RARE EUHEDRAL CALCITE CRYSTALS IN THE AMETHYST GEODES OF BRAZIL AND URUGUAY

The tholeiitic basaltic flows of the Serra Geral Formation in Southern Brazil, Uruguay and Argentina are noted for their large (up to 4 meters), usually elongated, amethyst geodes resulting from the formation of a cavity by degassing of the flow and the subsequent infilling by dominantly siliceous minerals. The typical infilling is agate, colorless quartz crystals and then amethystine quartz. Celadonite may or may not be present as the first mineral in the sequence. Calcite formed throughout the whole crystallization sequence, but is spectacularly seen as late euhedral crystals in a small number of geodes. This collection has over 750 samples of calcite-baring geodes, with over 50 pieces being cathedral-style. Generally the calcites are yellow to colorless, though some show colored overgrowths, and some are green being inter-grown with the celadonite. Most are multi-generational crystals with up to 6 separate growth periods seen, and some reach over 25 cm in length. Crystal habits range from flat rhombohedra, elongate scalenohedra, long prismatic, to rare pseudocubic and pseudo-octahedra forms. Many twins varieties, scepters, phantoms, bowties and other unusual forms are also seen. Detailed information on the collection will be available in a book published to coincide with the Denver 2014 show.