金红石-金红石界面。使用HRTEM和高角度环形暗场扫描TEM方法结合能量色散X射线光谱法，我们在金红石析出物附近发现了钛铁矿片晶的残留物，这是钛铁矿高T形成的重要指示。初级铁锰铁钛矿晶体。在金红石晶体中观察到另一种类型的析出过程，其中赤铁矿沉淀物通过中间的Guinier-Preston区从金红石的富铁部分向铁素体轴向溶解，其特征是{101}金红石反射增加了三倍。与赤铁矿中的金红石析出物不同，金红石形式的{301}（R）垂直棒{030}（H）平衡界面的赤铁矿析出物。我们样品的总体组成表明，母铁素体-钛铁矿晶体中钛铁矿和赤铁矿的比例接近于Ilm（67）Hem（33），这是铁镁质岩浆铁钛矿特征的典型特征。钛铁矿片晶的存在表明主要固溶体在接近900摄氏度的条件下通过了混溶性间隙。随后的溶出过程是由亚铁的表面氧化和普通氧亚晶格内阳离子的迁移触发的。在对样品进行纳米结构分析的基础上，我们确定了三个连续的析出过程：（1）从初级铁素体-钛铁矿晶体中提取钛铁矿片晶;（2）从钛铁矿中提取金红石片晶;以及（3）从Fe-铁矿中提取赤铁矿沉淀物丰富的金红石片状。所有观察到的光轴反应似乎是温度和氧逸度fO（2）的组合函数。

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