Each Na is surrounded by Cl and each Cl is surrounded by Na. The structure of minerals is often seen in the shape of crystals. Crystal structure depends on the conditions under which the mineral forms. Polymorphs are minerals with the same chemical composition but different crystal structures. At high T atoms vibrate more, and thus distances between them get larger.
Crystal structure changes to accommodate the larger atoms.
At even higher T substances changes to liquid and eventually to gas. Liquids and gases do not have an ordered crystal structure and are not minerals. Increase in P pushes atoms closer together. This makes for a more densely packed crystal structure. The compound Al 2 SiO 5 has three different polymorphs that depend on the temperature and pressure at which the mineral forms.