In which of the following minerals, aluminium is not present?

Fluorspar

The correct answer is option 4. Fluorspar.

1. Cryolite: Cryolite \((Na_3AlF_6)\) is a mineral consisting of sodium (Na), aluminum (Al), and fluorine (F). Its chemical formula, \(Na_3AlF_6\), reveals the presence of aluminum along with sodium and fluorine. Cryolite is known for its use in the production of aluminum metal, where it serves as a solvent to lower the melting point of aluminum oxide during the Hall-Héroult process.

2. Mica: Mica is a group of sheet silicate minerals known for their characteristic layered structure. While the exact composition can vary depending on the specific type of mica, aluminum is a common element found in many mica minerals. Aluminum atoms are often present in the crystal lattice of mica minerals, contributing to their physical and chemical properties.

3. Feldspar: Feldspar is a group of rock-forming minerals that make up a significant portion of the Earth's crust. Feldspars are tectosilicates, meaning they have a three-dimensional framework of silica tetrahedra. Aluminum atoms commonly substitute for silicon in the crystal lattice of feldspar minerals, along with other elements like potassium, sodium, and calcium.

4. Fluorspar (Fluorite): Fluorspar, chemically known as fluorite \((CaF_2)\), is primarily composed of calcium and fluorine. Its chemical formula, \(CaF_2\), indicates that it consists of calcium \((Ca)\) and fluorine (F), with no aluminum \((Al)\) present in its composition. Fluorspar is often used as a flux in various industrial processes, particularly in metallurgy.

Based on this information, we can confirm that aluminum is not present in fluorspar (fluorite), making it the correct answer to the question.