Rustad James R, Zarzycki Piotr
Department of Geology, University of California, One Shields Avenue, Davis, CA 95616-8605, USA.
Proc Natl Acad Sci U S A. 2008 Jul 29;105(30):10297-301. doi: 10.1073/pnas.0801571105. Epub 2008 Jul 18.
Ab initio molecular dynamics and quantum chemistry techniques are used to calculate the structure, vibrational frequencies, and carbon-isotope fractionation factors of the carbon dioxide component [CO(2)(m)] of soil (oxy)hydroxide minerals goethite, diaspore, and gibbsite. We have identified two possible pathways of incorporation of CO(2)(m) into (oxy)hydroxide crystal structures: one in which the C(4+) substitutes for four H(+) [CO(2)(m)(A)] and another in which C(4+) substitutes for (Al(3+),Fe(3+)) + H(+) [CO(2)(m)(B)]. Calculations of isotope fractionation factors give large differences between the two structures, with the CO(2)(m)(A) being isotopically lighter than CO(2)(m)(B) by approximately 10 per mil in the case of gibbsite and nearly 20 per mil in the case of goethite. The reduced partition function ratio of CO(2)(m)(B) structure in goethite differs from CO(2)(g) by <1 per mil. The predicted fractionation for gibbsite is >10 per mil higher, close to those measured for calcite and aragonite. The surprisingly large difference in the carbon-isotope fractionation factor between the CO(2)(m)(A) and CO(2)(m)(B) structures within a given mineral suggests that the isotopic signatures of soil (oxy)hydroxide could be heterogeneous.
采用从头算分子动力学和量子化学技术，计算土壤（羟基）氧化物矿物针铁矿、水铝石和三水铝石中二氧化碳组分[CO(2)(m)]的结构、振动频率和碳同位素分馏因子。我们确定了CO(2)(m)纳入（羟基）氧化物晶体结构的两种可能途径：一种是C(4+)替代四个H(+) [CO(2)(m)(A)]，另一种是C(4+)替代(Al(3+),Fe(3+)) + H(+) [CO(2)(m)(B)]。同位素分馏因子的计算结果表明，两种结构之间存在很大差异，对于三水铝石，CO(2)(m)(A)的同位素比CO(2)(m)(B)轻约10‰，对于针铁矿，差异接近20‰。针铁矿中CO(2)(m)(B)结构的约化配分函数比与CO(2)(g)的差异小于1‰。三水铝石的预测分馏比高>10‰，接近方解石和文石的测量值。给定矿物中CO(2)(m)(A)和CO(2)(m)(B)结构之间碳同位素分馏因子的惊人差异表明，土壤（羟基）氧化物的同位素特征可能是不均匀的。
