Which one of the following materials contains calcium?

1. 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.

2. B

   Corundum

   Corundum is aluminium oxide, Al2O3, used as emery and ruby/sapphire when coloured. It contains no calcium, so it is wrong.

3. 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.

4. 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.