No. 2] Proc. Acad., 45 (1969) 109

26. Chrome Garnet from the Vicinity of Nukabira Mine, Hidaka Province, , Japan

By Takeo BAMBA,*'Kenzo YAGI,**>and Kenjiro MAEDA***) (Comm. by Jun SuzuKi, M.J. A., Feb. 12, 1969)

Introduction. Uvarovites or chrome garnets are of restricted occurrence, and are found only in special geologic environments, such as ultra-mafic rocks or some metamorphic rocks. In Japan chrome garnets only from Ochi, Kochi Prefecture (Ikawa, 1935; Harada,1943) and Akaishi Mine, Ehime Prefecture (Kitahara, 1951) have been described mineralogically and analyzed chemically. In the summer of 1967, during their field survey of the ultra- mafic rocks developed along the Nukabira river in Hidaka mountains, the present authors found several specimen of beautiful emerald green veins developed in rodingite mass, associated with the serpen- tine. Under the microscope the veins were found to consist mainly of grossularite, associated with minor amounts of chrome garnet. Mineralogical study on this chrome garnet is given here. Mode of Occurrence. These chrome garnet-bearing veins occur in the rodingite cutting through the serpentine mass in the upper stream of Pankehayushininara river, a tributary of Nukabira river. The geology of this district belongs to the so-called Kamuikotan metamorphic belt, and is composed of pre-Cretaceous sediments (schalstein, slate, limestone), and spilitized pillow lavas, probably of Jurassic age, intruded by serpentine and fresh peridotite as shown in Fig. 1 (Bamba, 1963). In the eastern part, Cretaceous sediments are developed, covering the older formations. Numerous chromite ore deposits, some of which were mined for many years, have been known in this serpentine mass, especially along the marginal parts of the main mass, which probably represents the higher "niveau" of the intrusion tectonic (Bamba, 1963). Numerous leucocratic dike rocks, such as albitite, quartz albitite or rodingite, are well-developed in these serpentine mass (Suzuki, 1953, 1954). The chrome garnet under investigation occurs as small dis- seminated emerald green crystal grains, forming either narrow

*) Hokkaido Branch, Geological Survey of Japan , Sapporo. **> Department of Geology and Mineralogy , Hokkaido University, Sapporo. > Chemical Section, Geological Survey of Japan , Kawasaki. 110 T. SAMBA, K. MAGI, and K. MAEDA [Vol. 45,

Fig. 1. Geological map of Northern Hidaka District, showing the locality of chrome garnet (after Barn- ba 1963). veinlets, 2-5 cm in width, or irregular net-works, within the rodingite in the vicinity of the Nukabira Mine. The veins are composed mainly of grossularite, associated with chrome garnet, the latter being about 10 percent in amount (Fig. 2). In addition chrome garnet sometimes forms thin films along the cracks in the massive chromite ores, but garnet of this type is not studied here. Microscopic observation. The chrome garnet forms usually well-developed, euhedral crystals within massive aggregates of fine- grained grossularite, diopside, zoisite and chlorite mineral, all of which are less than 0.2 mm in size. Chrome garnet is less than 1 mm, usually about 0.2-0.5 mm in diameter. It is light green or greenish yellow in color, and non-pleochroic. It has a remarkable birefrin- gence, and shows twinning and trilling, composed of several rhombic sectors as shown in Fig. 2. Minute inclusions of chromite and pleochroic titanite are not uncommon. Physical properties. The vein was first crushed into size between 150 and 200 mesh, and at first chromite, and then grossula- rite, were separated from the chrome garnet by means of Clerici solution. Since the separation from grossularite was not complete, the grains of chrome garnet were checked under the binocular microscope, and only pure grains free from grossularite were hand picked. Though it was noticed that both deep green and light green fractions were present, no separation was made for the two fractions. The total amount of the sample obtained attained 0.4 g. The specific gravity of the chrome garnet was determined as follows. First the grains were just suspended in Clerici solution, whose density was later determined by a pycnometer. It was noticed that some grains were lighter and some heavier than the solution. No. 2] Chrome Garnet from Nukabira, Hidaka 111

Fig. 2. Photographs showing the chrome garnet in rodingite. (a) Vein in the right-hand side characterized by disseminated dark spots is composed of chrome garnet (black) and grossularite (grey). (b) Photomicrographs of chrome garnet. (b-1) Hexagonal or irregular-shaped sections of chrome garnet in the matrix com- posed of grossularite, diopside, zoisite and chromite. Open nicols. (b-2) Chrome garnet crystals are composed of three or more rhombic sectors by repeated twinning. Crossed nicols. 112 T. BAMBA, K. YAGI, and K. MAEDA [Vol. 45,

The average specific gravity of the chrome garnet was determined as 3.623. Refractive index was measured by means of standard glasses, and special dispersion liquid, composed of methylene iodide, sulfur and phosphorous yellow, by the method proposed by Sueno (1933). Since the birefringence is noticed, the higher and the lower values of indices show rather wide difference. nl 1.806±0.004 n2 1.814±0.004 n2 - ni 0.008 X-ray powder data obtained at the condition of CuKo, 30 KV, 25 mA are given in Table I, as compared with the data of a chrome garnet from Orford, , listed in the A.S.T.M. identification card 7-70.

Table I. X ray powder data of chrome garnets from Nukabira and Orf ord, Canada

The correspondence is fairly good, though some peaks are missing in the present garnet. The cell-dimension was calculated by the (400) peak with an internal standard of silicon, as ao 11.88A. Chemical composition. Pure sample was chemically analysed with the results shown in Table II, together with the atomic ratios on the basis of 0=12.00. From these data the chemical formula of

0 No. 2] Chrome Garnet from Nukabira, Hidaka 113

Table II. Chemical composition of chrome garnets from Nukabira, Japan, and other localities of the world

the chrome garnet is determined as follows : \C~2 95Mg0.O4Mn0.005)3.00(A11.31Fe038Cr0.36)2,05\ 12,86A10.14)3.00o12.00 which is in close agreement with the ideal formula of garnet M3+ M2+ Si3012. The molecular composition is calculated as follows : Uvarovite 18.0 Andradite 19.0 Grossularite 61.2 Spessartite 0.3 Pyrope 1.5 (mol percent). The garnet is composed of dominantly grossularite molecule, while uvare ovite is present only as subordinate molecule. Using the standard physical properties of garnet group given by Deer et al. (1962), the physical properties of this garnet were calcu- lated as follows from the molecular composition by the method pro- posed by Kozu et al. (1940). n 1.786, as 11.91A, D 3.701. Genetic consideration. Chrome garnets usually do not occur in the banded or disseminated chromite ores, both of which are regarded to be of orthomagmatic origin, but they are usually found in the massive chromite ores, regenerated from the primary chromite ores by the hydrothermal effect (Bamba, 1963), suggesting the im-

0 114 T. BAMBA,K. YAGI,and K. MAEDA [Vol. 45,

portance of the chrome-bearing hydrothermal solutions. Glasser and Osborn (1958) have shown experimentally that uvarovite is stable under subsolidus conditions in the system CaO- Cr203-Si02 at atmospheric pressure up to 1370°C, where it decomposes into a-CaSiO3 (pseudowollastonite) and Cr203. This implies the possibility of formation of uvarovite from the reaction between calcium silicates and Cr203-bearing solutions. It is concluded, therefore, that the chrome-bearing hydrothermal solutions derived from the regeneration of the orthomagmatic chro- mite ores were introduced into the rodingite, and formed chrome garnet from the reaction between the pre-existing grossularite and diopside and chrome-bearing solutions. Acknowledgements. The present authors wish to express their thanks to Dr. T. Sueno for his kind advice for preparing the special dispersion liquid for index measurement, and to Mr. J. Yaj ima for his help to prepare the microphotographs of the chrome garnet.

References

Bamba, T. (1963): Genetic study on the chromite deposits of Japan (in Japanese). Rept. Geol. Surv. Japan, No. 200, pp. 68 (p. 38-51). Deer, W. A., Howie, R. A., and Zussman, J. (1962): Rock-forming minerals, 1, pp. 333 (p. 77). Eskola, P. (1933): On the chrome minerals of Outokumpu. Bull. Comm. Geol. Finlande, No. 103, 26-44. Gasser, F. P., and Osborn, E. F. (1958): Phase equilibrium studies in the system CaO-Cr203-Si02. J. Am. Ceram. Soc., 41, 358-367. Harada, Z. (1943): On chrome-bearing minerals of Japan. II (in Japanese). J. Jap. Assoc. Min. Petr. Econ. Geol., 29, 12-23. Ikawa, M. (1935): Chrome garnet and radioactive limonite (in Japanese). Our Minerals, 4, 417-419. Kitahara, J. (1951): Chrome garnet from the Akaishi Mine, Ehime Prefecture (in Japanese). Mineral. Geol., 4, Nos. 3-4, 79-81. Knorring, 0. von, (1951): A new occurrence of uvarovite from northern Karelia in Finland. Min. Mag., 29, 594-601. Kozu, S., Takeuchi, T., and Omori, K. (1940) : A new method for calculating chemical composition of garnets from their physical properties, with special reference to chemical composition of garnets from Mitsuishi and other localities (in Japanese). J. Jap. Assoc. Min. Petr. Econ. Geol., 23, 163-193. Sueno, T. (1933) : On the use of standard glass powders in refractive index deter- mination. Am. Min., 18, 421-430. Suzuki, J. (1953) : The hypabyssal rocks associated with the ultra-basic rocks in Hokkaido, Japan, Comptes Rendus de la Dix-Neuvieme Session Alger, 131-137 (1952). (1954): On the rodingitic rocks within the Serpentinite masses of Hokkaido, Hokkaido Univ. Fac. Sci., 8, 419-430,