Mg-phengite in carbonate rock syngenetically formed from hydrothermal fluid: Micro-textural evidence and mineral chemistry

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 Al^VIAl^IV ↔ (Mg, Fe)^VISi^IV. 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.