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o Hydrothermal environment associated with mafic volcanism. (chapter 12) - Large tubular geode from Rio Grande do Sul, Brazil. Crystals face central void and grade to clear quartz toward agate substrate (gray) of geode margin. Long dimension of geode is 24 cm but this is a section normal to the longest direction of the geode tube. Broadening of amethyst crystals due to Brazil twinning on {010} and {011} with a {110} composition surface. 
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