Correct answer: (c) Gypsum
Explanation
- A
China clay
China clay (kaolinite) is a hydrous aluminium silicate, Al2Si2O5(OH)4. It has no calcium as an essential constituent, so it is not the key.
- B
Corundum
Corundum is aluminium oxide, Al2O3, used as emery and ruby/sapphire when coloured. It contains no calcium, so it is wrong.
- C
Gypsum
Gypsum is calcium sulphate dihydrate, CaSO4·2H2O. Calcium is an essential part of the mineral. That is why (c) is the official key among the four materials.
- D
Talc
Talc is a hydrous magnesium silicate, Mg3Si4O10(OH)2. It is not a calcium mineral, so it is not the key.
Summary. Official key is (c). Gypsum is CaSO4·2H2O and therefore contains calcium. China clay and talc are aluminium/magnesium silicates; corundum is Al2O3.
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