Abstract
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.
