Reaction sequence, polysomatic faults and chemical compositions in the manganese pyroxenoids from the Campiglia Marittima skarn

The late crystallisation stages of Campiglia Marittima skarn are characterised by an increase in Mn-activity at the meta-somatic front. Mn-pyroxenoids became stable with respect to johannsenite. The pyroxene-pyroxenoid textures indicate either pyroxenoid overgrowths on johannsenite or pyroxenoid pseudomorphs after johannsenite. TEM investigation of the replacement sites reveals that the pyroxene-to-pyroxenoid replacement mainly consists of a topotactic solid-state reaction involving the movement of a reaction front through the original johannsenite crystal (bulk reaction mechanism). Due to the fast kinetics of skarn formation, pyroxenoids are chemically heterogeneous and highly defective, with numerous chain periodicity faults. The crystallographic relationship (001) Pxd //(111) Jh is confirmed. Johannsenite-pyroxenoid solid-state transformations appear to occur at temperatures lower than 260°C. In contrast with previous assumptions, AEM data and crystal chemistry indicate that the maximum calcium content of pyroxmangite is 2/7, rather than 1/7. As a consequence, the AEM data plot along a normal and chemically collinear polysomatic trend. Moreover, it is suggested that minor concentrations of iron and magnesium in the system can affect the pyroxene → pyroxenoid transformation, determining which pyroxenoid is stable in the replacement sequence.