Crystal chemistry of turkestanite (Dara-i-Pioz massif, Tajikistan)

Abstract 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.%): K 2 O 4.13(6), CaO 8.1(1), Na 2 O 2.3(1), ThO 2 25.8(4), UO 2 3.6(4) and SiO 2 55.9(1). The averaged crystal-chemical formula for the studied turkestanite is (Th 0.84 U 0.12 ) Σ 0.96 (Ca 1.24 Na 0.65 ) Σ1.89 (K 0.75 ☐ 0.25 ) Σ1.00 Si 8 O 19.72 (OH) 0.28 . Single-crystal structural refinement of turkestanite gave tetragonal, space group P 4/ mcc , a = 7.5708(3) Å, c = 14.7300(11) Å, V = 844.27(6) Å 3 and Z = 2. Luminescence of the uranyl ion (UO 2 ) 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.