Here's why:

* Alum is a double sulfate salt with the general formula XAl(SO₄)₂·12H₂O, where X is a monovalent cation.

* Aluminum sulfate (Al₂(SO₄)₃) is the primary component of alum and provides the aluminum cation.

* The monovalent cation (X) is typically potassium (K⁺) for potassium alum, but other options include:

* Sodium (Na⁺) for sodium alum

* Ammonium (NH₄⁺) for ammonium alum

Therefore, the primary cation required for alum crystal formation is aluminum (Al³⁺).