neoformational

Neoformational is an adjective used to describe processes and products of neoformation—the creation of new minerals or crystalline phases during post-formational alteration of rocks, soils, and sediments. The term is common in geology, mineralogy, and soil science, and it emphasizes end products that did not exist in the rock’s original formation but arise through chemical, physical, or biological alteration after emplacement. Neoformation contrasts with primary minerals that crystallized during initial solidification and with detrital minerals that were derived from preexisting sources.

Neoformation can occur through a range of processes, including chemical weathering, diagenesis, metamorphism, hydrothermal or low-temperature aqueous alteration, and biologically mediated mineralization. It often yields minerals with chemistries or crystal structures not present in the unaltered material, such as secondary clay minerals, iron oxides formed during weathering, or newly precipitated cement minerals in sedimentary rocks and soils. In soils, neoformation commonly refers to clay mineral formation and oxides that develop in situ, contributing to soil properties like cation exchange capacity and color.

The concept aids interpretation of alteration histories, palaeogeography, and fluid–rock interactions, but its application varies by discipline and terminology. Some researchers prefer terms like secondary mineral formation or alteration halos to describe similar phenomena. See also secondary minerals, weathering, diagenesis, metamorphism.