Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
J Struct Biol. 2012 Dec;180(3):458-68. doi: 10.1016/j.jsb.2012.09.004. Epub 2012 Sep 16.
{110} Twin densities in biotic aragonite have been estimated quantitatively from the peak widths of specific reflections in powder X-ray diffraction (XRD) patterns, as well as direct confirmation of the twins using transmission electron microscopy (TEM). Influence of the twin density on the peak widths in the XRD pattern was simulated using DIFFaX program, regarding (110) twin as interstratification of two types of aragonite unit layers with mirrored relationship. The simulation suggested that the twin density can be estimated from the difference of the peak widths between 111 and 021, or between 221 and 211 reflections. Biotic aragonite in the crossed-lamellar microstructure (three species) and nacreous microstructure (four species) of molluscan shells, fish otoliths (two species), and a coral were investigated. The XRD analyses indicated that aragonite crystals in the crossed-lamellar microstructure of the three species contain high density of the twins, which is consistent with the TEM examination. On the other hand, aragonite in the nacre of the four species showed almost no difference of the peak widths between the paired reflections, indicating low twin densities. The results for the fish otoliths were varied between the species. Such variation of the twin density in biotic aragonites may reflect different schemes of crystal growth in biomineralization.
生物文石中的孪晶密度可通过粉末 X 射线衍射 (XRD) 图谱中特定反射峰的半高宽定量估计，也可通过透射电子显微镜 (TEM) 直接确认孪晶。利用 DIFFaX 程序模拟了孪晶密度对 XRD 图谱中峰宽的影响，将（110）孪晶视为两种类型文石单元层的层间交错，具有镜像关系。模拟表明，可通过 111 和 021 或 221 和 211 反射之间的峰宽差异来估算孪晶密度。本研究对软体动物贝壳的交错层状微结构（3 种）和珍珠层状微结构（4 种）、鱼类耳石（2 种）和珊瑚中的生物文石进行了研究。XRD 分析表明，3 种贝壳交错层状微结构中的文石晶体含有高密度的孪晶，这与 TEM 检查结果一致。另一方面，4 种珍珠层中的文石几乎没有配对反射峰之间的峰宽差异，表明孪晶密度较低。鱼类耳石的结果在不同物种之间存在差异。生物文石中孪晶密度的这种变化可能反映了生物矿化中晶体生长方案的不同。
