The decomposition of felspar is represented as K₂Al₂O₃.6H₂O + CO₂ + nH₂O = (the formula is printed as in the question paper)
- AK₂CO₃ + Al₂O₃.2SiO₂.2H₂O + 4SiO₂.nH₂OCorrect
- BK₂CO₃ + Al₂O₃.SiO₂.2H₂O + 4SiO₂.nH₂O
- CK₂CO₃ + Al₂O₃.2SiO₂.H₂O + 4SiO₂.nH₂O
- DK₂CO₃ + Al₂O₃.SiO₂.H₂O + 4SiO₂.nH₂O
Solution
Concept
Kaolinisation of felspar
Formula
K₂O·Al₂O₃·6SiO₂ + CO₂ + nH₂O → K₂CO₃ + Al₂O₃·2SiO₂·2H₂O (kaolinite) + 4SiO₂·nH₂O
Step-by-step solution
1. Orthoclase felspar K₂O·Al₂O₃·6SiO₂ reacts with carbonated water. 2. Potassium is removed as K₂CO₃ and the clay mineral kaolinite Al₂O₃·2SiO₂·2H₂O is formed. 3. Balance of silica: 6 − 2 = 4 SiO₂ is released as colloidal silica 4SiO₂·nH₂O.
Explanation
Felspar weathers by hydrolysis and carbonation into kaolin (china clay), soluble potassium carbonate and free silica. Kaolinite contains 2 SiO₂ and 2 H₂O per Al₂O₃, which fixes option (a).
Why the other options are wrong
Options (b), (c) and (d) show kaolin with the wrong silica or water ratio (SiO₂ or H₂O count), which does not balance the six silica units in orthoclase.
Shortcut
Kaolin = Al₂O₃·2SiO₂·2H₂O.
Common mistake
Writing the kaolin formula with 1 SiO₂ or 1 H₂O.
Correct answer: A) K₂CO₃ + Al₂O₃.2SiO₂.2H₂O + 4SiO₂.nH₂O