Comment on Lykova et al. (2024): “Natromelansonite, Na₃Zr[Si₇AlO₁₉]⋅4–5H₂O, a new member of the rhodesite mero-plesiotype series from Mont Saint-Hilaire, Quebec, Canada”

In this communication, we present a brief response to Lykova et al. (2024) who, in a paper in volume 88 https://doi.org/10.1180/mgm.2024.1 (this journal), describe the occurrence of the new mineral natromelansonite, Na₃Zr(Si₇AlO₁₉)⋅4–5H₂O, and provide commentary on the related species, melansonite (Na,□)□₂KZrSi₈O₁₉⋅5H₂O. The comments in question revolve primarily around the roles and abundances of Al and Na in both minerals, and how their incorporation is best represented in their idealised formulae.

Lykova et al. (2024) state that Al is an essential component of both melansonite and natromelansonite and should thus be reflected in the ideal formulae. With respect to Na, they propose that the ideal formula of melansonite contains two Na apfu rather than one, as was presented in our initial definition of the mineral.

We reiterate there is no unequivocal evidence that Al is an essential component of melansonite, occupying a unique crystallographic site, but rather, it only substitutes for Si. Secondly, though the true Na content of melansonite is probably higher than were originally reported (this being simply explained by the thin nature of the crystals analysed and the volatility of Na under the electron beam), it cannot reconcile the doubling of the Na apfu for melansonite, as was suggested by Lykova et al. (2024).