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锆石柱面中Hf、Y的配位差异性及其对晶型的控制效应

周东山, 汪相   

  1. 南京大学 地球科学系, 江苏 南京 210093
  • 收稿日期:2000-06-20 修回日期:2000-06-20 出版日期:2000-06-20 发布日期:2000-06-20

Different Coordination between Hf4+ and Y3+ on Prismatic Faces of Zircon and Its Morphological Constraint

ZHOU Dong-shan, WANG Xiang   

  1. Department of Earth Sciences, Nanjing University, Nanjing 210093
  • Received:2000-06-20 Revised:2000-06-20 Online:2000-06-20 Published:2000-06-20

摘要: 天然锆石的形态并不简单地依照PBC理论发育,它同时受到生长温度 、熔体扩散系数等物理参数的影响,以及置换Zr的杂质离子的种类和浓度等化学因素的制约 ,即杂质离子选择性地置换Zr而降低晶面的生长速度。通过对{100}和{110}柱面的半定量分 析发现,Hf4+、Y3+离子同O2-离子的成键数目在{100}与{110}生长层 上是不同的,且Hf-O的键强比Zr-O的大,而Y-O的键强比Zr-O的小。如果假定晶体与岩浆熔体并未达到真正的平衡,而是各晶面与岩浆熔体分别达到平衡,按热力学中浓度与能量变化的指数律去处理Hf和Y在{100}与{110}晶面上的配分可以得到,{100}晶面上趋于富Hf贫 Y,{110}晶面上趋于富Y贫Hf,从而导致富Hf的锆石上{100}优先发育,富Y的锆石上{110}优 先发育。

Abstract: The morphology of naturally grown zircon does not simply confor mto Ha rtman's PBC theory. It is often affected by the physical parameters such as grow th temperature, diffusion coefficient etc. Furthermore, it is ubiquitously restr icted by the anisotropic growth retardarce as the result of the anisotropic subs titution of various ions. After semi-quantitative analysis on {100} and {110} p rism faces, it can be found that the number of chemical bonds between Hf4+ or Y3+ and O2-- on {100} face (n{100} ) differs from that of {110} face (n{110}), and that the streng th of Hf-O, Zr-O and Y-O declines orderly. Because the growth of zircon is ge nerally slow enough, it can be assumed that local equilibrium exists between magmaticmelt and various single crystals during crystallization of zircon. Using the general therm odynamic equation of energy and concentration, the concentration difference of H f4+ and Y3+ ions on {100} and {110} faces can be formulated as: Cx{110}/Cx{100} = A{100}/ A{110}exp[(n{100}-{110})/RT×(EX-O – EZr-O)/RT] where X refers to Hf4+ or Y3+ ions, Zr refers to Zr4+ ions, O refers to O2- ions, E refers to the energy of the chemical bonds between the subscript ions, C and n refer to the concentration. o f subst itutional ions and number of formed chemical bonds on the subscript crystal face respectively. A is related to growth mechanism of zircon, but its differenc e between {100} and {110} faces of zircon is generally small enough to be ignore d. Thus, the concentration difference is mainly determined by the difference in coordination structure of zircon crystal faces and the difference in energy of c orresponding chemical bonds.   When reference data of the energy for Hf-O, Zr-O and Y-O bonds are cited, it can be drawn that Hf tends to enrich itself on {100} face, while Y tends to enrich itself on {110} face. As the result of growth retardarce of the impurity ions, { 100} will be preferentially developed in Hf-rich ziron, while {110} will be pre f erentially developed in Y-rich zircon, which agrees well with previous statisti c s. Finally, as the anisotropic substitution of Hf and Y on {100} and {110} is de pendent on the temperature strongly and with a concise mathematical correlation, a potential geothermometer based on the above relation is proposed.