Condizioni di cristallizzazione del polimorfo 3T delle miche diottaedriche

Blasi, Achille ; Blasi De Pol, Carla

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 55 (1973) no. 6, pp. 719-725

Résumé

3T and 2M1 polymorphs of dioctahedral micas coexist in granites of the central nucleus of the Argentera Crystalline Massif, Maritime Alps (Blasi and Blasi De Pol, 1973). The occurrence of 3T dioctahedral micas in these granites confirms indications on the crystallisation conditions of such a variant inferred from natural findings, from experimental syntheses, and from structural investigations. The most important factors leading to the crystallisation of the 3T polymorph are the $\sideset{^{VI}}{^{2+}}{\operatorname{R}} \sideset{^{IV}}{^{4+}}{\operatorname{Si}} \rightarrow \sideset{^{VI}}{^{3+}}{\operatorname{Al}} \sideset{^{IV}}{^{3+}}{\operatorname{Al}}$ substitutions (where $\sideset{^{VI}}{^{2+}}{\operatorname{R}}$ is mainly Mg and Fe2+) and moderate values of temperature, that affects ordering of (Si, Al) over the tetrahedral sites. These factors, however, are to be considered as a permissive rather than a requisite control, the 3T polymorph being in nature mainly coexisting with other variants. The pressure influence appears to be rather insignificant, while the activity of the fluids plays a role that, at this moment, is far from being completely understood.