The pyroxene-spinel polysomatic system

Abstract

Abstract Modules 〈P〉 and 〈S〉 which taken separetely constitute the structures of pyroxene and spinel respectively, may give rise, in combination, to a wide polysomatic system ranging between these two minerals as end members. The system is firstly discussed on the basis of complex layers 〈PmSnPm〉, built up by somatic parts of both spinel and pyroxene, from the 〈(P/2)S(P/2)〉 modules, which are the OD layers giving rise to the sapphirine family, to the general 〈Pm/2SnPm/2〉, which includes 〈(P/2)Sn(P/2)〉 (characterizing the structure of calcium aluminium ferrite and SFCA-I compound) and 〈PSP〉 (characterizing the surinamite-type structures). Afterwards the system is discussed on the basis of 〈Pm〉 and 〈Sn〉 layers, with derivation of a new polytypic family closely related to that of sapphirine. Finally the possible stacking sequences of single modules 〈S〉, giving rise to the spinelloid series and the stacking and condensation of single 〈P〉 modules, giving rise to the whole biopyriboles series, are presented and discussed.