Original paper

Petrogenetic significance of Na-K white mica mineralogy: Recent advances for metamorphic rocks

Guidotti, Charles V.; Sassi, Francesco P.

European Journal of Mineralogy Volume 10 Number 5 (1998), p. 815 - 854

151 references

published: Oct 5, 1998
manuscript accepted: Mar 6, 1998
manuscript received: Mar 21, 1997

DOI: 10.1127/ejm/10/5/0815

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Abstract 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 petrogenetic significance for understanding metamorphic rocks. Several aspects of these micas are considered in different sections: (1) Compositional space is discussed in terms of deviations from end member Ms and Pg formulas, as: (i) Ms- Pg substitution; (ii) celadonite substitutions; (Hi) a hypothetical "ferrimuscovite" substitution; (iv) margarite substitution; (v) deficiency in the XII site; (vi) occurrence of other minor or trace components which may have a petrological significance. (2) Analytical problems are discussed in terms of instrumental difficulties 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; (Hi) chemical implications for the solid solution behavior; (iv) crystal chemical effects of the minor and trace substitutions; (v) new crystal structure refinement work. (4) With regards to polytypism, 2M{ is the most common polytype in these micas, being typical of: (i) phengite-poor Ms over their whole P-T stability field; (ii) phengite-rich Ms formed at whatever T under low to medium P. 37 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 37 stacking, rather than the phengitic composition. (5) Petrological factors are considered which demonstrate how rock bulk composition controls mica chemistry. (6) The effect of the rock oxidation state on muscovite and paragonite chemistry is discussed: (i) redox state as indicated by the opaque minerals present is, for a given P-T-aAl2O3, the main control on the extent to which VIAl is replaced by Fe3+; and (ii) for a given P-T redox state, aAl2O3 (as indicated by the silicate mineral 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 geothermobarometry. (7,8) Geothermometry and geobarometry involving Ms and Pg are extensively discussed, with regards to petrologic rationale plus limits of validity; e.g. it is shown that the Ms-Pg geothermometer cannot be used at P > 0.8 GP. (9) A petrogenetic grid involving key AKNa reactions including Ms and/or Pg is discussed in order to show the petrologic effects of non-AKNa components substituting into the micas. (10) Rock-fluid interaction during metamorphism is discussed in order to show how the stability and chemistry of Na-K micas is also affected by the changing chemistry of the fluid phase.