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I have seen Basicity to be calculated as $\mathrm{B} = wt\%\:\ce{CaO}/wt\%\:\ce{SiO2}$ particularly in slag bascity/acidity calcuations. Now I do not think that $\ce{CaO}$ is the most basic oxide that we have or $\ce{SiO2}$ is the most acidic oxide then why are they calculated accordingly? On a lighter note in my geology classes, I have been taught to classify minerals as acidic on the basis of Silica content. Are they related? Please explain with close reference to context.

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  • $\begingroup$ The title question does not make much sense. Aside of that, it is not about being so basic ( high B ) nor so acidic ( low B ), but about being relatively basic or acidic. Extracts of rocks or soil with high(low) B have relatively high(low) pH. $\endgroup$ – Poutnik Aug 11 '19 at 14:50
  • $\begingroup$ @Poutnik But why $CaO$ and $SiO_2$ are only chosen? $\endgroup$ – user586228 Aug 11 '19 at 15:08
  • $\begingroup$ It is an empirical parameter based on the major representatives of basic and acidic items, easy to determine. $\endgroup$ – Poutnik Aug 11 '19 at 16:39
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What is so acidic...and basic...?

Nothing.

This derives from the historical perception that silica in geological systems and in melts was in the form of silicic acid ($\ce{H2SiO4}$) and the alkali and alkali earth elements were considered as bases.

We now know that in high temperature silicate liquids (that eventually solidifies into slag, or rocks, or glass) there is no acid–base chemistry, at least in the form that we know it from low-temperature aqueous acid–base chemistry. However, the usage of the terms acid and base in relation to rocks and slags still persists, primarily with older professors (who still teach) or in certain parts of the world, such as Russia. Nowadays geologists usually prefer the term "felsic" over the term "acidic" for silica-rich rocks.

Nonetheless, the measure of $\ce{CaO}$ and $\ce{SiO2}$ contents in slags, glasses and rocks is still useful for various reasons. Therefore, the ratio is still calculated and used, and the old name persists. A slightly better parameter, called optical basicity, takes into account the compositional variability other than $\ce{CaO}$ and $\ce{SiO2}$, by adding other oxides such as $\ce{Na2O}$ and $\ce{K2O}$ into the calculation. It is widely used and has many applications.

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