Nonstoichiometric hydrogen contents in common rock-forming hydroxyl silicates

Abstract

H⁺ and Fe³⁺ have been analyzed in conjunction with electron microprobe studies of over 500 petrologically well-constrained rock-forming Fe-bearing hydroxyl silicates. H⁺ and Fe³⁺ vary widely in the minerals studied. In most cases, biotite, muscovite, and chlorite are deficient in H⁺ relative to the generally assumed stoichiometries. Results indicate that oxysubstitution (or deprotonation) involving only H⁺ and Fe³⁺ exchange is rare, although the related substitution of H⁺_mineral (Fe, Mg)²⁺_mineral → (Fe³⁺, A1³⁺, $\text{1}\text{2}\text{Ti}{}^{\mathrm{4+}}$, etc.)_mincral + $\text{1}\text{2}\text{[H}{}_2\text{]}{}_{\mathrm{gas}}$ is extremely common. In biotites from metapelites in western Maine, this relationship can be expressed quantitatively as $\text{H}{}^{\mathrm{+}}\text{= −1.120 ± 0.302 × (6 −}{}^4\text{Si}{}^{\mathrm{4+}}\text{+}{}^6\text{Fe}{}^{\mathrm{3+}}$ + ⁶A1³⁺ + ⁶Ti⁴⁺ × 2)× + 4.9006 ± 0.2547 ⁶A1³⁺ + ⁶Ti⁴⁺ × 2)× + 4.9006 ± 0.2547 for a 24-oxygen formula unit. Similar equations will be developed for other hydroxyl phases. This mechanism accounts for the great majority of R³⁺ and R⁴⁺ substitution (apart from the Tshermak substitution), reducing or eliminating the need to invoke vacancies for charge balance and should also be considered in formulation of site substitution models, geothermobarometry and any other applications dependent on full characterization of mineral compositions.