Preiswerkite and Na-(Mg,Fe)-margarite in eclogites

Preiswerkite and Na-(Mg,Fe)-margarite are two unusual micas very rare in Na ture. They have been observed together in two eclogite occurrences (La Comp ointrie, France; Liset, Norway) as retrogression products in coronae or sym plectites around kyanite. The chemical compositions and some physical prope rties of these micas are presented. The possible solid solutions and the co nditions of stability are discussed. The preiswerkites display slight solid solution towards phengitic muscovite and Na-phlogopite. On the other hand, there is negligible solid solution towards more aluminous compositions; Al -IV less than or equal to 4 appears to be a composition limit for natural ( K,Na)-micas. The margarites have an unusual Na-(Mg,Fe)-rich composition. Th ey can be considered as a solid solution of about 2/3 mol% of margarite and 1/3 mol% of the theoretical end-member Na-2(Mg,Fe)(1)Al-4(VI) [Si4Al4IV]O- 20(OH)(4) ("Mica L"), with a possible substitution towards paragonite. The Marg(2/3) Mica L-1/3 compositon (i.e. NaCa2(Mg,Fe)(0.5) Al-6(VI)[Si6Al6IV]O -30(OH)(6)) might represent a paraticularly stable crystallographic configu ration and could be considered as a true end-ember. Many "sodian" margarite s decribed n the literature, are infact, complex solid solutions between ma rgarite, paragonite and Marg(2/3) Mica L-1/3 The rarity of these micas is n ot related to extreme or unusual P-T conditions. They seem to be related to unusual chemical compositions, appearing in H2O-saturated Na-Al-rich Si-po or systems, principally, if not only, at greenschist- or amphibolite-facies P-T conditions. Moreover, they are subject to crystallographic constraints whereby the high proportion of Altetrahedra create considerable distortion which prevents the entry of K into the interlayer site, thus necessitating Na (preiswerkite or ephesite) or Ca (margarite or clintonite) instead.