The Mineralogy and Petrology of Manganese-Rich Rocks From St. Marcel, Piedmont, Italy*

Manganese-rich metamorphic rocks con- taining violan from St. Marcel, Piedmont, Italy for- med 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 py- roxene, 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 re- lation. 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 com- ponent in the dioctahedral mica is buffered at a maximum by coexistence with phlogopite, microcline, and quartz. Various phase relations are used to estab- lish 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.