The mineralogy and petrology of manganese-rich rocks from St. Marcel,Piedmont, Italy

Manganese-rich metamorphic rocks containing violan from St. Marcel, Piedmont, Italy formed under blueschist facies conditions, yielding an unusual suite of minerals including omphacite-albite-quartz, braunite, microcline, hollandite, piedmontite, and strontian calcite. Violan, a violet-colored pyroxene, is shown to be a manganoan omphacite having a primitive unit cell, and is found in the same sample as diopside, possibly indicating a solvus relation. A manganoan phlogopite and a celadonitic muscovite coexist in one sample with microcline. The occurrence of celadonite and phlogopite is the first from the blueschist facies and the celadonite component in the dioctahedral mica is buffered at a maximum by coexistence with phlogopite, microcline, and quartz. Various phase relations are used to establish the P and T of equilibration at 8±1 kbar and 300±50 °C, respectively, while the oxygen fugacity is shown to have been very high, for these temperatures, as consistent with braunite+quartz and the presence of piedmontite. Contribution No. 343, from the Mineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, MI 48109, USA