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3T sericite has been found in altered wall rocks of the Porgera gold deposit in Papua New Guinea. It occurs as fine-grained, clayey material forming aggregates of minute flakes replacing both argillaceous matter in calcareous siltstone and igneous components in breccia. It is closely associated with quartz, dolomite and/or calcite, subordinate pyrite and fluorite. Chemically the material is K-dominant dioctahedral mica with appreciable octahedral Fe and Mg (exactly intermediate between ideal muscovite and phengite). Although intermixed with 2M1 sericite in small quantities, the X-ray powder diffraction pattern is satisfactorily indexed with a hexagonal cell: refined cell parameters by least squares calculation are a=5.203(1)Å, c=29.945(7)Å and V=702.04(26)Å3, which are in good agreement with the phengitic composition. The chemical composition is correlated with infrared and thermal properties.
Crystallinity, chemical composition (phengitic substitution) and modes of occurrence are variable among common 1M and 2M1 and rarer 3T sericite minerals in this deposit, reflecting diversity of genesis and origin. Local activity of high temperature hydrothermal to pneumatolytic fluid may be responsible for the formation of the 3T sericite, which is extremely well crystallized and closely associated with fluorite.
