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.