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

机译：硅灰石和蒙脱石之间的钙钛矿反应边缘的实验生长：体积扩散控制的证据

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摘要

Growth rates of monomineralic, polycrystalline akermanite (Ca_2MgSi_2O_7) rims produced by solid-state reactions between monticellite (CaMgSiO_4) and wollastonite (CaSiO_3) single crystals were determined at 0.5 GPa dry argon pressure, 1,000–1,200℃ 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℃ 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 lm indicating diffusion-controlled rim growth. The effective bulk diffusion coefficient D_(eff,MgO)~(Ak) is calculated to 10~(-15.8±0.1) m~2 s~(-1). Between 1,000℃ and 1,200℃, the effective bulk diffusion coefficient follows an Arrhenius law with E_a = 204 ± 18 kJ/mol and D_0 = 10~(-8.6±1.6) m~2 s~(-1). Akermanite grains display a palisade texture with elongation perpendicular to the reaction interface. At 1,200℃, average grain widths measured normal to elongation, increase withthe square root of time and range from 0.4 to 5.4 lm leading to a successive decrease in the grain boundary area fraction, which, however, does not affect D_(eff,MgO)~(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 effectiveMgObulk diffusion coefficient and experimentally determined volume diffusion data for Mg and O in akermanite from the literature. There is sharp contrast to the MgO–SiO_2 binary system, where grain boundary diffusion controls rim growth.
机译：在0.5 GPa干氩压力，1,000–1,200℃和5分钟至60 h的条件下，确定了由蒙脱石（CaMgSiO_4）和硅灰石（CaSiO_3）单晶之间的固相反应产生的单矿物多晶钙钛矿（Ca_2MgSi_2O_7）边沿的生长速率内部加热的压力容器。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℃且运行时间为5分钟至60小时之间，轮辋生长遵循抛物线速率定律，轮辋宽度在0.4到16.3 lm之间，表明扩散受控制。有效体积扩散系数D_（eff，MgO）〜（Ak）计算为10〜（-15.8±0.1）m〜2 s〜（-1）。在1,000℃至1,200℃之间，有效体积扩散系数遵循Arrhenius定律，E_a = 204±18 kJ / mol，D_0 = 10〜（-8.6±1.6）m〜2 s〜（-1）。天青石晶粒显示出栅栏纹理，其伸长率垂直于反应界面。在1200℃下，平均晶粒宽度测量为垂直于伸长率，随时间的平方根增加，范围从0.4到5.4 lm，导致晶界面积分数连续减小，但不影响D_（eff，MgO） 〜（Ak）在可检测的程度。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由体积扩散控制。我们对有效MgObulk扩散系数的估计与文献中实验确定的钙钛矿中Mg和O的体积扩散数据之间的一致性证实了这一点。与MgO–SiO_2二元体系形成鲜明对比，后者的晶界扩散控制了边缘的生长。

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