CATALYTIC EFFECT OF BIOTITE - FORMATION OF HYDROGARNET LENSES

Hydrogarnet lenses and lamellae occur parallel to the basal cleavage o f biotites in a garnet-amphibolite (Bamble, Norway), and a biotite-hor nblende granitic gneiss (Rondonia, Brazil). Neither chemical nor textu ral evidence indicate that the hydrogarnet lenses grew at the expense of their host biotite. Host biotites show a small increase in Fe2+/(Fe 2+ + Fe3+) ratios toward the hydrogarnet lenses. It is proposed that h ydrogarnet lenses formed due to the catalytic effect of the biotite; t hey likely precipitated where the amount of Al(IV) in biotite has a lo cal maximum with respect to the amount of tri- and tetravalent cations at the octahedral site, resulting in increased adsorption of H+ to th e biotite surface, and consequently decreasing of the pH of the inters titial fluid, causing the precipitation of minerals between the biotit e basal cleavage.