For phyllosilicates, a layer (see Fig. 1) contains one or more tetrahedral sheets and an octahedral sheet. There are two types of layers, depending on the ratios of the component sheets: a “1:1 layer” has one tetrahedral sheet and one octahedral sheet, whereas a “2:1 layer” has an octahedral sheet between two opposing tetrahedral sheets. Quot Guggenheim et al. (2006).
See references therein.
Cf., plane, sheet, tetrahedral sheet, octahedral sheet