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Title:
``Pseudosinhalite'', a new hydrous MgAl-borate: synthesis, phase characterization, crystal structure, and PT-stability
Authors:
Daniels, Peter; Krosse, Sigrid; Werding, Günter; Schreyer, Werner
Publication:
Contributions to Mineralogy and Petrology, Volume 128, Issue 2/3, pp. 261-271 (1997). (CoMP Homepage)
Publication Date:
00/1997
Origin:
SPRINGER
Abstract Copyright:
Springer-Verlag Berlin Heidelberg 1997
DOI:
10.1007/s004100050307
Bibliographic Code:
1997CoMP..128..261D

Abstract

The new synthetic phase Mg2Al3O[BO4]2(OH) provisionally named ``pseudosinhalite'' is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO4], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4-40kbar at temperatures that increase with pressure ( 650-->900°C), and it breaks down at higher temperatures to sinhalite+corundum+H2O. At P>=20kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a=7.455 (1) Å, b=4.330 (1) Å, c=9.825 (2) Å, β=110.68 (1)°, and space group P21/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5-O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800cm-1 and 3500cm-1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10kbar, 745°C and 35kbar, 950°C, giving a slope of about 8°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO2(OH).
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