Calciborite from the Fuka Mine, Okayama Prefecture, Japan

Journal of MineralogicalCalciborite and Petrological from Fuka, Sciences, Okayama, Volume Japan 109, page 13─ 17, 2014 13

Calciborite from the Fuka mine, Okayama Prefecture, Japan

* * * ** Shoichi Kobayashi , Tamami Ando , Akiko Kanayama , Mitsuo Tanabe , *** † Shigetomo Kishi and Isao Kusachi

* Department of Earth Sciences, Faculty of Science, Okayama University of Science, Kita-ku, Okayama 700-0005, Japan ** 2058-3 Niimi, Niimi 718-0011, Japan *** 534 Takayama, Kagamino-cho, Tomata-gun 708-0345, Japan † 509-6 Shiraishi, Kita-ku, Okayama 701-0143, Japan

Calciborite was found as a veinlet or a mass in crystalline limestone associated with gehlenite-spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Calciborite occurs as milky white aggregates up to 1 mm in diameter with shimazakiite, fluorite, bornite and calcite. An electron microprobe analysis of calciborite gave an em-

pirical formula (Ca0.999Mn0.001Co0.001)Σ1.001B1.999O4 based on O = 4. The unit cell parameters are a = 8.373(2), b = 13.811(8), c = 5.012(4) Å. The mineral is optically biaxial (−), α = 1.594(2), β = 1.654(2), γ = 1.672(2) and −2 2Vcalc. = 56°. The Vickers microhardness is 177 kg mm (50 g load), and the Mohs hardness number is 3.5. The measured and calculated densities are 2.88(2) and 2.881 g cm−3, respectively. The calciborite from the Fuka mine was probably formed by a reaction of boron-bearing fluids with limestone at a temperature between 250 and 300 °C.

Keywords: Calciborite, Calcium borate, Skarn, Fuka

INTRODUCTION Fuka mine.

Calciborite, CaB2O4, is a very rare mineral of calcium bo- OCCURRENCE rates. The mineral was first identified as a new mineral reported by Petrova (1955). It was found in deep drill cores Calciborite was discovered closely associated with shima- at the contact of Middle Devonian limestone with quartz zakiite (Kusachi et al., 2013) in crystalline limestone diorite intrusive in the Novofrolovo skarn type copper de- close to gehlenite-spurrite skarns at the Fuka mine, posit in the Turinsk district, Central Urals, Russia. It was Okayama Prefecture, Japan (34°46′N, 133°26′E), which associated with calcite and dolomite, with minor amounts were formed as pyrometasomatic products of limestone. of garnet, magnetite, and pyroxene. The mineral also Calciborite occurs as aggregates, up to 1 mm in diameter, filled fissures in limestone and was observed to replace of anhedral crystals. Calciborite is formed with small calcite. Malinko et al. (1963) re-examined the type mate- amounts of calcite and bornite as a veinlet up to 5 mm rial, and gave the chemical formula of CaB2O4. Subse- width or as a mass up to 2 cm in diameter in a block of quently, Shashkin et al. (1970) determined the crystal aggregates of shimazakiite, calcite and fluorite in the crys- structure of the mineral from the type locality. Moreover, talline limestone. The other associated minerals are borate powder X-ray diffraction data were obtained by Shashkin minerals such as sibirskite (Kusachi et al., 1997), nifonto- et al. (1971). vite (Kusachi and Henmi, 1994), uralborite (Kusachi et During a mineralogical survey of gehlenite-spurrite al., 2000) and an undetermined hydrous calcium borate, skarns at the Fuka mine, calciborite was found as a veinlet and cuspidine (Kusachi et al., 1977) and bornite. or a mass in crystalline limestone. This is the first occurrence of calciborite in Japan and the second occurrence in PHYSICAL AND OPTICAL PROPERTIES the world. The present paper deals with the mineralogical properties and mode of occurrence of calciborite from the Calciborite is milky white with a vitreous luster in hand specimens. In thin section, the mineral is colorless, trans- doi:10.2465/jmps.130619 parent as shown in Figure 1. Cleavage is recognized per- S. Kobayashi, [email protected] Corresponding author fect on {110}, though Petrova (1955) described no cleav- 14 S. Kobayashi, T. Ando, A. Kanayama, M. Tanabe, S. Kishi and I. Kusachi

Figure 1. Photomicrograph (plane-polarized light) of calciborite. Figure 2. Infrared absorption spectra of calciborite. Abbreviation: Cb, calciborite. the wave number range from 4000 to 400 cm−1, as shown Table 1. Optical and physical properties of calciborite from the in Figure 2. The numerous absorption bands in the range Fuka mine from 1320 to 600 cm−1 are characteristic to the borates. The spectrum of calciborite shows no absorption bands

for vibrations of the H2O, OH and CO3.

X-RAY CRYSTALLOGRAPHY

The X-ray powder diffraction data of calciborite from the Fuka mine were obtained by using an X-ray diffractome- ter (Rigaku RINT-2500V) with graphite-monochroma- tized Cu-Kα radiation generated at 40 kV and 200 mA. The cell parameters calculated from the powder data are a = 8.373(2), b = 13.811(8), c = 5.012(4) Å. They are in close agreement with those obtained by oscillating crystal X-ray diffraction study and refining by the powder meth- * 2Vcalc. od by Shashkin et al. (1971). The data for calciborite from 1. Fuka, Okayama Prefecture, Japan. The present work. the Fuka mine are shown with those from the Novofrolo- 2. Novofrolovo, Turinsk, Central Urals, Russia. Petrova (1955). vo mine of type locality in Table 2. The a and b-axes of calciborite from the Fuka mine were slightly shorter, age. Optically, the mineral is biaxial negative with whereas c-axis was slightly larger than those reported by refractive indices α = 1.594(2), β = 1.654(2) and γ = Shashkin et al. (1971), respectively. 1.672(2), giving a calculated 2V = 56°. It is non-pleochro- ic and non-fluorescent. The density was measured as CHEMICAL COMPOSITION 2.88(2) using a heavy liquid (Clerichi solutions) and calculated as 2.881 g cm−3 based on the empirical formula The chemical composition was determined by means of an and refined unit cell parameters. The mean Vickers micro- electron microprobe (JEOL JXA-8900R; WDS mode, 15 hardness value was 177 kg mm−2 (ranging between 161 kV, 12 nA and 5 μm beam diameter). A pure specimen of and 196 kg mm−2) on load of 50 g. In Table 1, the optical takedaite, nifontovite, rhodonite and CoO were used as and physical properties are compared with those reported standard materials for Ca, B, Mn and Co, respectively. The in the original description (Petrova, 1955). The data are average of 10 analytical points is given in Table 3, togeth- very close to those from the type locality except cleavage. er with the data by Malinko et al. (1963) for comparison. Calciborite is easily soluble in dilute hydrochloric acid. Petrova (1955) first reported the chemical formula of

The infrared absorption spectrum of calciborite from calciborite to be Ca5B8O17 from analytical data using pick- the Fuka mine was measured by the KBr matrix method ing material, which contained small amounts of dolomite, using an infrared spectrometer (JASCO FT/IR-410) for garnet and pyroxene. Subsequently, Malinko et al. (1963) Calciborite from Fuka, Okayama, Japan 15

Table 2. X-ray powder diffraction data of calciborite

1. Fuka, Okayama Prefecture, Japan. The present work. 2. Novofrolovo, Turinsk, Central Urals, Russia. Shashkin et al. (1971). re-examined the type material, and revised the ideal for- limestones from the Novofrolovsk copper deposit of the mula CaB2O4. The empirical formula of calciborite from skarn type in the Turinsk district, Central Urals. The the Fuka mine (based on O = 4 apfu), (Ca0.999Mn0.001 metaborate minerals are characterized by a constant molar

Co0.001)Σ1.001B1.999O4, is very close to the ideal formula. ratio CaO:B2O3 = 1:1. Calciborite, uralborite (Kusachi et al., 2000), nifontovite (Kusachi and Henmi, 1994), frolo- DISCUSSION vite (Kusachi et al., 1995a), pentahydroborite (Fujiwara et al., 1982) and hexahydroborite (Kusachi et al., 1999) were

Calciborite, ideally CaB2O4, shows the highest B2O3 con- also confirmed from the Fuka mine (Fig. 3). Though there tent among the calcium borate minerals from the Fuka was hexahydroborite and no korzhinskite in the Fuka mine (Fig. 3). Shashkin et al. (1971) represented endo- mine, the mineral assemblages are similar between Fuka genic calcium metaborate group such as calciborite, kor and Novofrolovsk. Optical and physical properties of zhinskite, uralborite, nifontovite, frolovite, and pentahy- metaborate group from the Fuka mine were collectivity droborite, which have been discovered in skarn-formed shown in Table 4. The molecular numbers of water within 16 S. Kobayashi, T. Ando, A. Kanayama, M. Tanabe, S. Kishi and I. Kusachi

Table 3. Chemical composition of calciborite

Figure 3. Minerals in the system CaO-B2O3-H2O from the Fuka mine. 1, takedaite; 2, shimazakiite; 3, calciborite; 4, sibirskite; parasibirskite; 5, uralborite; 6, nifontovite; 7, olshanskyite; 8, frolovite; 9, pentahydroborite; 10, hexahydroborite.

therefore crystallized at a temperature between 250 and 300 °C. Calciborite from Fuka is formed as a veinlet and a mass in crystalline limestone. From this mode of occurrence, it is very likely that calciborite from the Fuka mine was primarily formed by a reaction of boron-bearing fluids with limestone.

ACKNOWLEDGMENTS

* Analyzed spots. We would like to thank Prof. H. Akashi of Okayama Uni- 1. Fuka, Okayama Prefecture, Japan. The present work. 2. Novofrolovo, Turinsk, Central Urals, Russia. Malinko et al. versity of Science for help to use the IR spectro-photome- (1963). ter. We thank Dr. R. Miyawaki and Dr. Y. Banno for their insightful and constructive comments. This work was the minerals varies from 0 (calciborite) to 6 (hexahydro- supported in part by a Grant-in-Aid for the Scientific Re- borite). The refractive indices, birefringence, hardness search (No. 24540522) from the Ministry of Education, and density of calciborite were relative high, and these Science, Sports and Culture, Japan. values decreased with increase in water content. Hart and Brown (1962) reported that the mineral REFERENCES generated at a low temperature contained much water in the system CaO-B2O3-H2O. Therefore, the stabilized tem- Fujiwara, T., Takada, M., Masutomi, K., Isobe, T., Okada, H., Na- perature of calciborite may be higher than another miner- kai, I. and Nagashima, K. (1982) Pentahydroborite als in the metaborate group. Schäfer (1968b) synthesized CaB2O4·5H2O. Chigaku Kenkyu, 33, 11-20 (in Japanese). Hart, P.B. and Brown, C.S. (1962) The synthesis of new calcium the needle shaped single crystals of CaB2O4 from borate compounds by hydrothermal methods. Journal of Inor- Ca(OH) and H BO at temperatures between 270 and 2 3 3 ganic and Nuclear Chemistry, 24, 1057-1065. 300 °C under a saturated water pressure. On the other Kusachi, I., Henmi, C., Saburi, S. and Henmi, K. (1977) Cuspi- hand, takedaite, Ca3B2O6, which discovered at the Fuka dine from Fuka, the Town of Bitchu, Okayama Prefecture. mine, is anhydrous mineral as well as calciborite. Kusachi Mineralogical Journal, 13, 165-170 (in Japanese). et al. (1995b) described that takedaite would appear to be Kusachi, I. and Henmi, C. (1994) Nifontovite and olshanskyite from Fuka, Okayama Prefecture, Japan. Mineralogical Maga- formed at a temperature around 250 °C based on the re- zine, 58, 279-284. sult of synthetic experiment by Schäfer (1968a). In view Kusachi, I., Henmi, C. and Kobayashi, S. (1995a) Frolovite from of these results, calciborite from the Fuka mine would Fuka, Okayama Prefecture, Japan. Mineralogical Journal, 17, Calciborite from Fuka, Okayama, Japan 17

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