Keywords: 
• Mutual dilution mixture /
• End-member /
• Element abundance constant /
• Abundance matrix

Abstract: This article gives a definition of the mutual dilution mixture:if a sedimentary rock is formed by mixing of two end-members; the concentrations of element m in the end-members A and B are M_a and M_b, respectively, and M_２aM_b≥0;the concentrations of element P in the end-members A and B are Pa and Pb, respectively;then the concentration relationship between elements M and p in a sedimentary rock series formed by mixing of the end-members A and B is defined as the mutual dilution mixture.The element abuneances(x and y)in a rock with petrogenesis of mixture obey the law of the fractional linear function:where a,b, c and d are abundance relationship constants which are only related to the abundances (x₁,y₁;x₂,y₂) of two known rocks (1 and 2)in the rock series and equal to the mirror ∑transformation (or mixture transformation)of them:Based on the type of the element abundance relatioship of two known rocks with petrogenesis of binary mixture and by listing the column matrix of abundances and rearranging the column matrix by a certain model, the abundance relationship constants could be calculated conveniently. If the ratio of conements b and c is negative,then the element abundance relationship between y and x is a mutual diluhon mixture.Based on the element abundances of the two end-members in a sedimentary rock series, themutual dilution mixture (MDM) can be devided into three types:Pure MDM, half pure MDM and impure MDM.For a sedimentary rock series, provided the element concentrations of any two rocks with different compositions are known, the element abundances of the end-members of the sedimentary rock can be surely estimated, in turn the compositions of materials in the end-members can be figured out.Applying the theory and method discussed here, the data of the sandstone-claystone series in the northern Sichuan, obtained from neutron activation analysis, were dealt with. The resultsindicate that the concentration relationshipe between CaO and Al₂O₃ and between Na₂O and K₂O belong to the mutual dilution mixture. The components of the end-members in the sandstone claystone series of the northerm Sichuan are estimated based on the equation of the mixture (mirror ∑) transformation:1)one end-member is the complex of pure carbonate,siliceous materials and organic carbon the other is pute clay component. 2)the potassium-bearing materials in pure clay component are mainly the hydro-muscovite with minor alkalifeldspar and clay minerals;the mean composition of odium-bearing components is similar to that of the andesine.