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Crystals of hochleitnerite were analysed using wavelength-dispersive spectrometry on a JEOL JXA 8500F Hyperprobe operated at an accelerating voltage of 15 kV and a beam current of 2.2 nA. The beam was defocused to 10 µm. Analytical electron microprobe (EMP) results (average of 12 analyses on 12 different crystals) are given in Table 1. There was insufficient material for direct determination of H2O, the presence of which was indicated by the low EMP analysis sum of oxides and confirmed by Raman spectroscopy, so it was based upon the crystal structure analysis with 15 H2O per formula unit. The Fe2+ / Fe3+ ratio was based on the crystal structure, with Fe2+ assigned with all other divalent elements (Mg and Mn2+) to the M1 site, and the remaining iron as Fe3+ was assigned to the M2 and M3 sites.

The EMP results were normalised to 9 cations per formula unit and 33 (O + F), giving the atomic fractions K1.00Mn1.51Fe0.492+Fe1.193+Ti1.62Al0.15P4.00O32.77F0.23H30.27. The least-squares program OccQP (Wright et al., 2000) was used to optimise the site occupations for the M1 to M3 sites based on the refined site scattering and bond lengths from the crystal structure refinement combined with the chemical analyses, giving the following structural formula: A[(H2O)1.00K1.00]Σ2.00 M1(Mn1.512+Fe0.492+)Σ2.00M2(Ti1.084+Fe0.733+Al0.15)Σ1.96M3(Ti0.544+ Fe0.463+)Σ1.00(PO4)4.00[O1.50F0.23(OH)0.27]Σ2.00(H2O)10⋅4H2O. The corresponding end-member formula is [H2O]2Mn2Ti3(PO4)4O2(H2O)10 ⋅ 4H2O.

Although this formula was approved by the IMA CNMNC (IMA-2022-141), it was criticised by some voting members because it does not include K, which is co-dominant with H2O at the A site. The problem arises because of almost complete mixing of Ti and Fe3+ at the crystalochemically similar M2 and M3 sites, so the constituent assignments at these two sites depend sensitively on the location of the minor Al. With Al located at the M3 site rather than at the M2 site, the program OccQP gives dominant Fe3+, rather than Ti at the M3 site, and leads to the alternative end-member formula [K(H2O)]Mn2Ti2Fe(PO4)4O2(H2O)10 ⋅ 4H2O. To overcome the problem, common in paulkerrite group minerals, of very strong mixing of Fe3+ and Ti at the M2 and M3 sites, the compositions of these two sites were merged. The resulting empirical chemical formula can hence be rewritten as A[(H2O)1.00K1.00]Σ2.00M1(Mn1.512+Fe0.492+)Σ2.00M2+M3(Ti1.624+Fe1.193+ Al0.15)Σ2.96(PO4)4.00[O1.50F0.23(OH)0.27]Σ2.00(H2O)10 ⋅ 4H2O. This formula disregards the Ti-Fe3+ disordering over M2 and M3 and leads to the end-member formula [K(H2O)]Mn2(Ti2Fe)(PO4)4O2(H2O)10 ⋅ 4H2O, which requires K2O 4.79, MnO 14.44, Fe2O3 8.13, TiO2 16.26, P2O5 28.90 and H2O 27.448, with a total of 100.00 wt %. This revised formula was approved by the IMA-CNMNC in a paulkerrite-group mineral nomenclature proposal (revised proposal 22-K-bis). The details of this nomenclature, based on site-total-charge method (Bosi et al., 2019), will be reported separately, but the result for the merged M2 and M3 site compositions is illustrated by the ternary diagram shown in Fig. 4. This diagram shows the possible end-member compositions based on the occupation of (M2)2M3 by Al, Ti and Fe3+. The empirical composition of hochleitnerite, shown by the cross, is located in the compositional field for the end-member (Ti2Fe).