New Data on the Isomorphism in Eudialyte-Group Minerals. II. Refinement of the Aqualite Crystal Structure at 110 K

Abstract

The crystal structure of aqualite from the Khibiny massif, a high-oxonium eudialyte-group mineral, has been refined by X-ray diffraction analysis using a set of low-temperature (110 K) experimental data. The trigonal-cell parameters are found to be a = 14.1435(4) Å, c = 31.410(1) Å, V = 5441.4(4) ų, sp. gr. R3m. The structure is refined to the final reliability factor R = 0.046 in the anisotropic approximation of atomic displacements using 1173F > 3σ(F). The idealized formula of the mineral (Z = 3) is (H₃O)₉Na₃ (K,Sr,Ba,Ce)₂Ca₆Zr₃Fe³⁺[Si₂₄O₇₂](OH,Cl,H₂O)₅. Low-temperature refinement explained the low symmetry of aqualite from the Inaglinskii massif and revealed some new structural features of the mineral. Oxonium occupies mainly the N1–N4 sites, partially the N5 void, and the N6 site. A unique feature of aqualite (and its Mn analog, ilyukhinite), which was not previously observed in eudialytes and is considered for the first time in this study, is the presence of inversely oriented complex anion, which significantly affects the N4 site occupancy.