Manganoan berthierine, Meyers Pass, New Zealand: occurrence in the prehnite-pumpellyite facies

Manganoan berthierine, sometimes only a few layers thick, is preserved interlayered with manganoan chlorite (chamosite) in small recrystallized patches in cherty metapelagites matamorphosed in the prehnite-pumpellyite facies at Meyers Pass, Torlesse Terrane, South Canterbury, New Zealand. The Mn occupies 11-12% of the octahedral sites, making this the most Mn-rich berthierine reported so far. The ratio (Fe+Mn)(Fe+Mn+Mg), 0.67-0.70, is at the lower end of those of previously reported occurrences, making these amongst the most Mg-rich berthierines known. Manganoan berthierine and interlayered manganoan chlorite also make up ~11-13% of the cryptocrystalline groundmass in which the berthierine is believed to have crystallized under early diagenetic conditions near a deep-granulite ocean sediment interface. The occurrence of berthierine in the recrystallized patches lends support to the possibility of a stability field at the temperature of prehnite-pumpellyite facies recrystallization in association with chlorite of similar but not identical composition.