(i) As a general term, refers to either a discrete non-expansible mica of detrital or authigenic origin or to the micaceous component of interstratified systems, as in illite-smectite. If used to refer to the species, it should meet the following requirements: a) The micaceous layers ideally are non-expansible; b) the octahedral sheet is dioctahedral and aluminous; c) the interlayer cation is primarily potassium; and (4) the composition deviates from that of muscovite in two main ways: 1) A phengitic component is present in which substitution of R2+ cations for octahedral Al is balanced by addition of tetrahedral Si beyond the ideal Si:Al ratio of 3:1 for muscovite. This substitution gives the octahedral sheet an overall negative charge of about 0. 2 to 0. 3 per formula unit. 2) Interlayer vacancies or water molecules amounting to about 0. 2 to 0. 4 atoms per formula unit are compensated by additional tetrahedral Si cations beyond those required by the phengitic component. Where reference is made to the species illite, a clear statement should be made to that effect in order to avoid confusion with the general usage. (ii) In soil taxonomy, the presence of a 1 nm x-ray diffraction peak and greater than or equal to 4% K2O is used to denote the presence of illite.