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 H 2 O, powder X-ray diffraction, single-crystal X-ray structure analysis, 139 La NMR, 57 Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are a = 15.9247(13) Å, c = 11.9661(10) Å, space group P 4/ nnc . The structure model was solved and refined to R 1 = 3.35% for 1757 independent observed reflections with I > 4 σ ( I ). Manaevite-(Ce) is the first vesuvianite-group mineral with the species-defining role of REE 3+ at the X 3 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 (H 4 O 4 ) 4‒ ↔ (SiO 4 ) 4‒ associated with the Z 1 and Z 2 sites. The incorporation of REE 3+ into the crystal structure of manaevite-(Ce) proceeds via the substitution schemes 2Ca 2+ ↔ Th 4+ + □ and 3Ca 2+ ↔ 2REE 3+ + □, which results in the formation of vacancies at the X 3 site and the presence of H 2 O molecules.