Calcian serandite in a magnesioriebeckite-quartz schist from the Mitsuishi district, Hidaka Province, Hokkaido

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1 J. Japan. Assoc. Min. Pets. Econ. Geol. 79, , 1984 Calcian serandite in a magnesioriebeckite-quartz schist from the Mitsuishi district, Hidaka Province, Hokkaido TOSHIRO MORIKIYO Department of Geology, Faculty of Science, Shinshu University, Matsumolo 390, Japan Calcian serandite was found in a magnesioriebeckite-quartz schist from Mitsuishi, Hidaka Province, Hokkaido. The schist is composed of magnesioriebeckite, epidote, calcian serandite, albite, aegirine and quartz. Chemical composition and physical properties of the mineral are presented together with the results of chemical analyses of magnesioriebeckite, pectolite and the host whole rock. The calcian serandite seems to be formed as a primary metamorphic mineral judging from its mode of occurrence. It is assumed that the appearance or disappearance of the mineral in the crystalline schist depends on the chemical composition of the host rock under the condition of the epidote-glaucophane schist facies. This is the first finding of calcian serandite from metamorphic rocks. Introduction Serandite is the manganese analogue of pectolite, and calcian serandite has an interme diate composition between pectolite and seran dite. Pectolite has been already reported as a vein mineral in the epidote-amphibolite from the Mitsuishi district, Hidaka Province, Hok kaido, by Suzuki et al. (1974) and Harada and Hariya (1984). In this district, various kinds of metamor phic rocks are exposed as xenolithic blocks in the serpentinites as shown in Fig. 1. The metamorphic rocks are grouped into the follow ing types: epidote-amphibolite (hornblende+ epidote+albite+chlorite }muscovite }gar net }biotite), greenschist (actinolite+epidote+ albite+chlorite+muscovite }riebeckite } aegirine }stilpnomelane }pumpellyite), magnesioriebeckite-quartz schist (magnesio ribeckite+quartz+epidote+calcian serandite+albite+aegirine+magnetite), metagabbro (chlorite+albite }prehnite }pumpellyite). From the magnesioriebeckite-quartz schist, I found calcian serandite. In this paper, the mode of occurrence, physical properties and chemistry of the mineral are presented. Description of the rock samples Calcian serandite-bearing specimen was taken from a block on mountain slope 3km north-northeast of Mitsuishi (1 in Fig. 1). The rock is dark green and very fine-grained. It is rather homogeneous, and schistosity or bedding is not recognized. Original texture such as clastic or igneous one is not observed, but it seems to be a kind of ferruginous siliceous sedimentary rock. Under the microscope, the rock is composed of magnesioriebeckite, quartz, albite, aegirine and calcian serandite with a subordinate amounts of epidote, magnet ite and clay mineral. Magnesioriebeckite is a small acicular and columnar crystal with the size less than 0.01 ~0.06mm. Its color is vivid bule-green to violet. Calcian serandite is an euhedral and prismatic crystal, and is scattered (Manuscript received July 9, 1984)

2 504 Toshiro Morikiyo Fig. 2. Photomicrograph of calcian serandite from specimen in plane light. Bar: 0.1mm. Cs, calcian serandite; Mr, magnesioriebeckite; Qz, quartz. Fig. 1. Geological map of the Mitsuishi district and localities of the specimens studied. 1: calcian serandite-bearing rock (speci men ). 2: pectolite-bearing rock (specimen ). Compiled after Ishibashi (1939) and Mori kiyo (1979). in the fine-grained matrix composed of magnesioriebeckite, quartz and albite as seen in Fig. 2. It is colorless and average grain size is 0.16 ~0.04mm. Many tiny inclusions of low index material, which can not be identified, are contained, and the periphery of crystal is some times altered to brown clayey minerals. Epidote is present but is a very small amount. Quartz is abundant and shows a saccharoidal texture. These minerals do not show any reac tion texture among each other, except for between calcian serandite and clayey minerals. Therefore, magnesioriebeckite, quartz, albite, aegirine and epidote are in equilibrium with calcian serandite during the metamorphism, which seems to be in the epidote-glaucophane schist facies. Pectolite-bearing rock has been found from the quarry at the south of Horai-san (2 in Fig. 1). The rock is a dark green medium-grained epidote-amphibolite and contains large radiat ing aggregates of pectolite. This consists of blue-green hornblende, epidote, albite, chlorite, pectolite, muscovite and pumpellyite with a subordinate amounts of sphene, rutile and apatite. Pectolite is a colorless large poikilo blastic crystal with a length of several centi meters and includes hornblende, albite and pumpellyite. Aggregates of dull yellowish green chloritic mineral have been formed along the periphery. Pumpellyite forms aggregates of wedge-shaped grains and occurs as veinlets. The original mineral assemblage seems to be musc ovite-chlorite-albite-epidote-hornblende, which is for the albite-epidote-amphibolite facies (Morikiyo, 1979). Pectolite, pumpellyite and chloritic mineral would be formed in the later stage of the metamorphism, of which condition seems to be lower than that of the albite-epidote-amphibolite facies. Mineral description and discussion Calcian serandite was separated from the other minerals in the powdered sample by means of the heavy liquids with a centrifuge. Densities of the heavy liquids were measured with the specific gravity indicators, and the

3 Calcian serandite from Mitsuishi district 505 Table 1. X-ray diffraction data 1: Pectolite (Schaller, 1955). 2: Pectolite from Mitsuishi (specimen ). 3: Manganoan pectolite (Schaller, 1955). 4: Calcian serandite (Schaller, 1955). 5: Calcian serandite from Mitsuishi (specimen ). s: strong, ms: moderately strong, mw: moderately weak. Experimental condition: CuEa 40kv, 20mA, scanning speed 0.5 /min. densitiy of calcian serandite was measured to ite. Mn/Ca ratio of calcian serandite exceeds be 3.15 } There is no compositional difference from X-ray powder data of calcian serandite and pectolite are presented in Table 1 together with the data of Schaller (1955). The data for pectolite is in good accord with Schaller's data, and that of calcian serandite from this district shows an intermediate value between man ganoan pectolite and calcian serandite by Schaller. Refractive indices of calcian serandite were determined to be ƒ =1.641, ƒà=1.648, ƒá = Vz is about 48 and the elongation is nearly parallel to Z and extinction angle is usually small. Calcian serandite and magnesioriebeckite were chemically analysed as shown in Table 2. The analytical result of pectolite from locality 2 in Fig. 1 is also presented in Table 2. Atomic ratios of the minerals were calculated on the anhydrous basis of O=17 for calcian serandite and pectolite, and O=23 for magnesioriebeck grain to grain. Whether a crystal is chemi cally zoned or not could not be determined because of the small size. Pectolite is almost free from MnO and is very close to the end member of pectolite. ƒâd value of this pectolite has been reported to be -314% by Kuroda et al. (1979). Pectolite usually occurs as a hydrothermal mineral in serpentinites and in dolerites. It is also reported in alkaline igneous rocks and in kimberlites. However, it has rarely been re ported from metamorphic rocks except for some calcium-rich rocks and skarns (Deer et al., 1978). Occurrence of calcian serandite and serandite is rare and they have been found mainly from alkaline igneous rocks, such as nephelin syenite in Greenland (Winther, 1901), phonolite in Queensland, Australia (Carr and Phillips, 1976), lujavrite in Lovozero, Kola pen

4 506 Toshiro Morikiyo Table 2. Chemical analyses * EPMA analyses. ** Calculated based on the assumption that Fe+3/Fe+2=1. Fig. 3. A(Al+Fe+3-3K)-C(Ca)-F(Fe+2+Mn+ Mg)-N(Na) diagram. Abbreviation for minerals: Pect, pectolite; Cal Ser, calcian serandite; Ser, serandite; Ab, albite; Aeg, aegirine; Rie, riebeckite; Gl, glauco phane; Tr, tremolite; Act, actinolite; Parag, paragonite; Pump, pumpellyite; Ep, epidote; Chl, chlorite. Minerals are indicated by atomic ratios of A-C-F-N. Star, chemical composition of calcian se randite-bearing rock. Triangle: calcian serandite-free riebeckite-quartz schists and riebeck ite-al bite-epidote-actinolite schist. insula (Semenov et al., 1976) and sodalite syenite in St. Hilaire, Canada (Semenov et al., 1976; Philpotts, 1974). Recently, pure seran dite has been reported from the stratiform manganese ore deposits of the Tanohata mine, Japan, and its crystal structure has been deter mined (Takeuchi et al., 1976). Calcian seran dite has not yet been found from metamorphic rocks. Metamorphic minerals, which are common in the epidote-glaucophane schist facies rocks are shown in the A (Al+Fe+3-3K)-C(Ca)- F(Fe+2+Mn+Mg)-N(Na) diagram (Fig.3). Mineral paragenesis of the calcian serandite bearing rock is indicated by the triangular pyramid (Ep-Rie EGl-Ab Aeg-Cal Ser) within the diagram and chemical composition of the host rock is plotted within the pyramid. For comparison, chemical composition of riebeckite-albite-epidote-actinolite schist from Mitsuishi and that of riebeckite-quartz schist from other areas of the Kamuikotan metamorphic belt are also plotted in the dia gram. These rocks are considered to belong to the epidote-glaucophane schist facies and the paragenesis, epidote-albite-riebeckite is stable. Chemical composition of calcian serandite bearing schist is plotted in the N side of the plane Ep-Ab Aeg-Rie Gl, whereas those of calcian serandite-free schists are plotted between A and the plane. From this figure, it is considered that the appearance or disappear ance of calcian serandite depends on the chemi cal composition of the host rock, especially Na content under the condition of the epidote glaucophane schist facies. Acknowledgements: I would like to

5 Calcian serandite from Mitsuishi district 507 Table 3. Chemical analyses of whole 1: Calcian serandite-bearing magnesioriebeckite-quartz schist (Specimen ). 2: Riebeckite-bearing albite-quartz schist from Kamietanbetsu (Suzuki and Suzuki, 1959). 3: Aegirine augite-bearing riebeckite-quartz schist from Kamuikotan (Suzuki and Suzuki, 1959). 4: Aegirine-bearing riebeckite-albite-epidote-actinolite schist from Mitsuishi (Ishibashi, 1937). express my appreciation to Prof. Y. Kuroda of Shinshu University for helpful suggestions and critical reading of the manuscript. I am also grateful to Dr. K. Tazaki of Instiute for Ther mal Spring Research, Okayama University and Dr. M. Inomata of Geological Institute, Tokyo Nogyo Daigaku for their help in the analytical work. References Carr, G.R. and Phillips, E.R. (1976), An occurrence of eudialyte and manganoan pectolite in a phonolite dyke from southeastern Queensland. Mineral. Mag., 40, Deer, W.A., Howie, R.A. and Zussman, J. (1978), Rock-Forming Minerals, Vol. 2A: Single Chain Silicates (2nd ed.). pp. 668, Longman, London. Harada, Z. and Hariya, Y. (1984), Minerals in Hokkaido. pp Geol. Surv. Hokkaido (in Japanese). Ishibashi, M. (1937), On the metamorphic rocks of the Mitsuishi district, Hidaka Province. J. Geol. Soc. Japan, 44, (in Japanese). Ishibashi, M. (1939) On some metamorphic rocks of the Mitsuishi district, Hidaka Province. Bull. Geol. Surv. Hokkaido, No. 10, 7-34 (in Japanese). Kuroda, Y., Suzuoki, T. and Matsuo, S. (1979), The lowest ĉd found in hydrous silicate, pectolite. Nature, 279, Morikiyo, T. (1979), Zonal structure of garnet from the metamorphic rocks in the Mitsuishi dis trict, Hokkaido. J. Assoc. Min. Petr. Econ. Geol., 73, (in Japanese). Philpotts, A.R. (1974), The monteregian province. In The Alkaline Rocks (Soerensen Ed.). John Wiley, New York, Schaller, W.T. (1955), The pectolite-schizolite-se randite series. Amer. Mineral., 40, Semenov, E.L, Maksimyuk, I.E. and Arkangels kaya, V.N. (1976), On the minerals of the pectolite-serandite group. Zap. Vses. Min. Obshch., 104, (in Russian).

6 508 Toshiro Morikiyo Suzuki, J. and Suzuki, Y. (1959), Petrological study of the Kamuikotan metamorphic complex in Hokkaido, Japan. J. Fac. Sci. Hokkaido Univ., Ser. IV, 10, Suzuki, M., Arita, K. and Niida, K. (1974), Guide book for the geological excursion: Mitsuishi - Cape Erimo-Hiroo. Geol. Soc. Japan, 29pp (in Japanese). Takeuchi, Y., Kudoh, Y. and Yamanaka, T. (1976), Crystal chemistry of the serandite-pectolite series and related minerals. Amer. Mineral., 61, Winthr, Chr. (1901), Schizolite, a new mineral. Medd. om Gronland, 24,