针铁矿-四方纤铁矿-水体系氧同位素分馏的实验研究  

An experimental study of oxygen isotope fractionations in goethite akaganeite water systems at low temperatures

作　　者：徐宝龙[1] 周根陶[1] 郑永飞[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,安徽合肥230026

出　　处：《地球化学》2002年第4期366-374,共9页Geochimica

基　　金：国家自然科学基金(49903001;40033010)

摘　　要：针铁矿是非常重要的三价铁氧化物之一,其氧同位素组成对于古环境再造具有很大的价值。以4种不同的铁化合物作为Fe3+离子的源物质,于30～120℃范围内,采用强迫水解方法,在不同同位素组成的水中分别实验合成针铁矿和四方纤铁矿。结果表明,以Fe(NO3)3·9H2O、NH4Fe(SO4)2·12H2O、Fe(SO4)3·7H2O为Fe3+源物质合成的是纯针铁矿,而以FeCl3·6H2O为Fe3+源物质合成的是四方纤铁矿。氧同位素分析显示,在30～120℃范围内实验测定的针铁矿-水体系和四方纤铁矿-水体系氧同位素分馏几乎不可区分,并且满足下列分馏关系:103lnα针铁矿-水=9.59×103/T-26.39103lnα四方纤铁矿-水=8.85×103/T-24.44实验测定的针铁矿-水体系氧同位素分馏不仅与前人实验结果一致,而且与增量方法理论计算相近。由于实验采用不同反应途径得到了一致的分馏结果,因此所测定的针铁矿-水体系氧同位素分馏代表了热力学平衡。Goethite is one of the most common naturally occurring iron (III) oxides and its oxygen isotope composition is of great value to reestablishment of paleoenvironmental information. Goethite and akaganeite are synthesized by the forced hydrolysis method in the temperature range of 30 to 120 ℃using four different kinds of ferric compounds and deionized water with different oxygen isotope composition as initial materials. As a consequence, goethite is obtained by the forced hydrolysis of Fe(NO3)3, NH4Fe(SO4)2, or Fe(SO4)3 solution, and akaganeite by the forced hydrolysis of FeCl3 solution. Oxygen isotope analyses show that there is negligible difference in oxygen isotope fractionation between goethite and akaganeite at the run temperatures, and the oxygen isotope fractionations in the systems of goethite water and akaganeite water fit the following equations, respectively:103 lnαgoethite water=9.59×103/T-26.39 103 lnαakaganeite water=8.85×103/T-24.44The experimentally determined oxygen isotope fractionations between goethite and water are not only in agreement with the experimental determinations of Yapp (1987), but also close to the theoretical calculations of Zheng (1998) by the increment method. The same fractionations between goethite and water are obtained from the different reaction paths in the present experiments, and the resultant fractionation equation for the goethite water system is interpreted to represent the thermodynamic equilibrium.

关 键 词：针铁矿 四方纤铁矿 强迫水解 氧同位素 分馏 三价铁 古环境 

分 类 号：P578.497[天文地球—矿物学] P597[天文地球—地质学]