# However what is so acidic about CaO and basic(no pun intended) about SiO2 while calculating basicity of slag?

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.

• 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. – Poutnik Aug 11 '19 at 14:50
• @Poutnik But why $CaO$ and $SiO_2$ are only chosen? – user586228 Aug 11 '19 at 15:08
• It is an empirical parameter based on the major representatives of basic and acidic items, easy to determine. – Poutnik Aug 11 '19 at 16:39

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.
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.