Various micro-textures seen in geode agates in volcanic rocks have been investigated at a level of nanometer scale, by correlating optical and electron microscopic observations. It has been shown that optically observable individual fibers in agate bands are composed of much finer fibers in which quartz crystallites, 8 to 100 nm in length, are aligned parallel to <11-20> or <10-10>, with the c-axes perpendicular to the fiber elongation. Both uniformly spaced systematic striations and “Runzelbänderung” in agate bands, and coarse quartz or amethyst crystals (CQ) radiating inward to open space of a geode were found to have essentially the same texture as that of ordinary agate bands, and were formed when growth conditions were stabilized. CQ represents the latest stage of formation of agate bands. In contrast, strata-form horizontal banding, i.e. Uruguay band, was found to consist of only euhedral quartz grains of 0.5 to 4 μm across, or only spherulites with diameter of up to 100 μm or both. They were precipitated due to gravity after the formation of agate bands and coarse quartz crystals. The spherulites in Uruguay bands consist of core and rim portions, and have different characteristics from those observed in agate bands. Radiating quartz crystals in the rim portion of spherulites in Uruguay bands are of length-slow type, whereas those in agate bands of length-fast type.
Based on these observations, it was suggested that quartz crystallites both in agate and Uruguay bands were precipitated from a hydrothermal solution invaded into a geode, in which polymerized embryonic particles with quartz structure attaining a size of the order of 10 nm were already present. Electrophoresis and charge polarity of such particles were suggested as a plausible cause to accelerate their agglutination in the form of fibers in agate bands. The embryonic particles which grew larger than a critical size above which electrophoresis became less effective remained in the solution in suspension and later precipitated as Uruguay bands. This model also gives an explanation to the ubiquitous occurrence of Brazil twinning in quartz crystals of agate and Uruguay bands.

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Microprobe analysis of ribbeite from the Kaso mine give SiO₂ 24.97, MnO 72.01, MgO 0.76, CaO 0.09, total 97.83%, yielding the anhydrous empirical formula: (Mn_4.89Mg_0.09Ca_0.01)_Σ4.99Si₂O_8.98 (basis: Si=2), closer to the theoretical formula than the original magnesian material. The X-ray powder pattern is indexed on an orthorhombic cell with a=10.768Å, b=15.800Å, c=4.832Å. It occurs as light reddish to purplish brown masses included in light grey tephroite layers accompanied by finely bedded rhodonite-quartz ore from Mukaiyama orebody of the Kaso mine. The ore retains the original fine bedded texture despite thermal metamorphism due to nearby or underlying acidic intrusive bodies. The other associates include sonolite, rhodochrosite and hausmannite, the last being apparently absent in Honkô orebody of the mine, where any primary Mn³⁺-bearing minerals has not been found to date. This is probably due to the difference in the original constituents of ores therefrom.

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Single crystals of two plagioclases, labradorite and bytownite, were hydrothermally treated at 200°C and 15 bar with changing HCl concentration of solution (M_HCl=1.0∼0.001). The reaction products were investigated by analytical scanning electron microscopy. The reaction product of the labradorite in 1.0 M_HCl solution was silica and those in 0.1, 0.01 and 0.001 M_HCl solutions were aluminosilicate with Si/Al=6.2, 0.38, 0.10, respectively. Those of bytownite in 1.0, 0.1, 0.01 and 0.001 M_HCl solutions were aluminosilicate with Si/Al=1.7, 0.58, 0.25 and 0.11, respectively. The difference of Si/Al between reaction products of those two specimens which were treated in the same condition may be due to the difference of the microtexture in the specimens.

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