Kaolinite is a di-octahedral 1:1 clay mineral, with each 1:1 layer consisting of an octahedral sheet of alumina bonded via shared oxygen ions to a tetrahedral sheet of silica. It has the chemical composition Al2Si2O5(OH)4, resulting in a net layer charge of zero . Calcining kaolinite in air at an optimal temperature range (typically within 600-800 °C) results in formation of meta-kaolinitea highly disordered form of kaolinite that is X-ray amorphous and highly reactive (relative to its un-calcined form) . Loss of hydroxyl groups distorts the Al octahedra, which causes significant structural disorder in the octahedral sheet; in turn, this results in distortion of the tetrahedral sheet as Si tetrahedra rotate to adapt to the new configuration . As a result, there is buckling of individual 1:1 layers, and a subsequent loss of long-term order in all crystallographic axes. Hereafter "meta-kaolinite" will be used to describe the disordered form of kaolinite, and "metakaolin" will be used to describe a clay which contains meta-kaolinite alongside associated minerals. Because of well-established limitations and ambiguities around the use of "crystallinity" to describe clay minerals, the term "structural order" will be used instead .