This is a book chapter on silica minerals. It includes sections on 1)structure, composition, and morphology, 2) chemical and physical properties, 3) abundance, distribution, and formation, and 4) their environmental significance.

The silica mineral group is the second most abundant mineral group in the Earth's crust, exceeded only by the silicate mineral group, which also has the Si–O tetrahedron as its basic building block. While in soil, because of their great abundance and chemical quiescence relative to most other minerals, silica minerals influence several environmentally important soil properties. This chapter describes the physical and chemical properties of silica, its abundance and formation, and its environmental significance with respect to adsorption capacity, respiratory diseases, and landscape stability pertinent to a nuclear waste repository. It also describes the structure, composition, and morphology of silica minerals. X‐ray diffraction is the most conclusive method for identifying silica minerals. With respect to soil chemical properties, quartz, which is ubiquitous in most soils, plays an important role because of its tendency to dilute cation exchange capacity, resulting from its low surface area.