NONSTOICHIOMETRIC HYDROGEN CONTENTS IN COMMON ROCK-FORMING HYDROXYL SILICATES

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)mineral2+ --> (Fe3+, Al3+, 1/2Ti4+, etc.)mineral + 1/2[H-2]gas is extremely common. In biotites from metapelites in western Maine, this relationship can be expressed quantitatively as H+ = {-1.120 +/- 0.302 x (6 - [4]Si4+ + [6]Fe3+ + [6]Al3+ + [6]Ti4+ X 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.