In the humite homologous series, nM2SiO4 · M1-xTix(F,OH)2-2xO2x (where n = 1 for nor-bergite, 2 for chondrodite, 3 for humite, and 4 for clinohumite, M = Mg ≫ Fe > Mn > Ca,Zn,Ni and 0 ≤ x ≤ 1), x appears never to exceed ∼0.5 Ti atoms per formula unit, because local electrostatic charge imbalances at the 3-coordinated (F,OH,O) anion and the 4-coordi-nated oxygen atoms increase very rapidly as Ti4+ substitutes for M2+, even with the concomitant substitution of 3-coordinated O2− for (F,OH)1−. Both electrostatic and geometric arguments suggest that Ti orders into the M(F,OH)O “layer” of the humite structures, i.e., into the octahedron, which is the smallest of all octahedra in all the humite minerals (cf. Fujino and Takéuchi, 1978).
Refractive indices, density, unit-cell dimensions, and volume are dramatically affected by the substitution of OH for F, and this has been studied as chemistry varies from 1.0 ≤ (F,OHIII/[(F,OH)III + OIV] ≤ 0.0 and 0.0 ≤ SiIV/2(F,OH,O) ≤ 1/8 from sellaite Mg(F,OH)2(n = 0) through the humites to forsterite (n = ∞). The volume per anion increases much more rapidly as OH substitutes for F in these minerals than would be expected on the basis of observed differences in individual M-(F,OH) distances. As Yamamoto (1977) noted, the effective radius of OHIII would be ∼0.06A larger than that of FIII based on individual bond lengths. However, if the grand mean M-(F,OH,O) distance minus the effective radius of the M cation is used, OHIII would have to be ∼0.15A larger, and if the effective anion radius calculated from the observed volume per anion is used, values ∼0.10A larger are expected. Pro-ton-proton repulsions, which increase dramatically both within a given mineral series (say F-to OH-chondrodite) and as n goes from 4 to 0, expand the unoccupied polyhedra for the hep anion array very much more than the occupied octahedra; physical parameters are accordingly affected.