Many dikes varying from monzodiorite to quartz monzonite occur in limestone at Fuka, Okayama Prefecture. From the modes of occurrence and chemical compositions of the dike rocks, it is likely that the rocks are products of the same magmatic origin and intruded into the limestone in the following two stages. In the first stage, a monzodiorite dike intruded and formed gehlenite-spurrite skarns as high-temperature metasomatic products. The dike is composed mainly of orthopyroxene, clinopyroxene, and plagioclase. The metasomatic temperatures using the two-pyroxene geothermometer range from 860°C to 930°C, which is the highest skarnization temperature among the skarns reported in the world. In the second stage, a melanocratic quartz monzonite dike first intruded, and formed the same skarns as the monzodiorite dike. The dike is composed mainly of plagioclase, K-feldspar, quartz, biotite, clinopyroxene, and hornblende. The geothermometric temperatures using coexisting hornblendes range from 690°C to 800°C. In succession, a leucocratic quartz monzonite dike intruded, and formed low temperature skarns containing Ca-bearing minerals such as garnet, vesuvianite, and wollastonite.
The monzodiorite dike characteristically contains large amounts of biotite and pyroxene hornfelses as xenoliths. The pyroxene hornfels is composed mainly of olivine, orthopyroxene, clinopyroxene, and plagioclase. For the pyroxene hornfels, the metamorphic temperatures ranging from 900°C to 1090°C were estimated from the two-pyroxene geothermometer. The pyroxene hornfels would, therefore, be formed by partial melting of biotite hornfels before high temperature skarnization.

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K-Ar ages were determined for the Shitara volcanic rocks, which belong to the middle Miocene Setouchi volcanic rocks in southwestern Japan arc. The Shitara volcanic rocks are mainly composed of alkaline volcanic rocks, whereas high-Mg andesites (HMAs), which are the main constituents of the Setouchi volcanic rocks of other districts, were not observed. The samples for K-Ar age determination were selected from intrusive rocks that belong to the latest magmatic activity in this district. The obtained ages range from 13.6±0.4 Ma to 12.6±0.4 Ma, and these ages are similar to those of the other districts. These indicate that the magma activity in the Shitara district ceased at about 13 Ma, and the origin of the Shitara volcanic rocks is closely related to that of the HMAs, although the petrological characteristics of the Shitara volcanic rocks are different from those of the HMAs.

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Crystal chemistry and structure of garnets are reviewed with special attention to the cation-cation repulsion across the shared edges of polyhedra. The cation-cation repulsion plays an important role in the stability of garnet structure, and this effect is the most prominent between tetrahedral and dodecahedral cations. The magnitude of the cation-cation repulsion is closely related to the degree of polyhedral distortion, based on the difference in length between the shared and the unshared edges in polyhedra, due to the structural geometric restriction. Especially, the cation-cation repulsion increases in garnet solid solutions owing to the decrease in the shielding effect when the structural geometric restriction brought by the substitution of the cations with different sizes forcibly stretches the shared edges compared with the unshared ones. This is a factor to destabilize the crystal structure. Both the tendency of cation distribution in Y₃Fe₅O₁₂ (YIG)-Y₃Ga₅O₁₂ (YGG) system and the symmetry in Mg₃Al₂Si₃O₁₂-MgSiO₃ system change with the compositional variations in these solid solutions. These changes are the good examples of the destabilization of the crystal structures due to the effect of the cation-cation repulsion.

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The theme of the 2001 symposium of the mineralogical society of Japan is silica minerals. Silica minerals are important in earth sciences and materials science. We chose mimeral species as the symposium theme because many fields of mineralogists can participate in discussion. The research fields related to the symposium are mineralogy, earth and planetary sciences, materials science, environmental sciences and biology.

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Except for a high pressure polymorph,silicas crystallize with the frameworks of corner-linked SiO₄ tetrahedra. Electrostatic valence at an atomic site is fully compensated by its neighbor atoms with opposite charges, and therefore, no foreign cation can enter into the frameworks. This is probably the cause of negligible long-range interactions in the silica structures. Many topologically different linkages are possible in the absence of the long-range interactions. As the result, we have silicas in more than 20 different phases. We review the recent structural problems in the low-pressure polymorphs of silicas, ie. quartz, tridymite and cristobalite. Silica clathrates are also introduced, stressing their potential importance in material sciences.

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The statically recrystallized quartz grains, formed by the heating of the intrusion, show several characteristic microstructures associated with the grain boundary migration. Most of the new grains were formed by the strain-induced migration (bulging) of the original grain boundaries. The new grains contain much fewer fluid inclusions than the old ones, and the boundaries of the new grains contain fluid inclusion arrays. Thus the migrating boundaries result in a change in distribution of the inclusions, may make the picture even more complicated, such as “left over grains” and a cloud of fluid inclusions.

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Silica minerals are poorly encountered in meteorites and the terrestrial planets. Especially, primitive meteorites, except enstatite chondrites, rarely contain silica minerals. This mainly reflects the solar elemental abundance that Mg is more abundant than Si. Unless the extensive magmatic fractionation took place for long time, silica minerals did not become common minerals in the planets. In spite of their rare occurrences, the polymorphs of silica minerals are significant to estimate the physical conditions of the formation processes of meteorites and planetary rocks.

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Amorphous SiO₂ such as bulk silica glasses and thin films have been one of key materials in modern optoelectronic industries. They are currently used in communication technologies as an optical fiber, fiber grating and fiber amplifier, thin films for electrical insulation in the dynamic random access memory (DRAM), and optical lenses and photo-masks in the excimer laser lithography. The property essential for the applications is its wide band gap amounting ∼9eV. However, bulk silica glasses commercially available and silica thin films do show photo-responses to sub-band gap lights like KrF emission and unexpected trapping of charges, and the behavior has strong dependency on the preparation histoty.
A great number of studies were carried out to clarify the relation between the properties and structural imperfections in the materials and formation mechanisms of the defects. There are two categories of the imperfections: one is dopant- or impurity-related imperfections and another is non-stoichiometry related defects. These defects constitute gap states in a-SiO₂. The structural identification was usually done by absorption- and emission-spectroscopy in visible-ultraviolet region and electron spin resonance (ESR). The experimentally proposed models were compared with the predictions by theoretical calculation of energy levels.
Recent development of excimer laser lithography technique led us to recognition that a latent member, which has been unnoticed because of no response to the optical absorption or emission in the visible-uv range and ESR absorption, exists in the family of active centers in a-SiO₂, that is a strained Si-O-Si bond originated from the planar three membered ring. In contrast, the puckered four membered ring is unstrained. Although it has been pointed out that there was a wide distribution in Si-O-Si bond angle from 90° to 180° by X-ray analysis or ²⁹Si solid state NMR, the physical and chemical responses of the Si-O-Si bonds with a particular bond angle could not be differrentiated.
Very recently it was clarified that a strained Si-O-Si bonds, in other words chemically excited bonds, has an optical absorption locating on the band edge. The excited bond can be scavenged by fluorine doping, because it is chemically reactive. In the present review we hope to show the unresolved optical and electric responses of silica glasses can be comprehensively understood by taking the presence of the strained bonds into consideration.

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Quartz is one of the most familiar minerals on the earth crust. It has been used as an ornament and for religious belief because of the beautiful shape and the transparency since ancient times. Nowadays, quartz is used for frequency-control devises from the stability. The major applications are watches, personal computers, mobile communication systems and so on. The world's annual production of quartz crystal is estimated at around 2500-3000×10³kg. The production in Japan amounts to 900×10³ kg, which accounts for one-third in the world. Japan is also ahead on the quartz devise technology, especially, for small and high precision devises. The uses and characteristics of quartz devises, the outline of synthetic quartz growth technique and the quality are introduced in this paper.

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At the earth's surface, divers organic-inorganic interactions are occurring and controlling the biogeochemical cycling. In this paper, we will review the experimental results on silica-organic interactions in aqueous solutions. The kinetics of dehydration-crystallization of hydrated amorphous silica into more crystalline forms during the diagenesis of siliceous marine sediments was studied and the influence of some biogenic organic compounds was evaluated. The transformation of amino acids on hydrous silica was also studied in order to test a new hypothesis on the origin of life: coupled dehydration-polymerization in silica-amino acid systems. The significance of organic-inorganic interactions in the fate of organic pollutants in the earth's surface will also be pointed out.

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Biomats (microbial mats) have been studied to elucidate and initiation of life and its evolution. Biomats are complicated architecture consisting of microbes and minerals. A microbe needs a suitable adjustment to severe working conditions to survive. In case of this study, in silicified biomats, Si-biomineralization work under strong acidic conditions. Here, a model of interaction cycles between Si and microbes in biomats is proposed, based on observations of modern Si-biomineralization, associated with red algae, Cyanidium caldarium-dominated silicified mats.

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