The question of Al content and distribution

The content of Al is 6.86 wt.% Al₂O₃ (average of 8 analyses; Lykova et al., Reference Lykova, Rowe, Poirier, Friis and Barnes2024) in natromelansonite and 6.29 wt.% Al₂O₃ (average of 8 analyses; Lykova et al., Reference Lykova, Rowe, Poirier, Friis and Barnes2024 [note there is a typo in Lykova et al., Reference Lykova, Rowe, Poirier, Friis and Barnes2024: 5.89 instead of 6.89; so the correct range is 5.53–6.89] and 6.32 wt.% Al₂O₃ (average of 15 analyses; Gore and McDonald, Reference Gore and McDonald2023) in melansonite. These numbers are very consistent and correspond to ~1 atoms per formula unit of Al.

In the structures of both melansonite and natromelansonite one of the Si-centred tetrahedra is larger than others which is reflected in lower bond-valence sums for one of the Si sites in both melansonite (Si3, 3.63 vu; Gore and McDonald, Reference Gore and McDonald2023) and natromelansonite (Si4, 3.65 vu; Lykova et al., Reference Lykova, Rowe, Poirier, Friis and Barnes2024).

These data are consistent with our model of ordering of Al atoms at one of the Si sites and support the Y³⁺ + Si⁴⁺ ↔ Zr⁴⁺ + Al³⁺ substitution mechanism to describe the relationships between melansonite/natromelansonite and monteregianite-(Y). Such ordering has also been described in the other members of the rhodesite mero-plesiotype series: delhayelite, hydrodelhayelite (Pekov et al., Reference Pekov, Zubkova, Chukanov, Sharygin and Pushcharovsky2009) and fivegite (Pekov et al., Reference Pekov, Zubkova, Chukanov, Zadov and Pushcharovsky2011).

The model supported by Gore and McDonald does not predict these data nor do they provide an alternative explanation for it. Furthermore, U-rich melansonite that Gore and McDonald used to illustrate variability of Al content in melansonite (4.21 wt.% Al₂O₃; Lykova et al., Reference Lykova, Rowe, Poirier, Friis and Barnes2024) is, in fact, a Ln-rich phase, which corroborates with the Y³⁺ + Si⁴⁺ ↔ Zr⁴⁺ + Al³⁺ substitution mechanism.

Na in the structure of melansonite

Gore and McDonald did not provide the refined occupancy factor for the partially-occupied Na site in melansonite in either the original publication (Gore and McDonald, Reference Gore and McDonald2023) or the Comment (Gore and McDonald, Reference Gore and McDonald2024). Furthermore, they stated that the occupancy at the site was fixed to satisfy the charge balance by the substitution mechanism Y³⁺ + Na⁺ ↔ Zr⁴⁺ + □. Thus, the number was fixed to fit their model; therefore, it cannot be used to test either of the models.