Original paper
The crystal structure of orlovite, KLi2Ti(Si4O10)(OF): the first example of the short-range order of Ti in true trioctahedral micas
Sokolova, Elena; Hawthorne, Frank C.; Agakhanov, Atali A.; Pautov, Leonid A.; Karpenko, Vladimir Yu.
European Journal of Mineralogy Volume 30 Number 2 (2018), p. 399 - 402
8 references
published: Aug 20, 2018
published online: Apr 27, 2018
manuscript accepted: Aug 1, 2017
manuscript revision received: Jul 4, 2017
manuscript received: May 23, 2017
Abstract
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) Å3. 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 Ti1.00 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 K1.00 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: (K0.98Rb0.03Cs0.01)1.02Li2.01(Ti0.94Nb0.02Fe0.02Al0.02)1.00Si4O10[O1.00F0.95(OH)0.05]2.00, Z = 2; D calc. = 2.814 g/cm3.
Keywords
orlovite • true trioctahedral mica • crystal structure • titanium • lithium • short-range order (SRO)