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-b earing hydroxyl silicates. H+ and Fe3+ vary widely in the minerals stu died. In most cases, biotite, muscovite, and chlorite are deficient in H+ relative to the generally assumed stoichiometries. Results indicat e that oxysubstitution (or deprotonation) involving only H+ and Fe3+ e xchange is rare, although the related substitution of H(mineral)+ + (F e, Mg)mineral2+ --> (Fe3+, Al3+, 1/2Ti4+, etc.)mineral + 1/2[H-2]gas i s extremely common. In biotites from metapelites in western Maine, thi s relationship can be expressed quantitatively as H+ = {-1.120 +/- 0.3 02 x (6 - [4]Si4+ + [6]Fe3+ + [6]Al3+ + [6]Ti4+ X 2)} + 4.9006 +/- 0.2 547 for a 24-oxygen formula unit. Similar equations will be developed for other hydroxyl phases. This mechanism accounts for the great major ity of R3+ and R4+ substitution (apart from the Tshermak substitution) , reducing or eliminating the need to invoke vacancies for charge bala nce, and should also be considered in formulation of site substitution models, geothermobarometry, and any other applications dependent on f ull characterization of mineral compositions.