Giovanna Giorgetti á Peter Tropper á Eric J. Essene Donald R. Peacor Characterization of non-equilibrium and equilibrium occurrences of paragonite/muscovite intergrowths in an eclogite from the Sesia±Lanzo Zone (Western Alps, Italy) Received: 3 February 1999 / Accepted: 19 October 1999 Abstract Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia±Lanzo Zone (Western Alps, Italy). The P-T conditions of this eclogite reached 570±650 °C and 19±21 kbar and the rocks show several stages of mineral growth during their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these rocks and show two texturally dierent occur- rences indicating equilibrium and non-equilibrium states between them. In one mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite  albite also replaced paragonite dur- ing a greenschist-facies overprint, as evidenced by un- ique across ± (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite ± paragonite solvus, whereas the compositions of the greenschist-facies micas lie outside the solvus and indicate disequilibrium. The TEM obser- vations of the textural relations of the greenschist-facies micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary. We pro- pose that muscovite formed through a dissolution and recrystallization process, since no gradual variations toward the muscovite ± paragonite interfaces occur and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diusion mechanism is highly unlikely at these low temperatures (300±500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H 2 O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas, which in turn may lead to misin- terpretations of thermometric results. Introduction Observations of metamorphic assemblages and experi- mental studies on white micas indicate the presence of a wide solvus between muscovite and paragonite (Eugster and Yoder 1955; Zen and Albee 1964; Eugster et al. 1972; Chatterjee and Flux 1986; Flux and Chatterjee 1986; Roux and Hovis 1996). The degree of Na substi- tution in muscovite coexisting with paragonite, and K in paragonite, mutually increases with increasing temper- ature, providing a potentially powerful thermometer (Rosenfeld et al. 1958; Blencoe et al. 1994). However, the fundamental assumption that allows estimates of intensive variables from mineral assemblages is that the mineral phases attained a state of equilibrium at some pressure and temperature conditions. Mineralogical, textural, and chemical heterogeneities usually re¯ect a lack of equilibrium, which is a common feature in polymetamorphic rocks (Essene 1982). Identi®cation of textural equilibrium is very important for sheet silicates, since they commonly occur as ®ne-grained intergrowths (Shau et al. 1991; Giorgetti et al. 1997) or mixed layer- ing (Frey 1969; Li et al. 1994b) at scales below electron microprobe resolution. Coexisting Na-K white micas have been described from low- to medium-grade metamorphic rocks throughout the world and they are widespread in blue- schist- and eclogite-facies assemblages (Guidotti 1984; Contrib Mineral Petrol (2000) 138: 326 ± 336 Ó Springer-Verlag 2000 G. Giorgetti (&) 1 Department of Geological Sciences, The University of Siena, Via Laterina, 8; I-53100 Siena, Italy Tel.: ++39-0577-263830; Fax: ++39-0577-263938 e-mail: giorgett@dst.unisi.it G. Giorgetti á P. Tropper á E.J. Essene á D.R. Peacor Department of Geological Sciences, University of Michigan, Ann Arbor MI 48109-1063, USA Present address: 1 Dipartimento di Scienze della Terra, via Laterina 8 53100 Siena, Italy Editorial responsibility: T.L. Grove