Crystal structure and chemical variability of the 23 Å phase, the orthorhombic polytype of the 11.5 Å phase, a major water carrier in the Earth's upper mantle

Geochemical cycles in the Earth's mantle are strongly influenced by the mobility and stability of hydrous phases, which play a key role in storing and transporting water into the deep Earth. Understanding the structural and thermodynamic properties of these phases is therefore essential to model the deep-water cycle. This study clarifies the crystallographic relationship between the 11.5 Å and 23 Å phases, both representing important DHMAS phases and two main water carriers in the subduction environment towards the mantle transition zone. We report the crystal structure of the 23 Å phase, revealing it as an orthorhombic-pseudohexagonal polytype (11.5 Å 2O) of the monoclinic 11.5 Å phase (11.5 Å 2M). Both structures share a common topology with the chemical formula Mg₆AlSi₂O₈(OH)₇. The 11.5 Å 2O polytype crystallizes in the Cmcm space group, with unit cell parameters a = 8.9783(3) Å, b = 5.2018(2) Å, and c = 22.9982(8) Å. Both polytypes exhibit similar Raman spectra and compressibility, suggesting that their stabilization is only weakly governed by pressure-temperature conditions. The 2M phase observed is currently limited to lower pressures (∼6.5 GPa), while the 2O polytype remains stable over a broader P-T range. Our experiments also demonstrate the capacity of the 11.5 Å phase to incorporate significant amounts of Fe (up to 1.13 atoms per formula unit) and Cr (with Cr/(Cr + Al) ratios up to 0.29), increasing its petrological relevance in Cr- and Fe-rich natural lithologies. These findings support the existence of this modular family of hydrous silicates capable of acting as efficient water carriers in different chemical systems in both cold and warm geotherms down to depths exceeding 200 km. This has broad implications for fluid transport and metasomatic processes in the mantle and subduction zones.