What is hydroxides in mineralogy ?

Hydroxides : definition

Hydroxides are a subclass of minerals whose structure is characterized by the presence of hydroxyl groups (OH)- or water molecules (H2O). This presence decreases the power of the binding forces of these structures. This results in a lower hardness than for oxides. Common hydroxides are minerals of iron (goethite), manganese (manganite) or aluminum (gibbsite, diaspore).