52. Iron-Rich Cordierite Structurally Close to Indialite by Miyoji SAMBONSUGI Geological Institute, Faculty of Arts And. Science

190 [Vol. 33,

52. Iron-rich Cordierite Structurally Close to Indialite

By Miyoji SAMBONSUGI GeologicalInstitute, Faculty of Arts and. Sciences,Fukushima University (Comm.by S. TsuBOI,M.J.A., April 1.2, 1957)

Introduction In the course of his geological investigation of the Abukuma plateau, northeast Japan, the writer's attention was drawn to numerous pegmatites intruding the granitic and gneissic rocks which form the foundation of this district. In 1950 the writer found a peculiar mineral from one of the above-mentioned pegmatites, at Sugama. From its appearance the mineral was first identified as scapolite by the writer (Sambonsugi, 1953), but after further observations it has become clear that the mineral belongs to an iron-rich variety of cordierite, and that it is structurally close to indialite, hexagonal polymorph of cordierite, first found by Miyashiro and Iiyama (1954) from the fused sediment in the Bakaro coalfield, India. Miyashir0 et al. (1955) suspected that a structural gradation may exist between the hexagonal lattice of indialite and the orthorhombic lattice of cordierite, viz, that there may be some varieties of cordierite structurally close to indialite, though all then known show marked structural difference from indialite. The mineral found by the writer from the Sugama pegmatite is the first example of cordierite structurally close to indialite. It is the purpose of this paper to describe the mode of occurrence, the optical properties, and the chemical composition of the mineral. Recent- ly, the optical properties and the unit cell dimensions of the mineral were studied by T. Iiyama (1956). X-ray and thermal studies of the mineral were carried out by Miyashiro (1957).

Mineralogical Composition of the Pegmatite Near the western margin of the Abukuma plateau, many pegmatites are found in zones which are arranged in echelon and extend to a distance of 120 km in the NS direction. The zone, the Sugama zone as named by the writer, which lies in the center of the area, is characterized by the occurrence of numerous intrusives of pegmatites which contain unusual amounts of aluminous minerals such as sillimanite, andalusite, corundum, etc. (Sambonsugi, 1953). The cordierite now under consideration was found in a pegmatite at Sugama, 6 km north of the Kawahigashi station of the Suigun railroad line. The pegmatite occurs in dike form with a width of 4 m, striking N 30°E and dipping NW30°, in the biotite-hornblende-granodiorite and No. 4) Iron-rich Cordierite Structurally Close to Indialite i 91 associated injection gneiss having a rude foliation with N 20- 30°W strike. Many xenoliths, which are considered to be of sedimentary origin, are enclosed in the granodiorite and gneiss. There develops a zone of clayey materials, 50 cm in width, between the pegmatite dike and adjacent granodiorite. The main constituents of the marginal zone of the pegmatite dike are perthite and quartz which form graphic intergrowth with each other. Minute sericite flakes develop on the perthite surfaces as an alternation product. The central zone of the dike is made up of quartz, plagioclase, and perthite, associating with a little sillimanite, tourmaline, muscovite, pyralspite garnet, allanite, lepidomelane, and cordierite. Most of the associated minerals occur in euhedral form, while feld- spars and cordierite are subhedral, and quartz fills up interstices between other minerals.

Fig. :1 Inclusion of sillimanite needles Fig. 2. Cordierite with parting parallel in cordierite (X90) to 001 and included sillimanite needles parallel to c-axis of the host Garnet grains, generally (;x90) rhombic dodecahedra, occur enclosed in cordierite and muscovite crystals. Some of the grains enclosed in cordierite are irregular in form (Fig. 3). The refractive index n of the garnet is 1.789 - 1.800, and the specific gravity d2°~is 4.094. The chemical composition of the mineral, accord- ing to the personal communication from Miyashiro, is as in Table I, in which is shown the calculation leading to the formula of the Fig. 3. Garnet included in cordierite, mineral. showing irregular outline (X90) 192 M. SAM$ONSUGI [Vol. 33,

Fig. 4. X-ray diffraction pattern of cordierite from the Sugama district

Table I. Chemical composition of pyralspite garnet

From the calculation as shown in the above table, the chemical composition of garnet now in question, may be represented by the following molecular formula: (Fe, Mn, Mg)3(Al, Fe)2S13012 This composition is in accordance with the general formula of garnet: X3 Y2 Z3 012 in which X = Ca, Mg, Fe, Mn, Y = Fe, Al, Cr, Ti, Z = Si. The mineral belongs to the pyralspite group. Sillimanite needles are enclosed in garnet and cordierite, with parallel arrangement. The needles are mostly orientated with their elongation parallel to the c-axis of cordierite. Lepidomelane is included in muscovite, and tourmaline is cut by veinlets of quartz. From these observations the order of crystallization of the minerals in the rock is inferred as follows: No. 4] Iron-rich Cordierite Structurally Close to Indialite 193

Properties of the Cordierite The cordierite is known to occur only in the central zone of the pegmatite from Sugama. It is in large crystals but rarely shows outlines characteristic of its system of crystallization. In marginal portion it is altered to chlorite and mica. Crystal faces of the cordierite are sometimes seen. The crystals, ranging from 10 to 20 cm in length, are deep green in color with a silky lustre on fresh fracture surface. Parting parallel to (001) is developed in the marginal portion of the crystals (Fig. 2). The optical constants, determined by Miyashiro, are as follows: a=1.560 f9=1.574 ry=1.578 (-)2V=55° Its hardness is 4.5, and specific gravity 2.659. The powder X-ray diffraction pattern (Figs. 4 and 5) of indialite has a single peak at about 29°(20), while that of the typical cordierite has two distinct peaks. The iron- rich cordierite, now in question, has a peak with an intermediate character. From the optical properties and X-ray diffraction pattern, it is shown that the mineral is closely related to indialite in its structure. A chemical analysis of the mineral by an analyst of the Geological Survey of Japan, gave the result as in Table II. The material Fig. 5. Partial X-ray diffraction patterns of used for the analysis contained a small amount indialite (a), cordierite of chlorite and mica as impurities, which could from Sugama district (b),di and typical cor- not be removed. erite from Japan (c) From Table II, the composition of the mineral is as follows: (Mg, Fe+°)~.A13(Al,S15)6018, 194 M. SAMBONSUGY [Vol. 33,

Table II. Chemical composition of in which cordierite the iron-rich Mg : Fe+2=1: 2 Al : Si=1: 5 This composition of cordierite is obtainable by replacing some of Mg- component of Mg-cordierite with Fe +2-component. As has been already mentioned, the pegmatite at Sugama is characterized by the presence of a com- paratively large amount of aluminous minerals. This fact suggests that the pegmatitic residual liquid would have been unusually enriched in alumina. It is likely that the granitic magma of the district incorporat- ed with aluminous sediments, such Analyst : Geological Survey of Japan as those represented by xenolithic masses in the schistose biotite-hornblende-granodiorite to produce the alumina-rich pegmatitic liquid.

Table III

Acknowledgements

The writer wishes to express his sincere thanks to Prof. J. Suzuki and Prof. Z. Harada at the Hokkaido University for their kind in- structions. His thanks are due to Prof. K. Omori at Tohoku Uni- versity, Dr. M. Kaneko, Director of the Geological Survey of Japan, and also to Mr. H. Haramura at Tokyo Institute of Technology, for their kindness in undertaking the chemical analyses. No. 4] Iron-rich Cordierite Structurally Close to Indialite 195

The writer is greatly indebted to Dr. A. Miyashiro, University of Tokyo, for the X-ray and optical studies of the cordierite, and for the criticism of the result, and also to Prof. H. Kuno of the same University for critical `reading of the manuscript.

References Ii.yama, T. (1956): Optical properties and unit cell dimensions of cordierite and indialite, Min. Jour., 1, no. 6, 372-394. Miyashiro, A. (1957): Cordierite-indialite relations, Am. Jour. Sci., 255, 43-62. Miyashiro, A., and Iiyama, T. (1954) : A preliminary note on a new mineral, indialite, polymorphic with cordierite, Proc. Japan Acad., 30, 746-751. Miyashiro, A., Iiyama, T., Yamasaki, M., and Miyashiro, T. (1.955): The polymorphism of cordierite and indialite, Am. Jour. Sci., 253, 185-208. Sambonsugi, M. (1953) : On the grani tic pegmatite deposits occurring in western margin of the Abukuma Mountain and its relation to the geological structure (in Japanese), Sci. Rep. Fac. Arts and Sci., Univ. Fukushima, 2, 1-26.