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Students of geology are introduced to in their petrology course, (or used to be), to the terms "acidic", "basic", and the associated term "intermediate" in relation to %$SiО­­­_{2}$ in igneous rocks, where, depending on whom you read:

  • Acidic: rocks with above 63%/65% silica (mostly feldspar minerals and quartz), e.g. granite.
  • Intermediate: rocks with 50%/55% - 63%/65% silica, e.g. syenite, trachyte.
  • Basic: rocks that are about 45 to 55% silica (mostly mafic minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.

Of course, if they go on to work with chemists in, say, an environmental geology practice, this causes no small amount of contention and sometimes confusion between the geo and the chemist, who, of course, understands the terms "acidic", etc. in the classic chemical sense, of $pH = -log [H^+]$, etc.

How did the terms "acidic" and "basic" come to be associated with %$SiO_2$ in igneous rocks, in contrast to the classical chemical definition?

Note to Responders: Apologies for my ignorance. It's been over 30 years since I studied and worked in (exploration) geology. So my knowledge of current terminology is a little rusty, especially since my major experience has been that of an geologist transmogrified into an electronic engineer.

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    $\begingroup$ I will add the "acidity" and "basicity" has no meaning when you're talking about igneous rocks. pH is a measure of the proton activity in aqueous fluids, and is most commonly understood in standard conditions (1 atm, 25 degrees). As you start heating your fluid, pH (while still exists) becomes less important as a measure to describe the properties of the fluid. When you're discussing really high T silicate liquids, pH has no meaning. Hydrogen is not dissociated at these temperatures. $\endgroup$
    – Gimelist
    Jul 13, 2017 at 10:08

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The idea came from the theory that silicic acid was the chief form of silicon occurring in rocks. Early attempts to classify minerals, placed some mineral specimens in groups based acid-base reactions and the hypothetical acid that mineral was derived. Sulfates, phosphates, nitrates , and tungstates etc.

This thinking was the result of 19th century chemistry research which attempted to explain chemical reactions in terms of acid-base reactions.

Much of this terminology is still in use today even though it is considered archaic now.

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  • $\begingroup$ Just a comment to add the modern terminology: we now use "mafic" for "basic" and "felsic" for "acid". These terms are based on modal minerals, ferromagnesian for the first (hence ma-ferric), feldspar and quartz for the second (hence fel-si). I recommend the reading of "Igneous Rocks: A Classification and Glossary of Terms" ( doi.org/10.1017/CBO9780511535581). $\endgroup$
    – Jean-Marie Prival
    Dec 10, 2019 at 9:34

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