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The geology of agate deposits / Геология месторождений агата
“Agate” is defined by the American Geological Institute Glossary (1960) “as a kind of silica consisting mainly of chalcedony in variegated bands or other patterns commonly occupying vugs in volcanic and some other rocks.” Dana’s Textbook of Mineralogy (1898) defines agate as “a variegated chalcedony. The colors are either (a) banded; or (b) irregularly clouded; or (c) due to visible impurities as in moss agate, which has brown moss-like or dendritic forms, as of manganese oxide, distributed through the mass. The bands are delicate parallel lines, of white, pale, and dark brown, bluish, and other shades; they are sometimes straight, more often waving, or zigzag, and occasionally concentric circular. The bands are the edges of layers of deposition, the agates having been formed by a deposit of silica from solutions intermittently supplied, in irregular cavities in rocks, and deriving their concentric waving courses from the irregularities of the walls of the cavity. The layers differ in porosity…” Agate is considered a semi-precious gemstone, prized by collectors. I have tried to describe each agate province according to plate tectonics, the stratigraphy, the “agatization” process, secondary mineralization, hydrothermal activity, “agate” stratigraphy, and agate types. The physical and chemical characteristics of agate deposits are described and correlated with environments and conditions of deposition. I have also included related silica deposits as well, including opal, chert, petrified wood, geodes, and thundereggs. I have selected examples and illustrations, geologic stratigraphic columns and maps, and photographs of petrographic thin sections in order to emphasize structural, chemical, and temporal controls. Accordingly, I have included plate tectonic–lithotectonic relationships in order to better explain and interpret the volcanic processes involved in agate deposits. <...>