Keywords: 
• illite/smectiteinterstratified cay minerals /
• diagenesis /
• crystallochemistry /
• layer arrangement /
• layer charge

Abstract: Irregular illite/smectite interstratified clay minerals(I /S)occur frequently in sedimentary strata.Three pure I/S samples from the sedimentary strata in Xinjing, China are analysed in this paper, and it can be seen that the illite layer in I/S has the thickness of 1. 00 nm, but the smectite layer has two typesin thickness, that is，1. 5 nm for a smectite layer with two layers of interlayer water molecule and 1.25-1. 26 nm for a smectite layer with a single layer of interlayer water molecule. It〔an be concludedfrom the above results that, during diagenesis, the smectite layers in I/S transform to illite layer by twosteps, that is，smectite layer with two layers of interlayer water smectite layer with one layer of inter-layer water illite layer. It can also be seen that the layer linking probabilities of the I/S are betweenthose of wholly random interstratification and those of simplely physical mixture of two types of crystal-liter, i. e.，p_II.I. P_II.I,P_II.I <1. 0. The structural formulae are calculated for I4 pure I/S samplesfrom the drills of the Fuyu oilfield in Olin Province according to their electron probe quantitative analyz-ing results. By regression analysis of 14 variables including elements and cbarge distribution among te-trahedral, octehedral, interlayer sheets of the I/S sampsles as well as X一ray diffraction results(ra ti o oftwo type layers) and the sample burial depth, it is concluded that along with the continuing of diagenesis(increasing of burial depths)，the contents of illite layers in I/S obviously incrase correlation coefficientbeing 0. 85)，and the substitutions of A1 for Si in the tetrahedral sheet and the tetrahedral charge as wellas the layer、barge also increase(the coefficients being 0. 77, 0. 077, 0. 085, respectively)·By plo t tingof the contents of Siversus K and Na⁺2Ca versus K per formula, it can be seen that the illite layer inthe I/S of sedimentary strata has a chemical co mpo sition chracteristic of muscovite with the layer charge of 0. 30of 0. 75, and the smectite layer has a〔hracter of montomorillonite with the layer charge of 0. 30