Discovery of Li–bearing sodium amphibole from the Sanbagawa belt, Japan

Abstract

Lithium–bearing sodium amphibole (Li₂O = 0.01–1.02 wt%) was found in a specimen of schistose manganese ore from the Iimori region in the Sanbagawa metamorphic belt, central Japan. The ore is composed mainly of quartz, albite, amphibole, Na to Na–Ca pyroxene, and braunite. The amphibole occurs as prismatic crystals with lengths of up to 400 µm and consists of a pale–green core and an orange–red rim observed in hand specimen. The chemical formulae of averaged compositions of the core and rim, based on 24(O, OH, F, Cl) with (OH, F, Cl) = (2 − 2Ti) atoms per formula unit, are
^A(Na_0.468K_0.448)_Σ0.916^B(Na_1.586Ca_0.393Mn²⁺_0.021)_Σ2.000
^C(Mg_3.896Mn²⁺_0.124Fe³⁺_0.657Al_0.182Ti_0.031Li_0.106Cu_0.004)_Σ5.000
^T(Si_7.936Al_0.064)_Σ8.000O₂₂^W[(OH)_1.771F_0.167O_0.062]_Σ2.000 and
^A(K_0.576Na_0.428)_Σ1.004^B(Na_1.759Ca_0.241)_Σ2.000
^C(Mg_3.143Mn²⁺_0.332Fe³⁺_0.782Al_0.247Mn³⁺_0.081Ti_0.053Li_0.353Cu_0.009)
_Σ5.000^TSi_8.000O₂₂^W[(OH)_1.737F_0.157O_0.106]_Σ2.000, respectively. Consequently, the core amphibole has an intermediate composition between magnesio–arfvedsonite and potassic–magnesio–arfvedsonite, whereas the rim amphibole is potassic–magnesio–arfvedsonite.