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

4:30 PM - 4:45 PM JST(7:30 AM - 7:45 AM UTC)

[R1-04] Reinvestigation of crystal chemistry of nakauriite

*Koichi MOMMA¹, Hideyuki Hayashi¹, Ritsuro Miyawaki¹, Shoichi Kobayashi², Shigetomo Kishi (1. Nat’l. Mus. Nat. Sci., 2. Okayama Univ. Sci.)

Keywords:

Nakauriite,Single crystal X-ray diffraction

Nakauriite was first reported from Nakauri mine, Aichi Prefecture, Japan (Suzuki et al., 1976). Its chemical composition was originally reported as Mn_0.019Ni_0.231Cu_7.770(SO₄)_3.904CO₃(OH)_6.232·48.4H₂O from analytical results of impure samples by subtracting chemical composition of chrysotile. However, possible contamination of sulfur in the above composition was pointed out in later studies (Peacor et al., 1982), and revised ideal formula, (Mg₃Cu²⁺)(OH)₆(CO₃)·4H₂O, was proposed (Chukanov and Vigasina, 2019) considering possible ignorance of Mg in the original description. Originally reported orthorhombic unit cell was also questioned and a monoclinic unit cell was obtained by single-crystal X-ray diffraction (SXRD) study. Nevertheless, crystal structure of nakauriite was unknown up until now. Here we report SXRD study of nakauriite samples from Hokubo, Maniwa, Okayama Prefecture, Japan. Average chemical composition of the samples was reported in a previous study as (Mg_6.19Cu_1.77Ni_0.02Zn_0.01Fe_0.01)_Σ8.00(SO₄)_0.01(CO₃)_1.00(OH)_13.99·13.44H₂O (Nishikori et al., 2017). Powder XRD data was measured using a Gandolfi camera with diameter of 114.6 mm and Ni-filtered CuKα radiation (30kV, 20mA). SXRD experiments were carried out using a Rigaku Synergy Custom equipped with a rotating anode (MoKα, 50kV, 24mA) and a multilayer X-ray focusing mirror (VariMax).The main XRD diffraction lines [d Å (I/I₀)] are 7.28 (100), 2.358 (27), 3.642 (22), 2.391 (17), and 1.911 (16), consistent with the data for nakauriite. SXRD experiments revealed that the crystal is monoclinic, a = 7.8614(6), b = 6.1927(3), c = 9.5913(5), β = 111.955(6), twinned on the twinning plane {100}. The crystal structure was successfully refined to the R1 = 6.11% in the space group P2/m. The ideal formula derived from the crystal structure model is Mg₆Cu²⁺₂(OH)₁₄(CO₃)·4H₂O. We are planning to conduct compositional analysis and XRD measurement of the type specimen from Nakauri mine.

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