THE CRYSTAL STRUCTURE OF NALIVKINITE, A NEW LITHIUM MEMBER OF THE ASTROPHYLLITE GROUP

Abstract

The crystal structure of nalivkinite, ideally Li 2 Na Fe 2+ 7 Ti 2 (Si 8 O 24 ) O 2 (OH) 4 F, a new lithium member of the astrophyllite group, from the moraine of the Dara-i-Pioz glacier, the Alai mountain ridge, Tien-Shan Mountains, northern Tajikistan, triclinic, a 5.3745(6), b 11.9299(15), c 11.6509(14) A, α 113.325(3), β 94.524(2), γ 103.080(2)°, V 656.2(2) A 3 , space group P 1, Z = 1, D calc. 3.293 g/cm 3 , has been refined to R 1 = 6.3% for 2041 unique ( F 0 > 4σ F ) reflections, collected on a Bruker single-crystal P4 diffractometer with a 4K CCD detector and Mo K α X-radiation. Electron-microprobe analysis gives SiO 2 36.11, Al 2 O 3 0.58, Na 2 O 1.88, K 2 O 2.68, Cs 2 O 0.93, CaO 1.24, MgO 0.11, FeO 30.84, MnO 4.86, ZnO 0.23, TiO 2 9.48, Nb 2 O 5 2.40, Ta 2 O 5 0.61, ZrO 2 1.47, SnO 0.89, PbO 0.39, F 1.45, Li 2 O ICP–MS 1.30, (H 2 O) calc 2.75, sum 100.2 wt.% (H 2 O determined from crystal-structure analysis). The empirical formula based on 31 (O + OH + F) anions is (Li 1.14 K 0.75 Cs 0.09 Na 0.02 ) ∑ 2.00 (Na 0.78 Ca 0.22 ) ∑1.00 (Fe 2+ 5.64 Mn 0.90 Sn 0.09 Ca 0.07 Mg 0.04 Zn 0.04 Pb 0.02 □ 0.20 ) ∑7.00 (Ti 1.56 Nb 0.24 Zr 0.16 Ta 0.04 ) ∑2.00 [(Si 7.89 Al 0.15 ) 8.04 O 24 ] O 2 (OH) 4 F. The crystal structure of nalivkinite is topologically identical to that of triclinic astrophyllite. For nalivkinite, a general formula for the astrophyllite-group minerals, A 2 B C 7 D 2 ( T 8 O 24 ) O 2 (OH) 4 X 0–1 , ideally has A = Li; B = [10] Na; C = [6] Fe 2+ ; D = [6] Ti; T = [4] Si and X = F. In the nalivkinite structure, four T sites are mainly occupied by Si with minor Al, with T –O> = 1.623 A. The M (1–4) sites are occupied by (Fe 2+ 1.71 Ca 0.07 Pb 0.02 □ 0.20 ), Fe 2+ 2.00 , (Fe 2+ 1.10 Mn 0.90 ) and (Fe 2+ 0.83 Sn 0.09 Zn 0.04 Mg 0.04 ), with M (1–4)–O> = 2.164, 2.146, 2.140 and 2.140 A, respectively. The D site is occupied by (Ti 1.56 Nb 0.24 Zr 0.16 Ta 0.04 ), with D –O> = 1.959 A. There are two interstitial sites, A and B , ideally occupied by Li and Na. The B site is occupied by (Na 0.78 Ca 0.22 ), with B –O> = 2.612 A. The A site splits into two half-occupied sites, A (1a) and A (1b), with A (1a)– A (1b) = 1.28(4) A. The [13] A (1a) site is occupied by K, Li, Cs and Na (K 0.75 Li 0.14 Cs 0.09 Na 0.02 apfu ), with A (1a)–O> = 3.310 A. The [5] A (1b) site is occupied by Li (1.0 Li apfu ), with A (1b)–O> = 2.28 A. The aggregate content of the A site is (Li 1.14 K 0.75 Cs 0.09 Na 0.02 ) ∑ 2.00 or, ideally, Li 2 . In the structure, an O sheet of Fe 2+ octahedra and two heteropolyhedral (H) sheets of (Si 4 O 12 ) ribbons and Ti octahedra form an HOH block of composition Fe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F. The HOH blocks link via common vertices of Ti octahedra (F atoms) along [001] and contain interstitial cations at the A and B sites. Nalivkinite is a Li analogue of astrophyllite, and the two structures are related by the isovalent substitution Li (nalivkinite) ↔K (astrophyllite) at the interstitial A site.