Vanadium and barium-bearing green mica within coaly metapelite from the Ogcheon Supergroup, Republic of Korea

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Vanadium and barium-bearing green mica, first reported in Korea, has been found within the coaly metapelite from the Ogcheon Supergroup at the Deokpyeong and Hoenam area. The occurrence, optical property, chemical composition, crystal structure and formation environment of the green mica were systematically investigated. Concentrations of the V 2O 3 and BaO in green mica-bearing coaly metapelite range from 0.15 to 1.88 wt% and 0.55 to 6.66 wt%, respectively. The mode of occurrence and grain size of this mica are variable. Most of the green mica, ranging from 50 to 100 μm in length and from 5 to 10 μm in width, occur along foliation boundaries between graphite and quartz grains. The green mica predominantly occur as pseudo-hexagonal forms, pale to deep green in colour with flakes of crystals. Based on 2 M structure, unit-cell parameters of the mica are a=5.213–5.249 A ̊ , b=8.936–8.990 A ̊ , c=20.524–20.924 A ̊ and β=95.89–95.98°, which are higher than the normal 2 M1 muscovite. Representative chemical formula of the mica is (Na 0.1K 1.4Ba 0.5)(Al 2.8Ti 0.1V 0.5Fe 0.1Mg 0.5)(Si 6.0Al 2.0)O 22. The V and Ba possibly substitute for octahedral Al VI and K, respectively. The compositional ranges of V and Ba, on the basis of O=22, are from 0.31 to 0.68 and from 0.37 to 0.57, respectively. We suggest that the genesis of this green mica within the coaly metapelite could be interpreted as due to apparently related metamorphism. The Ba and V, enriched during diagenesis at the interface between the basinal brine and organic matter, possibly substituted for the K and Al VI in smectite and/or illite, respectively.