Natural Mineral Gels

amorphous minerals formed in an aqueous medium and containing varying amounts of water. They are frequently called colloidal minerals. Freshly formed natural mineral gels are very rich in water and resemble gelatinous or flocculent masses. With time they lose water and harden. Natural gels include silica, hydrated iron and manganese oxides, and ferrous sulfide. Among the hard mineral gels, the most common one is opal (Si0₂·nH₂0), which is found mainly in veins and mineral deposits of hot and warm springs. The typical solid gels produced by weathering include allophane (mAl₂O₃·nSiO₂·pH₂O) and delvauxite (hydrated iron oxyphosphate), as well as limonites and bog manganese.

The so-called metallocolloids—chalcedony (Si0₂), chrysocolla (CuSiO3·nH₂O), hydrogoethite (FeOOH·nH₂O), and some varieties of gibbsite—form from the products of crystallization of natural gels. Numerous natural mineral gel aggregates exhibit rounded external contours (so-called collomorphic structures). Natural mineral gels are most stable in the upper layers of the earth’s crust.