Multidisciplinary study of thorium mobility: formation of turkestanite and steacyite analogues, and structural insights using an XRD-directed microcrystal preparation technique.

Abstract

Minerals of the ekanite group typically contain Th, U or REE (rare earth elements) as primary structural constituents. The ^CNa analogues of turkestanite, ^ATh^B(Ca,Na)^C(K_1-x□_x)^T(Si₈O₂₀) (□ is vacancy), and steacyite, ^ATh^B(Ca,Na)^C(K_1-x□_x)^T(Si₈O₂₀), isostructural minerals of the ekanite group, were experimentally synthesized during hydrothermal alteration of chevkinite-(Ce). The experiment was conducted at 550°C, 200 MPa, at an oxygen fugacity approximately equivalent to the Ni-NiO (NNO) buffer with NaF and Ca(PO₄)₂ added to the hydrous fluid. Both phases formed as a replacement of chevkinite-(Ce) and earlier alteration products. Their identity was confirmed by electron probe microanalysis and electron backscatter diffraction as ^CNa analogues of turkestanite and steacyite. Further SCXRD, and Raman spectroscopy analyses confirmed that it is a new ^CNa analogue with respect to the ekanite mineral group. Raman spectroscopy revealed the presence of H₂O within the crystal structure. A dedicated FIB workflow was designed to extract single crystals ready for SCXRD analysis. Compositionally, the phase is Na rich and depleted in K, REE and Fe. The mean formula based on 20 O atoms can be written as ^A(Th_0.94U_0.03)_0.97^B(Na_0.96Ca_0.90Mn_0.11Ce_0.02Nd_0.01Fe_0.01)_2.0^C(Na_0.83K_0.07)_0.9^TSi_8.05O₂₀·0.1^C(H₂O). It crystallizes in space group P4/mcc with a = 7.4757 (2) Å, c = 14.9658 (7) Å, V = 836.38 (6) ų, and Z = 2. Compositional variation is represented mainly by the relationship Ca²⁺ + □ → 2Na⁺, where □ is a vacancy which can also be filled by H₂O during crystallization. The synthesis from this study represents the first record of ^CNa analogues of turkestanite and steacyite. A dedicated microcrystal selection technique is presented allowing for easy single-crystal X-ray diffraction.