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Article: "Skunk" calcite: mineral paragenesis in an amethyst geode from Ametista, Rio Grande do Sul, Brazil.
- Article from:
- The Mineralogical Record
- Article date:
- March 1, 2009
- Author:
CopyrightCOPYRIGHT 2009 The Mineralogical, Inc. This material is published under license from the publisher through the Gale Group, Farmington Hills, Michigan. All inquiries regarding rights should be directed to the Gale Group. (Hide copyright information)
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The famous amethyst geodes of Brazil and Uruguay rarely contain crystals of "skunk" calcite, wherein microcrystals of goethite form sharp, narrow black bands on rhombohedral calcite faces. Close study reveals the probable paragenetic relationships among goethite, lepidocrocite, calcite and two generations of amethystine quartz in a "skunk" calcite specimen; important uncertainties, however, remain.
INTRODUCTION
Amethyst-lined geodes occur in great numbers at certain horizons in Lower Cretaceous basalts in the Parana Basin. Such geodes, which reach weights of several hundred kilograms, are mined at numerous localities in Brazil's southernmost state, Rio ...