Transformation of titanomagnetite to titanomaghemite: A slow, two-step, oxidation-ordering process in MORB
Transformation of titanomagnetite to titanomaghemite: A slow, two-step, oxidation-ordering process in MORB
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钛磁铁矿向钛磁赤铁矿的转化：MORB 中缓慢的两步氧化排序过程
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发表时间：
1997
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影响因子：
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通讯作者：
R. Beaubouef
中科院分区：
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作者：
Weixin Xu;D. Peacor;W. Dollase;R. Van der Voo;R. Beaubouef
摘要对大西洋海底一系列枕状玄武岩中的磁性氧化铁进行了研究，以表征钛磁赤铁矿和确定磁赤铁矿化过程。距离扩张脊的距离和样品的年龄（括号内）分别为0-10（0-1）、160（9）、450（26）和900 km（70 Ma）。通过TEM和AEM观察到，在所有样品中，铁钛氧化物以1至10 μ m大小的树枝状和十字形晶体存在，具有相同的外观，并且在组成或结构上没有变化或显著不均匀性的迹象。参数从最年轻到最老逐渐变化，例如，居里温度= 180至360 °C;晶格参数= 8.466至8.361欧姆; Rietveld精修得到的每个晶胞的八面体阳离子数= 14.8至12.1; Mössbauer数据得到的300 K下的平均超精细（内部）场= 37至45 T。大的Ti含量（UV 60至UV 70）几乎恒定。SAED图案仅显示最老样品的超结构反射。最年轻的样品具有对应于几乎未氧化的钛磁铁矿的参数，而最古老的是近端元钛磁赤铁矿。中间样品部分改变，但没有显示超结构反射，这意味着缺乏显着的有序的空缺。因此，数据表明（钛）磁化过程有两个明显不同的组成部分：（1）Fe的氧化和损失，并产生无序空位，以及（2）空位的有序化。这些数据共同暗示了一个由晶体中Fe的固态扩散、Fe的氧化和空位的产生主导的过程，其中O最密堆积的骨架被保留，这与添加O或溶解和新结晶的模型形成鲜明对比。
Abstract Magnetic iron oxides in a sequence of pillow basalts that were dredged from the Atlantic Ocean floor have been studied to characterize titanomaghemite and to define the processes of maghemitization. Distances from the spreading ridge and ages (in parentheses) of the samples are 0-10 (0-1), 160 (9), 450 (26), and 900 km (70 Ma). Iron titanium oxides occur as 1 to 10 μm-sized dendritic and cruciform-shaped crystals with identical appearances in all samples and with no signs of change or significant heterogeneity in composition or structure as observed by TEM and AEM. Parameters change progressively from the youngest to the oldest, e.g., Curie temperature = 180 to 360 °C; lattice parameter = 8.466 to 8.361 Å ; number of octahedral cations per cell from Rietveld refinement = 14.8 to 12.1; mean hyperfine (internal) fields at 300 K from Mössbauer data = 37 to 45 T. The large Ti contents (Uv60 to Uv70) are nearly constant. SAED patterns show superstructure reflections only for the oldest sample. The youngest sample has parameters corresponding to nearly unoxidized titanomagnetite, whereas the oldest is near-end-member titanomaghemite. Intermediate samples are partially altered but display no superstructure reflections, implying a lack of significant ordering of vacancies. The data therefore show that the process of (titano)maghemitization has two distinctly different components: (1) oxidation and loss of Fe, with creation of disordered vacancies, and (2) ordering of vacancies. The data collectively imply a process dominated by solid state diffusion of Fe from the crystals, oxidation of Fe, and creation of vacancies wherein the O closest-packed framework is preserved, in sharp contrast to a model of addition of O or to dissolution and neocrystallization.