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basalt - hand/display specimen of typical non-vesicular basalt

$ 9.50

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This basalt is from a Tertiary flow east of the summit of Montgomery Pass in Mineral County, Nevada. It lives up to its description as being aphanitic, meaning the crystals are not visible to the naked eye. The roots are a, Greek for not, and phaneros, Greek for visible.

Basalt is rich in magnesium and iron, poor in silica, allowing flows to move and cool rapidly. Silica makes lava sticky, so unlike basalt flows, an andesite magma, with over 50% silica, is reluctant to flow and plugs up volcanic vents, leading to explosive eruptions, in contrast to the basaltic lavas of Hawaii, where tourists can approach a vent with considerably less risk.

The rapid cooling of this basalt makes the crystals of what are most likely plagioclase, pyroxine and olivine, too small to see. Aphanitic, somewhat dense, and black or dark gray are the characters a student would use to make the identification. 

Many basalt flows are vesicular, with large gas bubbles that were trapped when the lava cooled. We occasionally have examples in stock.

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, though andesite would be closer to the quartz side of the polygon. Their similarity can cause a student to identify andesite as basalt, acceptable, since accurate identifications are best made with thing sections and a petrographic microscope, allowing the percentage of quartz to be measured.




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