Why are oxide minerals generally hard, dense, and resistant to heat?

Oxide minerals are generally hard, dense, and resistant to heat (refractory) primarily due to the nature of their chemical bonding and atomic structure.1

Unlike minerals in the sulfide class, which exhibit a mix of bonding types, oxide minerals are characterized by strong ionic bonding. In these compounds, positively charged metal ions occupy the interstices of a structure based on close-packed oxygen atoms. This ionic interaction creates strong electrical forces between the ions.123

The physical properties of these minerals are directly tied to these bond strengths:
* Hardness and Melting Point: Because the atoms are joined by strong electrical forces, a significant amount of energy—in the form of heat—is required to separate them, resulting in high melting points and increased hardness.3
* Density: The structure of oxide minerals is typically based on the cubic or hexagonal close-packing of oxygen atoms, which allows for a dense arrangement of ions within the crystal lattice.2

In contrast, hydroxide minerals, which contain hydroxyl (OH)⁻ groups, generally have lower bond strengths, making them less dense and softer than their oxide counterparts.1