Which of the following corundum and cryolite are important ores ?
Aluminium
Both corundum and cryolite are minerals of aluminium — option 1.
| Mineral | Formula | Note |
|---|---|---|
| Corundum | Al2O3 | Crystalline alumina; extremely hard, 9 on the Mohs scale. Coloured by traces of impurity it forms ruby (chromium) and sapphire (iron and titanium) |
| Cryolite | Na3AlF6 | Sodium aluminium fluoride; the solvent used in the electrolytic extraction of aluminium |
| Bauxite | Al2O3·2H2O | The chief commercial ore of aluminium |
Why cryolite matters so much. Alumina melts at about 2,050 °C, which makes direct electrolysis of the pure oxide impracticable. In the Hall-Héroult process, alumina is dissolved in molten cryolite, and the mixture melts at roughly 950 °C — a saving of more than a thousand degrees. The cryolite also increases the conductivity of the melt. Aluminium is deposited at the cathode and oxygen at the carbon anode, which is slowly consumed :
\(\text{Al}^{3+}+3e^{-}\rightarrow\text{Al}\)
Natural cryolite, once mined in Greenland, is now nearly exhausted, and the industry uses synthetic cryolite.
The wider point about aluminium. It is the most abundant metal in the Earth’s crust, at about 8 per cent, yet it was a precious rarity until the 1880s because its oxide is so stable that neither carbon reduction nor ordinary chemical means will free the metal. Only electrolysis works, which is why aluminium smelting is enormously power-hungry and why smelters are sited near cheap electricity.
The other options : silver comes chiefly from argentite, Ag2S; tin from cassiterite, SnO2; and iron from haematite and magnetite.
Hence, the answer is Aluminium.
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