Reference:
▪ Sun, W., Huang, Ya-Xi, Li, Z., Pan, Y., Mi, J.-X. (2011): Hydrothermal synthesis and single-crystal X-ray structure refinement of three borates: sibirskite, parasibirskite and priceite.
Canadian Mineralogist,
49, 823-834.
Abstract:
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
P121/
c1 (no. 14), with
a 3.5624(7),
b 9.5225(19),
c 8.6231(17) Å, β 119.452(3)°,
V 254.72(9) Å3, and
Z = 4. Parasibirskite at 295 K crystallizes in space group
P121/
m1 (no. 11), with
a 6.6994(13),
b 5.4269(11),
c 3.5534(7) Å, β 93.048(11)°,
V 129.01(5) Å3, and
Z = 2. Priceite, (Ca2(H2O)[B5O7(OH)5]), at 173 K crystallizes in space group of
P121/
c1 (no. 14), with
a 11.580(3),
b 6.9844(19),
c 12.352(3) Å, β 110.573(9)°,
V 935.3(4) Å3, and
Z = 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.
Edited 1 time(s). Last edit at 07/29/2011 07:59AM by Marco E. Ciriotti.
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