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[R1P-10] Norrishite from Funakozawa mine and Kotamagawa mine, Iwate Prefecture, Japan
Keywords:Norrishite, Funakozawa mine, Kotamagawa mine
We found a large amount of norrishite as lithium-dominant mica from the Funakozawa mine and the Kotamagawa mine. Norrishite from both mine is commonly occurred as brown transparent plate or foil-like crystals in thin layers parallel to the schist. The appearance corresponds to that of so-called biotite, generally less than 0.5 mm in size, but as large as 2 mm. The norrishite-bearing layer is composed mainly of fine quartz and is accompanied by aegirine, amphibole, and braunite. Since the presence of lithium and trivalent manganese was evident from the single crystal XRD experiment described below, the EDS analysis was normalized to Al + Si = 4, and the lack of M sites was estimated as lithium. The chemical compositions were K1.02(Mn3+2.10Mg0.07Ti0.02Li0.81)Σ3(Al0.17Si3.83)Σ4O12.16 (Funakozawa mine), K1.00(Mn3+1.94Mg0.13Ti0.06Li0.87)Σ3(Al0.24Si3.76)Σ4O11.94 ( Kotamagawa mine). The crystal structure was examined by single-crystal XRD, and crystals with fewer inclusions from Funakozawa mine were used. Single-crystal XRD yielded lattice constants of a = 5.3024(2), b = 8.9520(4), c = 10.0812(5)Å, β = 98.191(4)°, and V = 473.65(3) Å3 in space group C2/m, with structure refinement converging at R1 = 4.3%. The M1 site was estimated as the occupancy of Li and Mn, resulting in Li0.944(4)Mn0.056. For the M2 site, occupancy was estimated using only Mn, resulting in Mn0.970(3). BVS calculations yielded M1 = 1.02, M2 = 3.08, and O4 = 1.68, which is consistent with the characteristics of norrishite, where M1 = lithium, M2 = trivalent manganese, and anhydrous. The M2 site is strongly distorted due to the Jahn-Teller effect by trivalent manganese, which is thought to be the cause of the refusal of other elements to the M2 site.
