A member of the kaolin group, which consists of the dioctahedral and aluminous rich 1:1 phyllosilicates. Nacrite has a chemical composition of Al2Si2O5(OH)4. Nacrite is distinguished from the other polymorphs, kaolinite and dickite, by the vacant octahedral site regularly alternating from layer to layer across “B” and “C” sites and by a different stacking sequence of layers (Zheng and Bailey, 1994). The “B” and “C” sites would be related by a mirror plane if both sites were occupied identically within the same layer, whereas the “A” site resides on the mirror plane. Dickite and nacrite have a similar alteration of vacant sites, but kaolinite does not. Nacrite differs from dickite by the different stacking sequence. In nacrite the stacking is similar to the 6R polytype, but the vacancy produces monoclinic symmetry. The choice of axes produces a two-layer, monoclinic structure. Nacrite is considered the high-temperature kaolin form, occurring in hydrothermal and pneumatolytic environments.
Cf., dickite, halloysite, kaolin, kaolinite