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首页> 外文期刊>Contributions to Mineralogy and Petrology >Experimental growth of åkermanite reaction rims between wollastonite and monticellite: evidence for volume diffusion control
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Experimental growth of åkermanite reaction rims between wollastonite and monticellite: evidence for volume diffusion control

机译：硅灰石与蒙脱石之间的奥克锰矿反应边缘的实验生长：控制体积扩散的证据

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摘要

Growth rates of monomineralic, polycrystalline åkermanite (Ca2MgSi2O7) rims produced by solid-state reactions between monticellite (CaMgSiO4) and wollastonite (CaSiO3) single crystals were determined at 0.5 GPa dry argon pressure, 1,000–1,200°C and 5 min to 60 h, using an internally heated pressure vessel. Inert Pt-markers, initially placed at the monticellite–wollastonite interface, indicate symmetrical growth into both directions. This and mass balance considerations demonstrate that rim growth is controlled by transport of MgO. At 1,200°C and run durations between 5 min and 60 h, rim growth follows a parabolic rate law with rim widths ranging from 0.4 to 16.3 μm indicating diffusion-controlled rim growth. The effective bulk diffusion coefficient Dtexteff,MgOtextAk D_{text{eff,MgO}}^{text{Ak}} is calculated to 10−15.8±0.1 m2 s−1. Between 1,000°C and 1,200°C, the effective bulk diffusion coefficient follows an Arrhenius law with E a = 204 ± 18 kJ/mol and D 0 = 10−8.6±1.6 m2 s−1. Åkermanite grains display a palisade texture with elongation perpendicular to the reaction interface. At 1,200°C, average grain widths measured normal to elongation, increase with the square root of time and range from 0.4 to 5.4 μm leading to a successive decrease in the grain boundary area fraction, which, however, does not affect Dtexteff,MgOtextAk D_{text{eff,MgO}}^{text{Ak}} to a detectible extent. This implies that grain boundary diffusion only accounts for a minor fraction of the overall chemical mass transfer, and rim growth is essentially controlled by volume diffusion. This is corroborated by the agreement between our estimates of the effective MgO bulk diffusion coefficient and experimentally determined volume diffusion data for Mg and O in åkermanite from the literature. There is sharp contrast to the MgO–SiO2 binary system, where grain boundary diffusion controls rim growth.
机译：蒙脱石（CaMgSiO 使用内部加热的压力容器，在0.5 GPa干氩压力，1,000–1,200°C和5分钟至60小时的条件下，测定了4 和硅灰石（CaSiO 3 ）单晶。惰性Pt标记最初位于蒙脱石-硅灰石界面，指示在两个方向上对称生长。这种和质量平衡的考虑表明，轮辋的生长受MgO的运输控制。在1,200°C且运行时间为5分钟至60小时之间，轮辋的生长遵循抛物线速率定律，轮辋宽度在0.4到16.3μm之间，表明扩散控制了轮辋的生长。有效体扩散系数D texteff，MgO textAk D_ {text {eff，MgO}} ^ {text {Ak}}计算为10 -15.8± 0.1 m 2 s -1 。在1,000°C和1,200°C之间，有效的体扩散系数遵循阿伦尼乌斯定律，E a = 204±18 kJ / mol，D 0 = 10 −8.6±1.6 m 2 s -1 。 Åmanmanite晶粒显示出栅状纹理，且伸长率垂直于反应界面。在1200°C下，垂直于伸长率测得的平均晶粒宽度随时间的平方根增加，范围从0.4到5.4μm，导致晶界面积分数连续减小，但这并不影响D texteff，MgO textAk D_ {text {eff，MgO}} ^ {text {Ak}}在可检测的范围内。这意味着晶界扩散仅占整个化学物质传递的一小部分，而边缘的生长基本上受体积扩散的控制。我们对有效MgO体积扩散系数的估算与文献中实验确定的åkermanite中Mg和O的体积扩散数据之间的一致性证明了这一点。与MgO–SiO 2 二元体系形成鲜明对比，后者的晶界扩散控制边缘生长。

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