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Does anyone know how one could extract Silica $\ce{SiO2}$ from a one or more of the following compounds in either ground or powder forms?

  • Plagioclase: $\ce{Ca2Al2Si2O8}$
  • Pyroxene: $\ce{(Ca,Mg,Fe)2Si2O6}$
  • Olivine: $\ce{(Mg,Fe)2SiO4}$

Ideally the process would only involve energy and mechanical processes rather than adding many other chemicals.

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    $\begingroup$ While we may write silica as $\ce{SiO2}$, it neither is a small molecule (like water), nor a salt (like sodium chloride). Instead, it forms networks of silicates. Think of little tetrahedra (en.wikipedia.org/wiki/Silicate) which may be connected with each other over their tips, or sharing a rim, or a face with the next one. So there is no simple solvent capable to extract these separately from the other metal ions mentioned for the three minerals. Silica isn't something you could attract with a magnet, either. $\endgroup$ – Buttonwood Jul 2 '20 at 22:07
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Mechanically speaking, it is not possible to separate $\ce{SiO_2}$ from these minerals. The only way to do it is carrying out a chemical transformation. Here is an example of such a treatment.

Mix a sample of one of these minerals with $\ce{Na_2CO_3}$ in excess, and heat it in a platinum crucible with a Bunsen burner. When the substance is liquid, wait $10$ minutes. Let it cool down. The mineral is now transformed into sodium silicate $\ce{Na2SiO3}$ plus some oxides and carbonates. If I choose the specially simple olivine $\ce{MgFeSiO_4}$ as an example, the following reaction may happen : $$\ce{MgFeSiO_4 + Na_2CO_3 -> Na_2SiO_3 + MgCO_3 + FeO}$$ The reaction is somewhat more complicated, because usually $\ce{FeO}$ is oxidized in air into $\ce{Fe_2O_3}$, and $\ce{MgCO_3}$ is partially decomposed into $\ce{MgO}$ and $\ce{CO_2}$. Furthermore, the sodium silicate $\ce{Na2SiO3}$is partly transformed into $\ce{Na4SiO4}$. Fortunately this side reactions do not really modify the rest of the process.

Anyway, put the crucible in a small beaker. Add $20$ mL water. Let it boil for a couple of minutes. When the substance is pulverized, add an excess of diluted $\ce{HCl}$. Heat on a water bath until the substance is dry. Add $2$ mL concentrated $\ce{HCl}$. The following reactions happen : $$\ce{Na_2SiO_3 + 2 HCl -> H_2SiO_3 + 2 NaCl}$$ $$\ce{MgCO_3 + 2 HCl-> MgCl_2 + CO_2 + H_2O}$$ $$\ce{Fe_2O_3 + 6 HCl-> 2 FeCl_3 + 3 H_2O}$$ After $10$ minutes, add some hot water and filtrate. $\ce{H_2SiO_3}$ remains on the filter. Calcium, magnesium and iron chlorides are soluble into water and are eliminated. Then $\ce{H_2SiO_3}$ can later be heated to obtain $\ce{SiO2}$ $$\ce{H2SiO3 -> SiO2 + H2O}$$ This is one way for obtaining $\ce{SiO2}$ from the proposed silicates. Other processes are known, using for example fluorhydric acid $\ce{HF}$. But, of course, using this acid is not recommended, because of its toxicity.

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