The minerals loose their luster and become soft.

It is one of the most common processes in nature and works with secondary minerals, such as aluminium oxide and iron oxide minerals and gypsum. (e.g.)

a) 2Fe2O3 + 3HOH    ->     2Fe2O3 .3H2O

   (Hematite) (Red)            (Limonite) (Yellow)

b) Al2O3 + 3HOH      ->     Al2O3 .3H2O

   (Bauxite)                      (Hyd. aluminium Oxide)

c) CaSO4 + 2H2O     ->      CaSO4 .2H2O

   (Anhydrite)                    (Gypsum)

d) 3(MgO.FeO.SiO2) + 2H2O  ->   3MgO.2SiO2.2H2O + SiO2 + 3H2O

     (Olivine)                                 (Serpentine)