What does mafic mean in geology ?

Mafic : definition

A mafic rock is an igneous rock containing between 45% and 52% by weight of SiO2. This poverty in silica is compensated by a richness in Ca, Mg and Fe (between 20% to 35% by weight of oxides on average). This chemistry results in the absence of quartz and the abundance of ferromagnesian minerals (biotite, amphiboles, pyroxene...).

Mafic volcanic rocks (basalts, andesites...) are very abundant and more common than maifc plutonic rocks (gabbros, norites...).

The term acid (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 "acid 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.