amygdule
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a·myg·dule
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amygdule
Secondary deposit of minerals found in a rounded, elongated, or almond-shaped cavity in volcanic rock. The cavities (vesicles) were created by the expansion of gas bubbles or steam within lava. Because gas bubbles tend to rise through the lava, amygdules are most common near the tops of flows. Many minerals have been found as amygdules, including some spectacular museum specimens of zeolites.
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A rounded mass of mineral formed in a gas cavity in a volcanic rock, a rock that solidified before all the gas bubbled out.
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Amygdule
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For part of the human brain, see Amygdala.
Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava (or other extrusive igneous rocks) are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Amygdules usually form after the rock has been emplaced, and are often associated with low-temperature alteration. Amygdules may often be concentrically zoned. Rocks containing amygdules can be described as amygdaloidal.
The word is derived from the Latin (amygdala) and Greek (αμυγδαλή) words for almond, reflecting the typical shape of an infilled vesicle.
"Amygdule" is more common in US usage, while "amygdale" is more common in British usage.
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American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
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Britannica Concise Encyclopedia
Britannica Concise Encyclopedia. © 1994-2012 Encyclopædia Britannica, Inc. All rights reserved. Read more
Peterson Field Guide to Rocks & Minerals
Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
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