Abstract: The powder diffractograms of twenty wet chemically analysed samples in the isomorphous triphylite-lithiophilite series and five synthesized members with Fe/(Fe + Mn) = 1, 0.75, 0.50, 0.25, and 0.0, were recorded. Their unit cell dimensions were accurately refined in order to find a reliable method for semi-quantitative determination of the divalent cation content of these minerals. A multivariate best fit analysis based on Kummell’s procedure shows the marked influence of Fe²⁺ and Mg²⁺ on the cell dimensions, as well as that of small amounts of Fe³⁺ substituting for Mn²⁺, following LiMn²⁺ → □ Fe³⁺. The best representation of the correlation between chemical composition and cell parameters is given by the equations:

a = 6.1041 − 0.0245 Fe_tot − 0.049 Mg²⁺

b = 10.4511 − Fe_tot − 0.082 Mg²⁺

c = 4.7400 − 0.0130(Fe²⁺ + Mg²⁺) − 0.025 Fe³⁺.

No evidence of non-linearity has been found for the variation of the three cell dimensions with the chemical composition. Assuming the absence of appreciable amounts of Mg²⁺, the following set of equations is proposed: Fe_tot = 41(6.104–a); Fe_tot = 35(10.451–b); Fe_tot = 77(4.740–c) in which the c dimension gives a relatively poor estimate.

Two sets of determinative graphs were constructed, one based on the cell parameter variation, and the other on the 311, 222, and 142 reflection angular positions, v. the total iron content of these minerals. These two methods, whose reliability is examined, can be used for determination of the divalent cation content, provided the samples contain less than about 0.5 wt.% Na₂O, 0.5 wt.% CaO, and 3 wt. % Fe₂O₃, and are homogeneous.