Journal
PHYSICS AND CHEMISTRY OF MINERALS
Volume 30, Issue 1, Pages 1-11Publisher
SPRINGER-VERLAG
DOI: 10.1007/s00269-002-0288-9
Keywords
hydrous olivine; water in nominally anhydrous minerals; hydrogen bonds; Ab initio; quantum mechanics structure refinement; DHMS phases
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Recently, the Hy-2a hydrous olivine (MgH2 SiO4).3(Mg2SiO4) occurring as nanometre-sized inclusions in mantle olivines has been found by TEM, and has been suggested to be a new DHMS phase (Khisina et al. 2001). A model of the crystal structure of Hy-2a has been proposed as a 2a-superstructure of olivine with one Me2+-vacant octahedral layer in the (1 0 0) plane per Hy-2a unit cell (Khisina and Wirth 2002). In the present study the crystal structure of Hy-2a hydrous olivine is optimized by ab initio calculations. The aims of this study are: (1) verification of the suggested models of Hy-2a hydrous olivine structure; (2) calculation of the most stable configurations. for Hy-2a structure with minimum static lattice energy, by assuming a possible formation of Me2+ vacancies in either M1 or M2 octahedral sites; (3) determination of the position of protons and hydrogen bonds in the Hy-2a structure. Several different possible configurations of the Hy-2a structure are optimized. The results support the idea of a stable olivine structure with ordered planar-segregated OH-bearing defects oriented parallel to (1 0 0). The data obtained indicate a preferred stability of the Hy-2a structure with the protons associated with M1 vacancies and bonded with O1 and O-2 oxygen sites. The relative energy values of the optimized Hy-2a structure configurations correlate as a rule with the average shifts of atoms from their positions in pure forsterite structure.
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