Phengite series is a dioctahedral solid solution between two end-members of muscovite [K1[Al2]VI[Al1,Si3]IVO10(OH)2] and celadonite [K1[(Fe3+,Al)1,(Mg,Fe2+)1]VI[Si4]IVO10(OH)2], which have a hetero-valent substitution of AlVIAlIV ↔ (Mg, Fe)VISiIV. In this study, we report a hydrothermaloriginated authigenic Mg-phengite-series mineral, which occurred as polycrystalline aggregates (Type 1), pore-fillings (Type 2) and well-crystallized lath form (Type 3) from the Haengmae Formation, a dolomite–pebble-bearing fine sand-sized dolostone, in South Korea. Based on microtextural observation, three types of Mg-phengite are associated with crystalline dolomite, and are followed by calcite precipitation as pore-filling, indicating that these should be formed by the influx of a Mg-rich hydrothermal fluid after the deposition of some clastic sediments and before calcitefilling. The structural formula based on O10(OH)2 shows that the number of Mg atoms per formula unit (apfu) of Mg-phengite ranges from 0.00 to 0.70 with no Fe, which is relatively high, compared with the previously reported metamorphic phengites. In REEs mineral chemistry, the Mg-phengites are characterized by the enrichment of REEs and by the particular enrichment of LREEs in the polycrystalline aggregates of Mg-phengite. It strongly suggests that the Mg-phengite should be formed by the infiltration of the highly evolved Mg-and REEs-enriched hydrothermal fluid into the clastic sedimentary rock (Haengmae Formation) as a strata-bound form, syngenetically or during early diagenesis.
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All Science Journal Classification (ASJC) codes
- Geotechnical Engineering and Engineering Geology