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<p>Topaz-OH phases, including structurally distinct but chemically identical topaz-OH I and topaz-OH II, are considered as potential water carriers in hydrated subducted sediments. In this study, Fe-bearing topaz-OH I was synthesized at 15.5 GPa and 1400&#xa0;°C, with a chemical composition of Al<sub>1.94±0.05</sub>Fe<sub>0.18±0.01</sub>Si<sub>0.69±0.02</sub>O<sub>6</sub>H<sub>2.88±0.13</sub>. Its crystal structure is similar to that of the ordered Al-endmember topaz-OH I (Al<sub>2</sub>SiO<sub>6</sub>H<sub>2</sub>), with both exhibiting the same <i>Pbnm</i> space group and comparable unit cell parameters. However, this structure contains evident vacancies in the tetrahedral sites and new occupied octahedral sites that are normally vacant in the ordered Al-endmember topaz-OH I. Our structural model represents a transitional phase between the Al/Si-ordered topaz-OH I and the fully Al/Si-disordered topaz-OH II. These findings suggest that structural disorder may play a key role in influencing the tetrahedral-to-octahedral occupancy ratio, the evolution of crystal structures, and the thermal stability of topaz-OH under the high pressure-temperature conditions of the deep mantle.</p>

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Structural disorder in Fe-bearing topaz-OH revealed by single-crystal X-ray diffraction

  • 期刊论文

  • 发表日期：

    2026年3月13日

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  • Baoyun Wang,
  • Meisu Xiang

摘要

Topaz-OH phases, including structurally distinct but chemically identical topaz-OH I and topaz-OH II, are considered as potential water carriers in hydrated subducted sediments. In this study, Fe-bearing topaz-OH I was synthesized at 15.5 GPa and 1400 °C, with a chemical composition of Al1.94±0.05Fe0.18±0.01Si0.69±0.02O6H2.88±0.13. Its crystal structure is similar to that of the ordered Al-endmember topaz-OH I (Al2SiO6H2), with both exhibiting the same Pbnm space group and comparable unit cell parameters. However, this structure contains evident vacancies in the tetrahedral sites and new occupied octahedral sites that are normally vacant in the ordered Al-endmember topaz-OH I. Our structural model represents a transitional phase between the Al/Si-ordered topaz-OH I and the fully Al/Si-disordered topaz-OH II. These findings suggest that structural disorder may play a key role in influencing the tetrahedral-to-octahedral occupancy ratio, the evolution of crystal structures, and the thermal stability of topaz-OH under the high pressure-temperature conditions of the deep mantle.