amethyst - cavity filling - hand/display specimen from Uruguay
Amethyst quartz SiO2
These amethyst crystal clusters formed as linings in gas bubbles in the flood basalts of the southern Paraná Magmatic Province. Recent 40Ar/39Ar dating shows these basalts erupted over a roughly 2 million year period beginning 135 ± 1 million years ago. The Paraná-Etendeka flood basalts cover an extensive area in southern Brazil and parts of Uruguay, Paraguay and Argentina in South America and in Namibia and Angola in southwestern Africa. Initiation of sea floor spreading, in the late Jurassic, split these flows and resulted in an asymmetrical distribution of these basalts, with the largest extent in southern South America.
This violet to purple variety of quartz owes its color to the presence of traces of iron in its crystal lattice. Natural gamma radiation causes the Fe+3 iron to lose an electron, forming an FeO4 color center which absorbs light in the visible spectrum, causing a material which is normally transparent to become colored.
Heating or ultraviolet radiation destroys the color centers. When heated to 300-400 degrees C, amethyst becomes yellow orange, resembling the quartz variety citrine. Prolonged exposure to sunlight will gradually fade amethyst.
The name amethyst comes from the Greek a (not) and methystos (drunken), since, in antiquity, amethyst was thought to ward off drunkenness. Rhea, mythical daughter of Gaia and Uranus, gave it to Dionysus, god of wine, to preserve his sanity from the vine.
These specimens were collected in the Los Catalanes gemological district at Artigas in northwestern Uruguay. This district, and the Ametista do Sul district in Rio Grande do Sul state in Brazil, are a world class source of giant amethyst geodes. Quartz and amethyst, deposited from hydrothermal fluids, lines cavities in altered basalt lavas in the Arapey Formation in Uruguay and the Serra Geral Formation in Brazil.
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