Reaction kinetics of dolomite rim growth
Reaction kinetics of dolomite rim growth
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DOI：
10.1007/s00410-014-1001-y
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发表时间：
2014-04
影响因子：
3.5
通讯作者：
V. Helpa;E. Rybacki;R. Abart;L. Morales;D. Rhede;P. Jeřábek;G. Dresen
中科院分区：
文献类型：
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作者：
V. Helpa;E. Rybacki;R. Abart;L. Morales;D. Rhede;P. Jeřábek;G. Dresen
在400℃、750~850℃、3~146h的条件下，在定向方解石(CaCO3)和菱镁矿(MgCO3)单晶接触处进行固相反应制备了白云石(CaMG[CO3]2)反应轮缘，测定了反应动力学。白云石反应圈显示出两个不同的微结构域。细长的白云石栅栏垂直于碳酸镁界面生长，长度约为6~41微米，同时在与碳酸钙接触处生长出宽5~71微米的等轴粒状白云石边缘。铂标记显示，原始界面位于颗粒状白云岩和栅栏状白云岩之间。除白云石外，在白云石反应边缘和方解石单晶之间形成了12-80微米厚的镁方解石层。所有反应产物对方解石反应物至少表现出轴向晶体关系，而在粒状白云石和镁方解石中占主导地位的是方解石。白云石颗粒通常表现为围绕等效轴之一旋转180°的生长孪晶。从物质平衡的角度推测，白云石的反应边是由氧化镁和氧化钙的反向扩散形成的。假设CaO和MgO的扩散活化能与温度呈Arrhenius型关系，则CaO和MgO的扩散活化能分别为Ea(CaO)x=1192±954 kJ/mol和a(MgO)x=1198±244 kJ/mol。
Reaction rims of dolomite (CaMg[CO3]2) were produced by solid-state reactions at the contacts of oriented calcite (CaCO3) and magnesite (MgCO3) single crystals at 400 MPa pressure, 750–850 °C temperature, and 3–146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the MgCO3interface with length ranging from about 6 to 41 µm. At the same time, a 5–71 µm wide rim of equiaxed granular dolomite grew at the contact with CaCO3. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12–80 µm thick magnesio-calcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of theequivalent axis. From mass balance considerations, it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius-type temperature dependence, activation energies for diffusion of CaO and MgO areEa(CaO) = 192 ± 54 kJ/mol andEa(MgO) = 198 ± 44 kJ/mol, respectively.