' and Fumiko SHIDO**' (Comm. by S

No. 3] 137

30. Finding of Jadeite from the Sanbagawa and Kamuikotan Metamorphic Belts, Japan By YOtarO SEKI*' and Fumiko SHIDO**' (Comm. by S. TsUBoI,M.J.A., March 12, 1959)

Jadeite is an experimentally proved high pressure mineral. Syn- thetic work of Robertson et al. (1957) showed that at room tempera- ture it is stable under pressures higher than about 1.4 kilobars, and that at higher temperatures still higher pressure is required for its ex- istence. Jadeite can coexist stably with quartz under pressures higher than 9 kilobars at room and higher temperatures. Recent studies by de Roever (1955), Bloxam (1956), and McKee (1958) show that jadeite is a mineral characteristic of glaucophanitic metamorphism, which represents high solid pressures. In connection with this, it is thought worthy of note that we found recently jadeite composing rocks of the Sanbagawa and Kamuikotan glaucophanitic metamorphic belts of Japan - a fact hitherto never recorded. Jadeite occurs commonly in various metamorphic rocks developed there, some- times in association with quartz. Jadeites from the Sanbagawa and Kamuikotan metamorphic belts were identified from the optical as well as from the X-ray data: a =1.650 -1.655, $ =1.657 -1.661, r =1.667 -1.673, optically positive, 2V=63 -75°; three characteristic peaks at 20=31.5°, 30.5°, 28.65° in the X-ray powder pattern, at practically identical positions as given by Yoder (1950). Detailed mineralogical and petrographical descriptions of the minerals are to be published elsewhere. Occurrence of jadeite from the Sanbagawa metamorphic belt. In crystalline schists of the Sanbagawa metamorphic belts in Kanto mountains (Seki, 1957 and 1958) and in Sibukawa district (Seki, Aiba, and Kato, 1959a and 1959b), both in central Japan, lawsonite occurs in association with glaucophane in some limited areas within the lower- grade part, which belongs probably to the lawsonite-pumpellyite-epidote- glaucophane subf acies of the glaucophane-schist f acies (Miyashiro and Seki, 1958). Jadeite was first discovered as a constituent of meta-gabbro in a lawsonite-bearing area of the Sibukawa district, which is chiefly composed of jadeite and pumpellyite, with a small quantity of chlorite, lawsonite, analcite, grossularite, quartz (?), and relict augite. Soon later, jadeite was found to occur also in many crystalline schists in the lawsonite- *> Department of Earth Sciences , Saitama University, Urawa. * *) Geological Institute , Faculty of Science, University of Tokyo. 138 Y. SEKIand F. SHIDO [Vol. 35, bearing areas of Kanto mountains and of Sibukawa district. Occurrence of jadeite from the Kamuikotan metamorphic belt. In the gorge of Kamuikotan in central Hokkaido, glaucophane, lawsonite, and pumpellyite occur widely in various crystalline schists, which probably belong to the lawsonite-pumpellyite-epidote-glaucophane subf acies. We found jadeite at many localities in the Kamuikotan gorge, of which the following three are representative: 1) Jadeite occurs in lawsonite-stilpnomelane-chlorite-jadeite- quartz-schist at the south-east foot of the Kamuikotan bridge near the west end of the gorge. 2) Jadeite occurs in a metamorphosed dyke in the western part of Meoto-iwa. The host rock is now mainly composed of relict augite and jadeite with a small quantity of chlorite, calcite, and albite. 3) Jadeite occurs in lawsonite-jadeite-white mica-chlorite-albite- quartz schist in the eastern part of the gorge. Besides, jadeite was found also in some Kamuikotan metamorphic rocks in an area, 8 km northeast of Mituisi-mati, Hidaka, Hokkaido.

References Bloxam,T. W. (1956): Jadeite-bearing metagreywackes in California, Amer. Miner- alogist, 41, 488-496. de Roever, W. P. (1955): Genesis of jadeite by low-grade metamorphism, Amer. Jour. Sci., 253, 283-298. McKee, B. (1958): Jadeite alteration of sedimentary and igneous rocks, Program, 1958 Annual Meetings of the Geol. Soc. America, etc., 108. Miyashiro, A., and Seki, Y. (1958): Mineral assemblagesand subfacies of the glaucophane-schist f acies, Jap. Jour. Geol. Geogr., 29, 199-208. Robertson, E. C., Birch, F., and MacDonald,G. J. F. (1957): Experimental determi- nation of jadeite stability relations to 25,000bars, Amer. Jour. Sci., 255, 115- 137. Seki, Y. (1957): Lawsonite from the eastern part of the Kanto Mountainland, Sci. Rept. Saitama University, ser. B, 2, 363-373. (1958): Glaucophaniticregional metamorphismin the Kanto Mountains, central Japan, Jap. Jour. Geol. Geogr., 29, 233-258. Seki, Y., Aiba, M., and Kato, C. (1959a): Metamorphic zoning of the Sanbagawa crystalline schist terrain in the Sibukawa district, central Japan (in Japanese with English abstract), Jour. Geol. Soc. Japan (in press). (1959b): Edenites from Sanbagawa crystalline schists in the Sibukawa district, central Japan, Jap. Jour. Geol. Geogr. (in press). Yoder, S. (1950): The jadeite problem, Amer. Jour. Sci., 248, 225-248.