高知市三谷産マンガンバビントン石

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Manganbabingtonite from Mitani, Kochi City, occurs as veinlets cutting massive magnetite ore involved in high pressure and low temperature crystalline schists within Kurosegawa Tectonic Zone. This is the first occurrence of this mineral in Japan. Two chemical ayalyses gave SiO_2 52.61,52.20; Al_2O_3 0.35,0.00; Fe_2O_3 13.53,13.90; FeO 1.76,5.13; MnO 13.21,6.99; MgO 0.00,0.33; CaO 17.71,19.20; totals 99.17,97.75%, after the assignment of total Fe to FeO and Fe_2O_3. These yield empirical formulae : (Ca_<1.80>Mn_<0.20>)_<Σ2.00> (Mn_<0.86>Fe^<2+>_<0.14>)_<Σ1.00> (Fe^<3+>_<0.97>Al_<0.04>)_<Σ1.01>Si_<5.00O>_14.51 and (Ca_<1.97> Mn_<0.03>)_<Σ2.00>(Mn_<0.53>Fe^<2+>_<0.41>Mg_<0.05>)_<Σ0.99>Fe^<3+>_<1.00> Si_<5.00>O_<14.49>, respectively, on the basis of total cation=9. The former has the highest MnO content known so far and the deficience of Ca, which is accounted for by the excess of Mn^<2+>. The X-ray powder diffraction pattern is indexed on a triclinic cell with a=7.492,b=12.32,c=6.701 A, α=85.2°, β=93.9°, γ=112.2°. The associated minerals include ilvaite, howieite, riebeckite, aegirine, greenalite, albite and potash feldspar besides magnetite. All the iron silicates in them are more or less manganiferous, but none of them is with Mn^<2+>>Fe^<2+>. This relation is uniquely seen in manganbabingtonite, which corresponds approximately to Fe^<3+>-rich derivative of manganoan hedenbergite. The present material owes the formation to the metamorphism of a manganiferous ironstone accompanying a minor limestone as the source of calcium, according to the comparison with the nearby ironstone localities in the same tectonic zone, where manganoan howieite or taneyamalite is found.
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高知市三谷産マンガンバビントン石

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