The decomposition of felspar is represented as (the options differ in the chemical formulae)
- AK₂AlO₃·6H₂O + CO₂ + nH₂O = K₂CO₃ + AlO₃·2SiO₂·2H₂O + 4SiO₂·nH₂O
- BK₂Al₂O₃·6H₂O + CO₂ + nH₂O = K₂CO₃ + Al₂O₃·2SiO₂·2H₂O + 4SiO₂·nH₂OCorrect
- CK₂Al₂O·6H₂O + CO₂ + nH₂O = K₂CO + Al₂O₃·2SiO₂·2H₂O + 4SiO₂·nH₂O
- DKAl₂O₃·6H₂O + CO₂ + nH₂O = KCO₃ + Al₂O₃·2SiO₂·2H₂O + 4SiO₂·nH₂O
Solution
Concept
Kaolinisation of orthoclase felspar
Formula
K₂O·Al₂O₃·6SiO₂ + CO₂ + nH₂O → K₂CO₃ + Al₂O₃·2SiO₂·2H₂O (kaolinite) + 4SiO₂·nH₂O
Step-by-step solution
1. Orthoclase felspar reacts with carbonated water. 2. Potassium leaves as potassium carbonate K₂CO₃, kaolinite Al₂O₃·2SiO₂·2H₂O is formed, and the rest of the silica is released as hydrated silica 4SiO₂·nH₂O. 3. Only option (b) writes all species with correct formulae (K₂CO₃ and Al₂O₃).
Explanation
Felspar weathers into kaolinite (a clay mineral), an alkaline carbonate and hydrated silica. Options (a), (c) and (d) contain incorrect formulae such as AlO₃, K₂CO and KCO₃.
Why the other options are wrong
(a) uses AlO₃; (c) uses K₂Al₂O and K₂CO; (d) uses KCO₃ and KAl₂O₃, none of which are valid formulae.
Shortcut
Kaolinite = Al₂O₃·2SiO₂·2H₂O; alkali carbonate = K₂CO₃.
Common mistake
Overlooking the wrong subscripts.
Correct answer: B) K₂Al₂O₃·6H₂O + CO₂ + nH₂O = K₂CO₃ + Al₂O₃·2SiO₂·2H₂O + 4SiO₂·nH₂O