Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
You may occasionally encounter older references that speak of "basic" and "acidic" igneous rocks. These now obsolete terms are synonyms for mafic and felsic rocks, respectively.
Before plate tectonics came along, accounting for granite was a real problem. First of all the sheer volume of granite intrusions presented the question where all the rocks displaced by the granite went. And then there was the question of how exactly granite formed. Fractionating magma in place to make granite would require many times as much mafic residue, which simply was nowhere to be found.
One proposed solution (in more senses than one) was "granitization." Granitization was metasomatism on a grand scale, in which solutions invaded the rocks and converted them to granite. Granitization had the virtue of requiring much less material since most of the ingredients of granite were already in place in the rocks already. It had the disadvantage of being unable to account for the sharp contacts of most granite bodies, and of course the origins of the fluids and the sources of their extra ingredients were still unexplained. Granitization does happen on a small scale but is no longer taken seriously as a mechanism for creating large granitic bodies. Among the agents of granitization were hypothetical "silicic acids."
We can write the following reactions:
Mineralogists will immediately recognize the Si:O ratios of the nesosilicates, sorosilicates, inosilicates (pyroxenes) and phyllosilicates. These classes of silicates were once called orthosilicates, pyrosilicates, metasilicates and disilicates respectively because of a presumed connection with the silicic acids. Rocks with a lot of silica had presumably reacted with a lot of silicic acid, hence the name "acidic." And of course if the silicon and oxygen in each acid go into minerals, the only thing left will be hydrogen ions, the very definition of an acid. Rocks that lacked silica, in contrast, must have been "basic."
Silicic acids do exist and can be made in the laboratory. In the oceans, orthosilicic acid is present and is utilized by diatoms for making shells. But the silicic acids do not play a major role in the formation of rocks.
There is one respect in which the acidic-basic nomenclature is still useful. Bases are much more effective at dissolving silicates than acids, and "basic" igneous rocks are much more effective at neutralizing acid rain than "acidic" rocks.
Created 21 February 2002, Last Update