The results of combined single-crystal X-ray diffraction, electron probe microanalysis, Fourier microspectroscopy, and photoluminescence spectroscopy study of crystals of turkestanite from the Dara-i-Pioz deposit, Tien-Shan Mountains, Tajikistan are reported. It is a single-layer sheet silicate belonging to the ekanite group with a steacyite structural type. Averaged major-element analysis provided (wt.%): K2O 4.13(6), CaO 8.1(1), Na2O 2.3(1), ThO2 25.8(4), UO2 3.6(4) and SiO2 55.9(1). The averaged crystal-chemical formula for the studied turkestanite is (Th0.84U0.12)Σ0.96(Ca1.24Na0.65)Σ1.89(K0.75☐0.25)Σ1.00Si8O19.72(OH)0.28. Single-crystal structural refinement of turkestanite gave tetragonal, space group P4/mcc, a = 7.5708(3) Å, c = 14.7300(11) Å, V = 844.27(6) Å3 and Z = 2. Luminescence of the uranyl ion (UO2)2+ is observed in turkestanite. In the excitation spectrum, the bands corresponding to a charge transfer transition from the 2p states of the ligand to the 5f state of uranium were found.

Turkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite–steacyite group was found in evolved peralkaline granitesfrom the Morro Redondo Complex, south Brazil. It occurswith quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO2, Na2O and K2O contentsof 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE2O3 abundances(5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite–steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluidscarrying Ca, the circulation of which wasenhanced by deformational events.