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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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A few of the dykes look to contain amygdules comprising abundant calcite, and small amounts of chlorite, amphibole, quartz, and chalcedony, locally in concentric arrangements.
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).
Botryoidal crusts about 1 mm thick line cavities in the nodules and cement fragments of crandallite in some specimens; some cavities are entirely filled, like amygdules.
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).
Minor grain types include chalcedonic microcrystalline quartz of indeterminate origin but possibly from amygdules in volcanic rock, polycrystalline microlitic and felsitic volcanic rock fragments (in addition to more abundant glassy volcanic rock fragments), equant opaque iron oxide grains that are probably magnetite, internally foliated fragments of metamorphic slate, and rare aggregate epidote grains.
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.
The presence of gonnardite in amygdules is problematic, however, a limited substitution of Ca for Na in natrolite was registered.
Gases from adjoining areas, where solidification has already penetrated more deeply, migrate towards the top half of the diapir and are ultimately frozen in place as a cluster of amygdules.
Sulfides are either confmed to amygdules or disseminated in the basalts.
White and in places red calcite and chlorite fill amygdules in part of the flow (of flows).
1939) described amygdules and geodes in the capping basalts, containing axinite, calcite, danburite, datolite, fluorite, goethite, hematite, pyrite, quartz, schorl and specular hematite.
5 to 3 mm in length set in a groundmass that includes flow-aligned plagioclase microlites and amygdules infilled with chlorite, epidote and calcite (Fig.