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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.]
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5 mm in length) arranged in fan-shaped or radial divergent aggregates (can be completely closed taking on a spherical appearance) as shown in Figures 6i and 6j However, it also occurs as acicular to elongated tabular crystals (up to 650 [micro]m in length) developing radial aggregates Figure 6k illustrates an example of zeotypes' spatial distribution in amygdules, thomsonite displaying the typical radial ibrous aggregates in the other part of the cavity wall, followed by natrolite occurring as fine fibers forming clusters sometimes growing on typical trapezohedral analcime crystals growing in the centre of the cavity Needle-like and pointed natrolite crystals (up to 2 mm in length) are wildly dispersed among pseudocubic chabazite crystals (Figure 6l).
Viljoen and Viljoen (1969) recognized that many high-Mg rocks in the Barberton greenstone belt were lava flows of significant lateral extent and thickness, with chilled of brecciated tops, amygdules, pillows and the distinctive quench textures that were subsequently named "spinifex" (after an Australian spiky grass--Triodia spinifex).
The MFU, [less than or equal to] 150 m thick, consists of numerous thin flows ([less than or equal to] 10-15) of fine-grained basalt with abundant amygdules arranged in a consistent zonal pattern (Aubele et al.