Question

Lapis Lazuli is
A) Ferrous sulphate
B) Copper sulphate
C) Sodium aluminosilicate
D) Zinc sulphate

Answer 1

Hint: Lapis Lazuli is a metamorphic rock. It is deep blue or purple in colour. Because of its intense colour Lapis Lazuli is used as a semi-precious stone. Lapis Lazuli is a mixture of minerals and lazurite is its main constituent.

Complete answer :
Lapis Lazuli is a mixture of minerals. It contains lazurite, calcite, sodalite and pyrite. Lazurite is the main constituent of Lapis Lazuli. Lapis Lazuli contains $40\% $ of lazurite.

We know that lazurite is a tectosilicate mineral. Lazurite contains sulphate, sulphur and chloride. The formula for lazurite is ${\left( {{\text{Na,Ca}}} \right)_{\text{8}}}\left[ {{{\left( {{\text{S,Cl,S}}{{\text{O}}_{\text{4}}}{\text{,OH}}} \right)}_{\text{2}}}\left( {{\text{A}}{{\text{l}}_{\text{6}}}{\text{S}}{{\text{i}}_{\text{6}}}{{\text{O}}_{{\text{24}}}}} \right)} \right]$.
Lazurite is deep blue, violet blue, greenish blue in colour because of the presence of ${\text{S}}_3^ - $ anion. The crystals of lazurite occur as dodecahedra or cubes.

Lazurite is a main constituent of Lapis Lazuli. Thus, Lapis Lazuli is also deep blue in colour. Lapis Lazuli is a rock and its crystals occur as a dodecahedra and are compact and massive. The colour of Lapis Lazuli are deep blue or purple mottled with white calcite and brassy pyrite. The lustre of Lapis Lazuli is dull i.e. Lapis Lazuli is non-lustrous. Lapis Lazuli contains lazurite which contains sodium, aluminium and silica. Thus, Lapis Lazuli is a sodium aluminosilicate.

Thus, the correct answer is option (C) sodium aluminosilicate.

Note:Lapis Lazuli is used to make a deep blue coloured pigment called ultramarine. Ultramarine is made by grinding Lapis Lazuli into a powder. Lapis Lazuli is used to make jewellery, ornaments, vases, carvings, etc. lapis Lazuli is synthesised commercially by Gilson process.