Topaz is a ______ of fluorine and aluminium.

silicate

The correct answer is option 4. silicate.

Topaz is a mineral that belongs to the silicate class. Silicates are minerals composed of silicon, oxygen, and other elements. In the case of topaz, it is a silicate of aluminum, meaning it contains aluminum in combination with silicon and oxygen, along with other elements.

The chemical formula of topaz is Al₂SiO₄(F,OH)₂. In this formula:

Al₂SiO₄ represents the silicate structure where aluminum (Al) is bonded with silicon (Si) and oxygen (O).

(F,OH)₂ indicates the presence of fluorine (F) and/or hydroxyl (OH) groups in the mineral.

The incorporation of fluorine into the topaz structure gives it specific properties and variations in color. Topaz can exhibit a range of colors, including yellow, orange, pink, blue, and colorless, depending on the presence of different impurities.

In summary, topaz is classified as a silicate mineral due to its composition, primarily containing aluminum, silicon, and oxygen in a silicate structure.