Recent progress in the studies on rock-forming minerals are reviewed from methodological viewpoints. One of the greatest progress in the studies on rock-forming minerals is the promoting of a better understanding of the solid solutions which is one of the essential properties of the minerals formed in the multi-component system. The progress depends on the methodologies which were established about the research on the solid solutions in the 1960s. They are mainly the thermodynamical approach to the complicated solid solutions, the improvement in the method of phase equilibrium experiments under high pressure and high temperature and also the development of analytical method on the fmetexture or microtexture which are frequently observed in rock-forming minerals. The progress mentioned above is beginning to lead to the study of the mineral behaviour, which is useful for understanding of the geological process that the minerals have undergone. It is necessary to consider the following tasks as further subjects in studies on rock-forming minerals; i. e., to find out new finetexture or microtexture in the rock-forming minerals, to characterize the minerals formed under several geochemical processes and also to systematize them with the object of the discovering historical law of the earth governing the formation of the minerals.

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Clinopyroxene in garnet Iherzolite from Alpe Arami, Bellinzona, Switzerland has been re-equilibrated at two different stages during its emplacement. The solubility of Al, Cr, Na and Fe atoms in clinopyroxene decreases systematically throughout the re-equilibration, while that of Ca and Mg atoms increases. Primary crystals of clinopyroxene rarely remain, and are surrounded by recrystallized olivine, orthopyroxene, clinopyroxene and Mg-hornblende. The primary crystals have optically visible broad clinoamphibole lamellae parallel to (100). Electron microprobe analyses show that the marginal part of clinopyroxene is depleted in Al, Cr, Na and Fe content due to diffusion of atoms while attaining the re-equilibration. The core of the crystal, in most part, has undergone the same chemical depletions to form the broad clinoamphibole lamellae. Primary chemical composition only occurs in restricted areas sufficiently far away from the clinoamphibole lamellae to be free from detectable chemical depletion by electron microprobe analysis. Even in such areas, X-ray and electron microscopic observations reveal that clinopyroxene has re-equilibrated in sub-microscopic domain size. This re-equilibration texture forms intercalation of thin clinoamphibole lamellae (generally up to 100 A thick), having a coherent interface to the host, and related to planar defects parallel to (010). This suggests that the fluctuation of chemical composition is generally limited to a scale from the unit-cell size to several hundred angstroms.

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Fine textures and fine structures of clinopyroxenes altered by metasomatism from the Akatani ore deposit were observed by high resolution electron microscope. Clinopyroxenes transformed into triple and double chain silicates (fibrous amphibole-like aggregates), and then to sheet structure (talc) during the metasomatism. Various kinds of fine textures and fine structures were observed in weakly altered clinopyroxenes and other biopyriboles. From the observations, the knowledge of alteration of clinopyroxene in metasomatism was renewed; that is, the intermediate products in the metasomatism (so called "fibrous amphibole") are the mixture of multistage products (double and triple chain structures, disordered structures of pyriboles, local superstructures and sheet structures) in local and nonequilihrium reaction.

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Phase equilibria in the join CaMgSi₂O₆-CaFe³⁺AlSiO₆-CaTiAl₂O₆ have been determined in air at 1 atm by the ordinary quenching method. At subsolidus temperatures the single phase field of clinopyroxene extends up to 19 wt. %. Even in the field of clinopyroxene + perovskite, the TiO₂ content in clinopyroxene continues to increase and attains 9.2 wt. % TiO₂ with 24.8 wt. % Al₂O₃. An interesting fact is that unusual clinopyroxenes which contain more A1IV than Silv are present in the CaFeAlSi0₆-rich region. At subliquidus temperatures the liquid is richer in Ti, Al, and Fe³⁺ than the coexisting pyroxene. The clinopyroxenes coexisting with liquid contain less TiO₂, A1₂0₃ and Fe₂0₃ than those crystallized at subsolidus temperatures. The system CaMgSi₂O₆-CaFe³⁺AlSiO₆-CaTiAl₂0₆ is relevant in evaluating the crystallization trend of clinopyroxenes in udersaturated alkalic rocks.

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The compositions of amphiboles of four samples which were collected from the Mt. Poroshiri area of the Western Meta-ophiolite zone of the Hidaka metamorphic belt were studied. The four samples (A, B, C & D) show different degrees of recrystallization, i. e. from olivine gabbro to massive amphibolite. The compositions of metamorphic amphiboles are all calciferous but sometimes range from tremolitic to pargasitic amphibole even in the same thin section. Tremolitic amphibole replaces primary clinopyroxene, whereas pargasitic amphibole occurs along the boundary between clinopyroxene and plagioclase or inside plagioclase. Moreover, amphibole with intermediate composition appears between the tremolitic and pargasitic amphiboles. The range of the amphibole composition narrows with the progress of recrystallization, i. e. from A to D. The variation of the amphibole compositions appears to be controlled by different local bulk compositions and by differences in the diffusional process.

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First, a brief review of the studies on staurolite as a metamorphic mineral is given, and the localities and modes of occurrence of staurolite in Japanese Islands are reviewed. And then the problems on the ZnO content in staurolite are discussed, especially dealing with the Mozambique metamorphic rocks in the Machakos area, central Kenya and with the Ryoke metamorphic rocks in the Hazu-Gamagori-Hongusan area, central Japan. (1) In the Machakos area, a characteristic belt consisting of the association of pelitic gneiss and amphibolite occurs continuously more than 20 km, and this pelitic gneiss is composed of kyanite, staurolite, almandine, biotite, muscovite, plagioclase and quartz. The ZnO content of staurolite of this pelitic gneiss reaches 3.79 % in maximum. Its content is, however, considerably variable even in the same horizon. The lower the content of FeO of staurolite is, the higher the content of ZnO becomes. Even in a same rock specimen, its content is remarkably variable. Staurolite inclusion in almandine porphyroblast is poor in ZnO; staurolite inclusion in muscovite crystal is rich in ZnO; poikiloblastic staurolite exhibits intermediate properties. There are several stages of staurolite formation during metamorphism. This fact suggests that the geochemical behaviour of Zn during metamorphism is governed by the petrochemical and petrological environment. (2) In the Hazu-Gamagori-Hongusan area, three regional metamorphic zones, characterized by the presence of andalusite, sillimanite and sillimanite-K-feldspar for each, are recognized, and an additional contact metamorphic zone is also developed around the granite intrusives. Staurolite schists have been found in all the zones except the sillimanite-K-feldspar zone. Staurolite occurs in the four modes. Despite the various modes of occurrence, staurolites in the schists with muscovite have higher Zn/(Fe+Mg+Zn) than those in the schists without muscovite. The difference of Zn content could be ascribed to the difference in advancing degree of the continuous staurolite-breakdown reaction during the progressive metamorphism.

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Zoned clinopyroxene and orthopyroxene found in the hornblende-dolerites of the Shiono-misaki Igneous Complex were examined. In the case of the clinopyroxene, the core part and the mantle part have contrasting Ca-content. In the core, Ca/Ca+Mg+Fe ratio is within the range from 0.42 to 0.39. On the other hand, in the mantle, this ratio is within 0.46 to 0.44. Both the core and the mantle part of the clinopyroxene can be concluded to have coprecipitated with the core and the mantle part of the zoned orthopyroxene, respectively. Such a Ca-enrichment trend of zoned clinopyroxene that have coprecipitated with orthopyroxene can be attributed to the abrupt increase of P_H2O in the magma chamber, and to the resorption of the existing crystal to a certain degree, followed by the formation of new pyroxene being stable under the higher P_H2O and somewhat lowered temperature. Some of the zoned clinopyroxene show idiomorphic shapes. Accordingly, above-mentioned increase of P_H2O was the event not restricted in the trapped liquid but yielded wholly in the magma chamber.

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