Location
Ametista do Sul, Alto Uruguai region, Rio Grande do Sul, South Region, Brazil
Brazilian Agate
There are at least 15 major Agate producing areas in Brazil. Twelve of these areas are situated in the five southern-eastern most States of the country, Rio Grande do Sul immediately North of Uruguay being the best known. Additional important Agate producing areas are in the States of Santa Catharina, Parana, Sao Paolo, and Minas Gerais. Two Agate producing areas are in the States of Bahia in East Central Brazil, and one other in state is Roaraima in Brazils extreme Northwest.
Agate Information
Silicon dioxide in its crystalline form is called Quartz, with the chemical formula of Si02. Chalcedony is microcrystalline quartz made up of twisted crystal fibres. Agate is banded chalcedony, but Agate may also contain the polymorphs crystalline Quartz, moganite (which forms on the outside of the Agate), and common opal. Agate has a microcrystalline structure composed of microscopically sized crystals. Most Agates contain water (unlike Quartz which is anhydrous), chemically bound with the silicon dioxide, but this amount is a small percentage and far less than in, say, common opal, which is a form of low temperature Quartz, and has no recognised crystal structure (i.e. it is amorphous).
Formation of Agates
Agate occurs as a filling within gas cavities and veins in certain lavas or as a replacement mineral within some sedimentary limestones and clay-stones. It is most commonly found within volcanic lavas, in particular andesites, some rhyolites and tholeiitic basalts. The most explosive volcanoes produce ashes or tuffs- a mixture of ash, lava and sediments torn from the surrounding rock by the eruption.
All these varieties of lavas are very poor in free silicon dioxide, and the fact that Agates are composed of silica has resulted in several theories being generated of where the missing silica originally came from. One of the most likely theories is that micro-shards of silica in glass explosively produced ashes and tuffs, devitrify and eventually release silica in the form of a watery gel that permeates through into underlying lavas via meteoric (atmospheric or rain) waters. Meteoric waters heated at depth, rise and dissolve silica en route to the surface. Other suggested sources of silica include decomposing vegetation or animal matter within the sediments deposited between eruptions
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