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A small gas bubble in igneous, especially volcanic, rock filled with secondary minerals such as zeolite, calcite, or quartz.

[Latin amygdala, almond (from its shape); see amygdala + (nod)ule.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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Natrolite is an extremely rare zeolite at the Fanwood quarry; specimens were discovered on one occasion as jumbles of acicular crystals, no longer than 1 cm, on prehnite in a diapiric amygdule. The crystals are white and exhibit the classic elongated orthorhombic prism with pyramidal terminations.
The principal secondary mineralization is found in the amygdules: gas pockets located in the upper parts of the first, second and third basalt flows and in the adjacent 20 cm of the overlying flows.
However, its position in the succession of other amygdule minerals is uncertain.
Two samples of trachyandesite representing two principal facies: DO-1-1 (fresh material without amygdales) and DO-1-2 (zeolitized sample with numerous amygdules), were taken for petrographic and geochemical studies.
Locally, it contains mineral-filled amygdules less than 1 cm in diameter.