Abstract
Energy calculations were used to locate H+ in selected humite minerals: norbergite, OH-end-member chondrodite, magnesian alleghanyite, titanian chondrodite, and titanian clinohumite. The calculations involved a search for the position of lowest energy for H+ in a cluster of up to 27 neighboring atoms. The influence on the H+ position of the exchanges F(OH)-1 and TiO2M-1(OH)-2 was explored by performing calculations on models with different local distributions of Ti4+, M2+, O2-, F-, and neighboring H+.
In all but one of 18 models, two energy minima were found near the hydroxyl oxygen. The calculated minima, 35 in all, are located in two clusters, corresponding to two kinds of observed H positions, H1 sites (Yamamoto, 1977; Fujino and Takéuchi, 1978) and H2 sites (Yamamoto, 1977). Calculated O-H distances are consistent with, or more realistic than, observed distances in OH-end-member chondrodite (Yamamoto, 1977) and titanian clinohumite (Fujino and Takéuchi, 1978).
Several conclusions may be drawn: (1) Energy calculations give realistic H positions for humite minerals. (2) Primary Hl-type energy minima were found only for models with hydroxyls on adjacent anion sites (i.e., neither Ti nor F in the local structural environment). Furthernore, when hydroxyl oxygens are on adjacent anion sites, one H is at an H2 site, and the other is at an Hl site. (3) Ti substitution favors H2 sites for hydrogens of nearby hydroxyls. (4) F substitution also favors H2 sites for hydrogens of nearby hydroxyls. (5) Structural distortions due to Mn substitution in chondrodite (e.9., magnesian alleghanyite) seem to have little or no special influence on the H positions.
