HYDROTHERMAL SYNTHESIS AND SINGLE-CRYSTAL X-RAY STRUCTURE REFIN

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• journal_title：The Canadian Mineralogist
• Contributor：Wei Sun ; Ya-Xi Huang ; Zucheng Li ; Yuanming Pan ; Jin-Xiao Mi
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.3.823
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：3
• firstpage：823
• section：Articles

摘要

<p id="p-1">Sibirskite, parasibirskite and priceite, synthesized in aqueous solutions at temperatures from 80 to 240°C, have been investigated by single-crystal X-ray refinements of the structure. Sibirskite and parasibirskite are dimorphs of CaHBO₃. Sibirskite at 295 K crystallizes in space group m>Pm>12₁m>/cm>1 (no. 14), with m>am> 3.5624(7), m>bm> 9.5225(19), m>cm> 8.6231(17) Å, β 119.452(3)°, m>Vm> 254.72(9) Åp>3p>, and m>Zm> = 4. Parasibirskite at 295 K crystallizes in space group m>Pm>12₁m>/mm>1 (no. 11), with m>am> 6.6994(13), m>bm> 5.4269(11), m>cm> 3.5534(7) Å, β 93.048(11)°, m>Vm> 129.01(5) Åp>3p>, and m>Zm> = 2. Priceite, (Ca₂(H₂O)[B₅O₇(OH)₅]), at 173 K crystallizes in space group of m>Pm>12₁m>/ cm>1 (no. 14), with m>am> 11.580(3), m>bm> 6.9844(19), m>cm> 12.352(3) Å, β 110.573(9)°, m>Vm> 935.3(4) Åp>3p>, and m>Zm> = 4. Our refinements allow determination of the positions of boron and hydrogen atoms in these minerals, which was not possible in previous powder XRD studies. In particular, the isolated [BO₂(OH)] groups in sibirskite and parasibirskite are characterized by a long B–OH bond and two short B–O bonds, different from the [BO₃] and [B(OH₃)] groups in nesoborates. Periodic density functional theory (DFT) calculations for sibirskite and parasibirskite support those results of structure refinements and suggest that parasibirskite undergoes a displacive phase-transition involving hydrogen ordering below 173 K. Sibirskite and parasibirskite left in their parental solutions at ambient conditions decompose to form calcite. p>