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I understand that basalt is darker in color and has less silica but do not understand how these reflect the underlying minerals/mineraloids (if mineraloids are even present in these rocks)???

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The technical difference is simply in the bulk percentage of silica, Basalts are relatively low in silica, less than 52% in total, the rest of it being composed primarily of iron and magnesium oxides which makes it quite low viscosity compared to other lavas. Andesite lavas contain from 57 to 63% silica by weight and are thus more viscous as they contain a smaller percentage of other rock forming oxides. The changes in viscosity often reflect differences in dominant mineralogy but primarily are due to the higher eutectic temperature of the more pure silica. There is also the fact that andesites contain more volatiles (particularly water and flourine) that evaporate near or at the surface taking heat out of the melt and freezing the flow, this often results in explosive eruptions. Rhyolites tend to be even wetter and are even higher in silica and viscosity resulting in very few lava flows and many extremely explosive eruptions.

The colour difference is due to the higher percentage of dark iron/magnesium oxides and silicates in basalt versus andesites' more silica rich chemistry.

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The first mineralogical difference is that hardly any quartz is found in basalt, but some amount of quartz is found in andesite.

Another distinction is that basalt has higher concentrations of magnesium/iron, thus presents olivine, whiles andesite has more amphiboles.

https://en.wikipedia.org/wiki/Amphibole#/media/File:Mineralogy_igneous_rocks_EN.svg this picture might help.

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  • $\begingroup$ Hmmm I have also read however that basalt doesn’t always have olivine $\endgroup$
    – Harrychink
    Commented Jul 6 at 23:36

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