An algorithm for the construction of petrogenetic grids; application to some equilibria in granulitic paragneisses

Abstract

Sequence proposed: balancing of the reactions and calculation of the slopes, arranging reactions about invariant points, determining and drawing the partially closed net. Geometrical and thermodynamic data are arranged and treated in a series of elementary matrices. This algorithm has been computerized to allow a direct drawing on a graphic plotter of the different invariant points and of the petrogenetic grid and its related residual net. This method is applied to some equilibria involving sillimanite-muscovite-K-feldspar-biotite-garnet-cordierite-orthopyroxene (+ quartz and water) in the systems Al ~2~ O ~3~ -K ~2~ O-FeO-MgO; Al ~2~ O ~3~ -K ~2~ O-FeO, and Al ~2~ O ~3~ -K ~2~ O-MgO. Particular emphasis is put on the geometrical relations between continuous and discontinuous reactions in P-T diagrams and on the physical significance of residual nets.\--Modified journal abstract.