Relation between unit cell parameters — d_OO1 and b-value of trioctahedral chlorites is detaily discussed. b-parameter of the unit cell allows to estimate the total content of the cations Fe, Mn, Cr, Ni. The method for determination of the chlorite composition using val chemical analysis is described. Coordinates of atoms in the unit cell and the distribution of octahedral cations is possible to calculate on the basis of the chlorite composition.