Minerals of the viridine hornfels from Darmstadt,Germany

Viridine containing the highest amounts of Mn₂O₃ detected thus far (up to 20.5 mol % “Mn₂SiO₅”) coexists in a metasedimentary hornfels with spessartine, Mn-phlogopite (mangan-ophyllite), Mn-phengite (alurgite), hematite, quartz and probably some primary braunite. In layers poorer in viridine spessartine is absent but piemontite appears as an additional phase. Microprobe analyses of all these phases are presented which indicate very strong fractionation of Mg and Mn in coexisting phlogopite and garnet, and of Fe and Mn in coexisting hematite and braunite. Sericitic aggregates consisting of phengitic muscovite and braunite are interpreted as retrograde alteration products of viridine, but might partly be pinitic alterations of a former Mg-rich cordierite. Due to the occurrence of the assemblage spessartine-viridine-quartz Mn-cordierite cannot have been a stable phase prior to retrograde alterations. In general the stability field of viridine is extended towards higher temperatures as compared to that of pure andalusite, Al₂SiO₅. Due to the coexistence of phlogopite and muscovite (phengite) the temperature of contact metamorphism cannot have exceeded some 550°–650° C depending on the prevailing water pressure.