allophane

Allophane is a group of naturally occurring hydrous aluminum silicate minerals that are not entirely amorphous but exhibit short-range order. These minerals are characterized by a dominance of Si-O-Al bonds, with most aluminum atoms in a tetrahedral coordination. Allophane's morphology varies, appearing as fine, rounded particles, ring-shaped particles, or irregular aggregates, with individual particles measuring about 30–50 angstroms in diameter. Its chemical composition typically shows a SiO₂:Al₂O₃ ratio between 1.0 and 2.0. The name "allophane" comes from the Greek word for "appearing otherwise," reflecting its varied appearance.12