Types of Occurrence of Nontronite and Nontronite-like Minerals in Soils

Abstract

Nontronite, the iron-rich dioctahedral mineral of the montmorillonite group, is rarely found in soils. A number of montmorillonite clays having a high content of iron oxide have been found in subsoils. These occurrences are located at Toowoomba, Queensland, Australia; Waipata, South Island, New Zealand; Molumolu, Fiji; and at a number of sites in the Hawaiian Islands. These clays occur as relatively pure mineral aggregates and therefore are well segregated from their matrix. The type of occurrence provided material of homogeneous chemical and mineral compositions. The iron oxide content of these clays ranged from 9 to 32 per cent and indicates a wide range of iron substitution in the octahedral position. These clays occur under a wide range of climatic conditions, as evidenced by rainfall variation of 13 to 120 inches per year. Likewise, they occur in both early and late stages of weathering and, because of that, they occur in different mineral associations of primary minerals, other 2: 1 clays, kaolin, iron oxides, and bauxite.