basalt - hand/display specimen of amygdaloidal basalt
Basalt is commonly vesicular, filled with empty bubbles that form when dissolved gases and water come out of solution as the pressure drops in a magma during an eruption, much as bubbles form in a shaken soda when the can is opened and the pressure is released.
Basalt is porous and if hydrothermal fluids pass through the flow, the vesicles can be filled with quartz, calcite or any of a number of zeolite minerals. The filled vesicles are called amygdales. A basalt with amygdales is amygdaloidal.
Amygdale's etymology is interesting. Amygdale is Greek for almond. In 15th century English, amygdales meant "the tonsils." The amygdales in this basalt are only almond shaped if you use a lot of imagination.
Zeolites are microporous aluminosilicates. The name is from the Greek zeo meaning to boil, and lithos, meaning stone. In 1756, Swedish mineralogist Axel Fredrik Cronstedt coined the name when he observed that strongly heating a zeolite with a blowpipe produced large amounts of steam from water it had absorbed. The mineralogy of the amygdales in this basalt is undetermined.
This basalt is from a Neogene (Quaternary) flow at the northern end of the Coso Range in Inyo County, California. As you can see from the QAPF diagram, the compositions of basalt and andesite can overlap and both are variable within the yellow polygon in the diagram.
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