PETROGENETIC SIGNIFICANCE OF NA-K WHITE MICA MINERALOGY - RECENT ADVANCES FOR METAMORPHIC ROCKS

In the last ten years much new work has been done on the petrogenetic significance of the Na-K white micas in metamorphic rock, both by the authors, and by numerous others. This paper reviews these new results. Special emphasis is given to the new advances on the Pg-Ms mineralogy and phase relations which the authors perceive as having direct petro genetic significance for understanding metamorphic rocks. Several aspe cts of these micas are considered in different sections: (1) Compositi onal space is discussed in terms of deviations from end member Ms and Pg formulas, as: (i) Ms-Pg substitution; (ii) celadonite substitutions ; (iii) a hypothetical ''ferrimuscovite'' substitution; (iv) margarite substitution; (v) deficiency in the XII site; (vi) occurrence of othe r minor or trace components which may have a petrological significance . (2) Analytical problems are discussed in terms of instrumental diffi culties and recognition of poor quality analyses. (3) Some aspects of the Na-K mica crystal chemistry are briefly considered: (i) effects of compositional variation on cell dimensions; (ii) explanations for the observed relationships; (iii) chemical implications for the solid sol ution behavior; (iv) crystal chemical effects of the minor and trace s ubstitutions; (v) new crystal structure refinement work. (4) With rega rds to polytypism, 2M(1) is the most common polytype in these micas, b eing typical of: (i) phengite-poor Ms over their whole P-T stability f ield; (ii) phengite-rich Ms formed at whatever T under low to medium P . 3T is less common, and typical of phengites from high P/T terrains. A high P/T ratio seems to be the necessary condition for the 3T stacki ng, rather than the phengitic composition. (5) Petrological factors ar e considered which demonstrate how rock bulk composition controls mica chemistry. (6) The effect of the rock oxidation state on muscovite an d paragonite chemistry is discussed: (i) redox state as indicated by t he opaque minerals present is, for a given P-T-aAl(2)O(3), the main co ntrol on the extent to which Al-VI is replaced by Fe3+; and (ii) for a given P-T redox state, aAl(2)O(3) (as indicated by the silicate miner al assemblage) is the prime control on the extent of this replacement. Ignoring Fe3+ in Ms causes significant underestimations of the Mg/Fe2 + ratio in Ms, with serious consequences for cation exchange geothermo barometry. (7,8) Geothermometry and geobarometry involving Ms and Pg a re extensively discussed, with regards to petrologic rationale plus li mits of validity; e.g. it is shown that the Ms-Pg geothermometer canno t be used at P > 0.8 GP. (9) A petrogenetic grid involving key AKNa re actions including Ms and/or Pg is discussed in order to show the petro logic effects of non-AKNa components substituting into the micas. (10) Rock-fluid interaction during metamorphism is discussed in order to s how how the stability and chemistry of Na-K micas is also affected by the changing chemistry of the fluid phase.