The crystal structure of orlovite, KLi2Ti(Si4O10)(OF): the first example of the short-range order of Ti in true trioctahedral micas

The crystal structure of orlovite, ideally KLi₂Ti(Si₄O₁₀)(OF), from the Darai‑Pioz alkaline massif, Tien‑Shan, Garmskii district, northern Tajikistan, was solved and refined in the space C2/m to R_1 =3.56% based on 729 independent reflections with F_o > 4σF, a = 5.198(4), b = 9.046(7), c = 10.093(8) Å, β = 99.53(2)°, V = 468.1(1.1) ų. In the O sheet of orlovite, there are two crystallographically independent M sites: M1 and M2. In many true trioctahedral micas with space group C2/m, the M1 site corresponds to the Wyckoff position c, with multiplicity of 2 and site symmetry of 2/m. In orlovite, M1 (= Ti) atoms are displaced from the inversion centre and hence the M1 site corresponds to the Wyckoff position i, with multiplicity of 4 and site symmetry of m. The M1 site is 50% occupied primarily by Ti, ideally Ti_1.00 per formula unit (pfu), with 〈M1–ϕ〉 = 1.977 Å. The two points of the M1 site occur 0.432 Å apart. Each M1 atom is coordinated by four O2 atoms, with M1–O2 = 2.009 Å and two A anions of the composition (OF); M1–A = 1.699 Å, where A ═ O and M1–A = 2.130 Å, where A = F. This is the first occurrence of short-range order of Ti in a true trioctahedral mica. The M2 site is occupied solely by Li, giving Li₂ pfu, with 〈M2–ϕ〉 = 2.106 Å. In the T sheet, there is one tetrahedrally coordinated T site occupied solely by Si, 〈T–O〉 = 1.624 Å. The interstitial ^[12]I site is occupied primarily by K, ideally K_1.00 pfu, with 〈I–O〉 = 3.075 Å. The empirical formula of orlovite was calculated on the basis of 12 (O + F), with the constraint F + OH = 1 apfu: (K_0.98Rb_0.03Cs_0.01)_1.02Li_2.01(Ti_0.94Nb_0.02Fe_0.02Al_0.02)_1.00Si₄O₁₀[O_1.00F_0.95(OH)_0.05]_2.00, Z = 2; D_calc. = 2.814 g/cm3.