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I read that the acidic character, or, better, persilicic character of a rock is determined by a content of more than $65\%\ \ce{SiO_2}$.

A rock is defined, by the texts available to me, as mesosilicic (or intermediate) when its $\ce{SiO_2}$ content is contained in the interval $52\%-65\%$, while it is called hyposilicic (or basic) when $\ce{SiO_2}<52\%$.

If I correctly understand the $\ce{SiO_2}$ contained in the rock as a part of a mineral other than pure $\ce{SiO_2}$, for example, the $\ce{Si_2O_4}$ part of anorthite $\ce{CaAl_2Si_2O_8}$, is calculated into the quantity of total silica. What I cannot understand at all, is whether mass or volume is taken into account.

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    $\begingroup$ Fred's answer is correct. A few notes though: I have never seen the per/meso/hypo silicic terms used in the professional literature (or any other literature for that matter). Just stick to acidic and basic (which have nothing to do with the chemical ideas of acid and basic, by the way). $\endgroup$ – Gimelist Jan 22 '15 at 8:01
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    $\begingroup$ @Michael Thank you very much for the comment, in particular for explicitly stating that "acidity" is used in this petrological context in a different way from its meaning in chemistry. I've found the per/meso/hypo- silicic terminology in Italian geological literature from the 70-80's . $\endgroup$ – Self-teaching worker Jan 22 '15 at 19:31
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According the website http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm from Tulane University silica content in igneous rock is based on mass. Below is copy of the relevant part of the web page where it states wt %, weight percent.

General Chemical Classifications

$\ce{SiO2}$ (Silica) Content

> 66 wt. % - Acid

52-66 wt% - Intermediate

45-52 wt% - Basic

< 45 wt % - Ultrabasic
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I'd like to add to Fred's answer, which is correct. Some more points:

If I correctly understand the $\ce{SiO_2}$ contained in the rock as a part of a mineral other than pure $\ce{SiO_2}$, for ex. the $\ce{Si_2O_4}$ part of anorthite $\ce{CaAl_2Si_2O_8}$, is calculated into the quantity of total silica.

This is not correct. When calculating that acidic or basic character of a rock, you have to take into account the entire rock, in what is known as a "whole rock" or "bulk rock" composition. You don't separate $\ce{SiO_2}$ of various minerals in the calculation.

What I cannot understand at all is whether mass or volume is taken into account.

As a general rule of thumb, rock and mineral chemistries are usually calculated using mass (commonly known as weight in the literature) or molar proportions, and rarely if ever as volume.

Also, I have never seen the per/meso/hypo silicic terms used in the professional literature (or any other literature for that matter). Apart from that, the term acidic originated long long time ago when it was thought that silica was present in the rocks as silicic acid. This is now known to be false, and in the past decades the terms acidic and basic are falling out of use. Silicic and mafic are more common nowadays. Note that obviously, this only works for igneous or metamorphic silicate rich rocks. You would not use that to describe rocks that are very poor in that stuff such as carbonatites, or sedimentary rocks such as phosphorites or evaporites.

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  • $\begingroup$ Thank you so much!!! I think I incorrectly expressed what I meant... Forgive my bad English. I meant: let $M_i$ be the molar mass of the element $i$; then I think that the percentage is calculated, in the example of a rock of pure $\ce{CaAl_2Si_2O_8}$, as $\frac{2(M_{\ce{Si}}+2M_{\ce{O}})} {M_{\ce{Ca}}+2M_{\ce{Al}}+2M_{\ce{Si}}+8M_{\ce{O}}}\cdot100\%$ since there are two moles of $\ce{SiO_2}$ for each mole of $\ce{CaAl_2Si_2O_8}$... Thanks again! $\endgroup$ – Self-teaching worker Oct 31 '15 at 13:34
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    $\begingroup$ We geologists don't like to think in terms of single atoms. For us anorthite is not composed of Ca, Al, Si and O. It is composed of SiO2, CaO and Al2O3. Using oxides makes everything much easier, and you have to calculate less. $\endgroup$ – Gimelist Oct 31 '15 at 22:06

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