R1: Characterization and description of minerals (Joint Session with The Gemmological Society of Japan)

[R1-P-13] Arsenmedaite from the Yamato mine, Kagoshima Prefecture, SW Japan

*Shunsuke Ohsumi, Daisuke Nishio-Hamane¹, Hiroki Oka², Masashi Tamura³, Kosuke Takagi⁴ (1. ISSP, Univ. of Tokyo, 2. OYO Corp., 3. Fac. Eng. Tech. Div., Mie Univ., 4. Grad. Sch. of Eng., Mie Univ.)

Keywords:arsenmedaite, Yamato mine

The Yamato mine in the Amami Oshima Island, Kagoshima Prefecture, southwestern Japan, is a well-known manganese mine as the type locality of haradaite (Watanabe et al., 1982) and occurrences of V and/or As-rich minerals such as roscoelite (Yoshimura & Momoi, 1964), goldmanite (Momoi, 1964), tiragalloite (Nakao et al., 2005), palenzonaite, nabiasite, tokyoite, vuorelainenite (Yamada et al., 2008), medaite, saneroite (Matsubara et al., 2013), poppiite (Niwa & Ishibashi, 2015), and reppiaite (Yamada et al., 2018). In this study, we report the first occurrence of arsenmedaite in Japan.Arsenmedaite, As⁵⁺-analogue of medaite, was reported in quartz veinlets cutting braunite ores from the Molinello mine, which is hosted within metacherts of the Northern Apennines. It is associated with calcite, As-rich medaite, rhodochrosite, talc, and ganophyllite.In the Yamato mine, arsenmedaite was found as from yellow to orange-reddish granular crystals, up to 1 mm in diameter, with a vitreous lustre in siliceous manganese ores, which consist mainly of massive hausmannite, rhodochrosite, and coarse-grained rhodonite, with absence of medaite. Tiny crystals of rhodonite are ubiquitously included in arsenmedaite. The occurrence of arsenmedaite in the Yamato mine differs from that in the type locality.The representative chemical composition by SEM-EDS is: SiO₂ 30.12, CaO 0.99, V₂O₅ 3.25, MnO 48.03, As₂O₅ 13.26, H₂O_calc. 1.01, total 96.65 wt%, and they gave the empirical formula as (Mn²⁺_6.04Ca_0.16)_Σ6.20[(As_1.03V_0.32)_1.35Si_4.47]_Σ5.82O₁₈(OH), assuming total cations = 18 and Mn = Mn²⁺. The empirical formula is slightly richer in As and V and less in Si than the ideal formula of arsenmedaite, Mn²⁺₆ As⁵⁺Si₅O₁₈(OH). Therefore, it is inferred that As and V partly substitute the Si-tetrahedral framework in arsenmedaite.Raman spectroscopy detected weak peaks at 340–360 and 994 cm^-1, moderate peaks at 644 and 658 cm^-1, and strong peaks at 859, 879, and 894 cm^-1. These Raman peaks are comparable with those of arsenmedaite (Biagioni et al., 2019) and medaite (Lafuente et al., 2015).Although a precise crystallographic analysis has not been completed yet due to its rarity and rhodonite inclusions, the occurrence of arsenmedaite possibly contributes to V/As-mineralization in the Yamato mine.