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Kurchatovite from the Fuka Mine, Okayama Prefecture, Japan

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Kurchatovite from the Fuka Mine, Okayama Prefecture, Japan Journal of Mineralogical and Petrological Sciences, Volume 112, page 159–165, 2017 Kurchatovite from the Fuka mine, Okayama Prefecture, Japan Ayaka HAYASHI*, Koichi MOMMA**, Ritsuro MIYAWAKI**, Mitsuo TANABE***, † ‡ Shigetomo KISHI , Shoichi KOBAYASHI* and Isao KUSACHI *Department of Earth Sciences, Faculty of Science, Okayama University of Science, Kita–ku, Okayama 700–0005, Japan **Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305–0005, Japan ***2058–3 Niimi, Okayama 718–0011, Japan †534 Takayama, Kagamino–cho, Tomada–gun, Okayama 708–0345, Japan ‡509–6 Shiraishi, Kita–ku, Okayama 701–0143, Japan Kurchatovite occurs as colorless granular crystals up to 1 mm at the Fuka mine, Okayama Prefecture, Japan. The mineral is associated with shimazakiite, calcite and johnbaumite. The Vickers microhardness is 441 kg mm−2 (50 g load), corresponding to 4½ on the Mohs’ scale. The calculated density is 3.23 g cm−3. Electron microprobe analyses of kurchatovite gave empirical formulae ranging from Ca0.987(Mg1.004Fe0.020Mn0.001)Σ1.025 B1.992O5 to Ca0.992(Mg0.466Fe0.523)Σ0.989B2.013O5 based on O = 5, and kurchatovite forms a continuous solid solution in this range. The mineral is orthorhombic, Pbca, and the unit cell parameters refined from XRD data measured by a Gandolfi camera are a = 36.33(14), b = 11.204(3), c = 5.502(17) Å, V = 2239(12) Å3.Itis formed by a layer of MgO6–octahedra, a layer of CaO7–polyhedral and B2O5 consisting of two BO3–triangles. The kurchatovite from the Fuka mine was probably formed by supplying Mg and Fe to shimazakiite from hydrothermal solution at a temperature between 250 to 400 °C. Keywords: Kurchatovite, Calcium magnesium borate, Skarn, Fuka INTRODUCTION During a mineralogical survey of the gehlenite–spur- rite skarns at the Fuka mine, kurchatovite was found as Kurchatovite, Ca(Mg,Mn,Fe)B2O5, is a calcium magne- fine several granular crystals in borate mineral aggregates. sium anhydrous borate. The mineral was first identified This is the first occurrence of kurchatovites in Japan. The as a new mineral by Malinko et al. (1966). It was found present paper deals with the mode of occurrence, and min- in the granitic rocks of the Solongo iron skarn mine of eralogical properties of kurchatovite from the Fuka mine. Siberia. It was associated with vesuvianite, garnet, svab- Moreover, we examined the crystal structure of Fe–rich ite, magnetite and sphalerite. In some occurrences, it is kurchatovite, which has not been reported. replaced by a fine–grained aggregate of szaibelyite, cal- cite and chlorite. The crystal structure of kurchatovite has OCCURRENCE been determined to Pc21b by Yakubovich et al. (1976). Moreover, Yakubovich et al. (1976) and Simonov et al. Kurchatovite was discovered from borate mineral aggre- (1980) showed that the compound CaMgB2O5 has two gates mainly composed of shimazakiite, Ca2B2O5 (Kusa- polytypes, kurchatovite and clinokurchatovite, and deter- chi et al., 2013) in crystalline limestone close to gehlen- mined in the space groups Pc21b and Pc21c, respectively. ite–spurrite skarns which were formed as pyrometaso- Subsequently it has been refined to Pbca by Callegari et matic products of limestone at the Fuka mine, Okayama al. (2003). Nikolaychuk et al. (1970) carried out hydro- Prefecture, Japan (34°46′N, 133°26′E). Kurchatovites oc- thermal syntheses with the compound CaMg(B2O5)in cur as several colorless granular crystal up to 1 mm in a temperature range of 250–690 °C in the CaO–MgO– diameter in a shimazakiite crystal (Fig. 1). The mineral is B2O3–H2O system with a B2O3 concentration of 8%. surrounded by black colored minerals consist of Fe, Mg, As, S, F, and Cl (Fig. 2). The other associated minerals doi:10.2465/jmps.170330 are calcite and johnbaumite, Ca5(AsO4)3(OH) (Kusachi et S. Kobayashi, [email protected] Correponding author al., 1996). 160 A. Hayashi, K. Momma, R. Miyawaki, M. Tanabe, S. Kishi, S. Kobayashi and I. Kusachi Table 1. Chemical compositions of kurchatovite Figure 1. Photograph of kurchatovite from the Fuka mine. Abbre- viations: Kur, kurchatovite and Shi, shimazakiite. * The Mg– and Fe–rich values show the highest Mg and Fe con- tent, respectively. ** Total was normalized to 100%, after deducting 10% of iden- tified impurities. *** The number of ions on the basis of O = 5 was determined by calculation based on wt%. 1. Fuka, Okayama Prefecture, Japan. The present work. 2. Solongo, USSR. Malinko et al. (1966). 3. Ural, USSR. Malinko et al. (1973). CHEMICAL COMPOSITION The chemical composition was determined by means of an electron microprobe (JEOL JXA–8230; WDS mode, 15 kV, 12 nA, and 5 µm beam diameter). The standard materials were; takedaite or calciborite (Ca and B), peri- clase (Mg), hematite (Fe) and manganosite (Mn). The – Figure 2. Photomicrograph (plane polarized light) of kurchatovite chemical analyses of kurchatovite near the Mg end mem- from the Fuka mine. Abbreviations: Kur, kurchatovite and Shi, – shimazakiite. ber and the most Fe rich one are given in Table 1, togeth- er with the data by Malinko et al. (1966, 1973) for com- parison. The empirical formula of kurchatovite from the PHYSICAL AND OPTICAL PROPERTIES Fuka mine (based on O = 5 apfu) varies depending on individual crystal, its composition is shown in the range Kurchatovite is colorless to pale gray in hand specimens. of Ca0.987(Mg1.004Fe0.020Mn0.001)Σ1.025B1.992O5 to Ca0.992 In thin section, the mineral is colorless to light brown and (Mg0.466Fe0.523)Σ0.989B2.013O5. In the range, kurchatovite transparent (Fig. 2). Cleavage is perfect on one direction is formed a continuous solid solution (Fig. 3). Kurchato- {100}. Although kurchatovite from Solongo (Malinko et vite from Solongo reported by Malinko et al. (1966) is al., 1966) and synthetic kurchatovite (Nikolaychuk et al., richer in Mn than Fe, whereas kurchatovite from the Fuka 1970) have been reported to fluoresce under long wave- mine is almost Mn–free. Some of them are kurchatovite length UV light, fluorescence was not confirmed for the with Fe / (Mg + Fe + Mn) (apfu) slightly exceeding 0.5 kurchatovite from the Fuka mine. The mean Vickers mi- and its chemical composition can be plotted within the crohardness is 441 kg mm−2 (ranging between 346 and area for the Fe–analogue of kurchatovite (Fig. 3). The 498 kg mm−2) under load of 50 g, which corresponds Fe–rich kurchatovite shows the highest iron content ever to 4½ on the Mohs’ scale. The calculated density of reported among the specimens in the world. kurchatovite from the Fuka mine is 3.17 g cm−3 based on the empirical formula and refined unit cell parameters X–RAY CRYSTALLOGRAPHY obtained from X–ray diffraction data. It is larger than 3.02 gcm−3 reported from Malinko et al. (1966). X–ray diffraction investigations were carried out for Kurchatovite from Fuka, Okayama, Japan 161 given in Table 4. Selected interatomic distances are sum- marized in Table 5. DISCUSSION The crystal structure of kurchatovite contains of 3 B2O5 units, pairs of corner–sharing 2 BO3 triangles (B1–B4, B2–B5 and B3–B6), 3 polyhedra of 7–coordinate Ca (Ca1, Ca2, and Ca3) and 3 octahedra of 6–coordinate Mg (Mg1, Mg2, and Mg3). The examined kurchatovite from the Fuka mine is rich in Fe, which substitutes more than 0.35 atomic fraction of Mg in the octahedral Mg site. However, it shows no apparent difference in the crystal structure from kurchatovite from Solongo. The refine- ment revealed almost no preference in the occupancy of Fe among the three Mg sites, i.e., Fe is randomly and equally substituting Mg atoms among the three distinct sites. Average bond distances of Mg sites of Fe–rich kurchatovite (Table 5) are slightly (0.005–0.007 Å) larger than those values reported by Callegari et al. (2003). While ~ 37% of Mg sites are substituted by Fe, the Figure 3. The chemical composition of kurchatovite from the change in bond distances is very small and average bond Fuka mine in terms of the components Mg–Fe–Mn. distances of Ca and B sites of Fe–rich kurchatovite (Table 5) are kept nearly identical to those reported by Callegari kurchatovite, having a size of 0.06*0.05*0.025 mm3 that et al. (2003). Therefore, it is suggested that the same crys- was picked from the thin section under a microscope. The tal structure would be kept even in the Fe–analogue of powder X–ray diffraction pattern of kurchatovite from the kurchatovite, CaFeB2O5. Fuka mine was obtained by using a Gandolfi camera, The formation process of the borate minerals from the 114.6 mm in diameter, employing Ni–filtered CuKα radi- Fuka mine is considered as follows: Anhydrous borate ation. The X–ray diffraction data is given in Table 2, with minerals such as takedaite and shimazakiite were primarily those from Solongo, Siberia (Malinko and Pertsev, 1983). formed by a reaction of boron–bearing fluids with lime- The unit cell parameters refined from the Gandolfi data stone; a subsequent hydrothermal solution altered the min- are a = 36.33(14), b = 11.204(3), c = 5.502(17) Å. These erals into hydrated borate minerals such as sibirskite, values are in good agreement with the unit cell param- CaHBO3 and uralborite, CaB2O2(OH)4 (Kusachi et al., eters of kurchatovite from Solongo, a = 36.29(8), b = 1999). In the aggregate of borate minerals including kurch- 11.120(2), c = 5.491(2) Å (Malinko and Pertsev, 1983). atovite, there are a lot of hydrothermal veinlets composed The single crystal X–ray diffraction data were obtain- of uralborite, sibirskite and calcite. Shimazakiite, Ca2B2O5 ed with the same fragment on a Rigaku R–AXIS RAPID associated with kurchatovite shows two polymorphisms, diffractometer using MoKα radiation monochromated and shimazakiite–4M and shimazakiite–4O (Kusachi et al., focused by a VariMax confocal multilayer mirror.
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