Secondary Minerals

minerals formed by the chemical disintegration or displacement of minerals evolved earlier. The formation of secondary minerals is common in nature, especially in the surface zones of the earth’s crust during the processes of hypergenesis, and is also linked to changes in the physicochemical parameters of mineral formation.

The transition into secondary minerals can occur without a change in the quality of the composition of the substance— for example, the transition of chalcopyrite CuFeS₂ into the secondary bornite CuFeS₄. More frequently, however, transition is accompanied by the removal and exchange of substance from the surrounding medium—for instance, the formation of covellite Cu₂S CuS₂ through chalcopyrite, malachite Cu₂[CO₃](OH)₂ through atacamite CuCl₂ 3Cu (OH)₂, and kaolin through feldspar. Secondary minerals can also be formed during a purely physical change in the crystalline structure of a substance, retaining its chemical composition: cubic chalcosine changes to rhombic chalcosine, high-temperature cubic leucite changes to low-temperature rhombic leucite.

There are various forms of secondary minerals. These are powdery or other masses or pseudomorphs which retain the external form of the primary mineral while the substance is replaced by a new one.