Question

Question: The clay mineral kaolinite$\left({\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right)$is formed by the weathering action of water containing dissolved carbon dioxide on the feldspar mineral anorthite$\left({\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\right)$. Write a balanced chemical equation for the reaction that occurs. The${\mathrm{CO}}_2$forms${\mathrm{H}}_2{\mathrm{CO}}_3$as it dissolves. As the pH is lowered, will the weathering occur to a greater or a lesser extent?

Short answer

The balanced reaction is,

${\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\left(\mathrm{s}\right)+{\mathrm{Ca}}_2+\left(\mathrm{aq}\right)+2{\mathrm{HCO}}_3^{-}\left(\mathrm{aq}\right)$

The weathering of feldspar occurs to a greater extent when the pH is lowered

Step-by-step solution

Step 1:

The clay mineral kaolinite $\left({\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right)$is formed by the weathering action of water containing dissolved carbon dioxide on the feldspar mineral anorthite $\left({\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\right)$. So the balanced reaction is,

${\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\left(\mathrm{s}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+2{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\left(\mathrm{s}\right)+{\mathrm{Ca}}_2+\left(\mathrm{aq}\right)+2{\mathrm{HCO}}_3^{-}\left(\mathrm{aq}\right)$

Step 2:

When carbon dioxide is dissolved in water it forms carbonic acid so,

${\mathrm{CO}}_2\left(\mathrm{g}\right){\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\to {\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)$

         

The weathering of feldspar occurs to a greater extent when the pH is lowered. Hence more kaolinite would be formed by the washing away of the ions of sodium, potassium, iron and magnesium.