What does acidic mean in geology ?

Acidic : definition

An acidic rock is an igneous rock containing more than 66% by weight of SiO2, hence the usual presence of quartz in significant quantities, and relatively poor in Mg, Fe and Ca (less than 15%), which is commonly translated by the absence or the low quantity of ferromagnesian minerals (biotite, amphiboles...).

Acidic rocks such as granites and granodiorites constitute a preponderant part of plutonic magmatic rocks, while volcanic acid rocks are rarer (rhyolite, rhyodacite, etc...). Acid rocks are sometimes referred to as supersaturated rocks, to express their high silica content.

The term acidic (unrelated to the activity of H+ ions, measured in terms of pH) goes back to the beginnings of mineralogy, at a time when scientists thought that silicates were real salts derived from the reaction of a hypothetical silicic acid with an equally hypothetical base. We had pushed the reasoning to the point of considering several types of silicic acids : orthosilicic for the peridots, metasilicic for the amphiboles and pyroxenes. In this context, the appearance of quartz rocks, the silicate, required large quantities of silicic acid, hence the term "acidic rocks", as opposed to rocks without quartz or rare quartz, called "basic".

This historical meaning has continued to the present day, as has the term "silicates", although it has long been established that they are not salts in the chemical sense of the term.