Insights into crystal chemistry of the vesuvianite-group: manaevite-(Ce), a new mineral with complex mechanisms of its hydration

Manaevite-(Ce), a new vesuvianite-group mineral has been investigated by means of electron microprobe, TGA and DSC and CHN analysis of H₂O, powder X-ray diffraction, single-crystal X-ray structure analysis, ¹³⁹La NMR, ⁵⁷Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are a = 15.9247(13) Å, c = 11.9661(10) Å, space group P4/nnc. The structure model was solved and refined to R₁ = 3.35% for 1757 independent observed reflections with I > 4σ(I). Manaevite-(Ce) is the first vesuvianite-group mineral with the species-defining role of REE³⁺ at the X3 site. The mineral contains two different kinds of hydroxyl anions. The first type of hydroxyl groups is associated with the O10 and O11 sites as observed for other vesuvianite-group members. Another type of OH groups is due to the hydrogarnet substitution (H₄O₄)⁴⁻ ↔ (SiO₄) ⁴⁻ associated with the Z1 and Z2 sites. The incorporation of REE³⁺ into the crystal structure of manaevite-(Ce) proceeds via the substitution schemes 2Ca²⁺ ↔ Th⁴⁺ + □ and 3Ca²⁺ ↔ 2REE³⁺ + □, which results in the formation of vacancies at the X3 site and the presence of H₂O molecules.