Re-investigation of the crystal structure of magnesium astrophyllite

The crystal structure of magnesium astrophyllite, ideally K2 Na [Fe4 Mg2 Na] [5]Ti2 Si8 O26 (OH)4, is monoclinic, sp. gr. C 2/m, a = 5.3327(2), b = 23.1535(9), c = 10.3775(4) Å, β = 99.615(1)°, V = 1263.3(1) Å3, Z = 2, Dcalc. 3.158 g cm−3 from Mt. Yukspor, Khibina alkaline massif, Kola Peninsula, Russia, has been refined to R1 3.2% on the basis of 1861 unique reflections (Fo > 4σF) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and MoKα-radiation. Electron microprobe analysis gave (wt.%): SiO2 39.72, Al2O3 0.24, Nb2O5 0.64, TiO2 13.11, FeO 18.86, MgO 6.72, MnO 4.21, CaO 0.65, BaO 0.13, Na2O 4.22, K2O 7.66, F 0.29, H2O 3.00 (calc. from structure refinement), O=F–0.12, total 99.33. The empirical formula is calculated on the basis of 30.2 (O+OH+F) anions: (K1.97Ba0.01)∑=1.98 (Na0.65 Ca0.14)∑=0.79(Fe3.182+ Mg2.02 Na1.00 Mn0.72)∑=6.92(Ti1.99 Nb0.06)∑=2.05 [(Si8.01 Al0.06)∑=8.07 O24] O2 (OH)4.03 F0.19, Z = 2. The main structural unit is the HOH block in which there are one close-packed octahedral (O) and two heteropolyhedral (H) sheets. There are four [6]-coordinated sites in the O sheet. The M(1,2,3, 4) sites are occupied by Na1.00, (Fe1.252+Mn0.75), (Fe1.202+Mg0.80) and (Mg1.25Fe0.752+), with 〈M(1,2,3,4)-O〉 = 2.372, 2.166, 2.113 and 2.094 Å, respectively. In the H sheet, there is one [5]-coordinated D site, occupied by (Ti1.97Nb0.03) with 〈D–O〉 = 1.907 Å, and there are two tetrahedrally coordinated T sites occupied primarily by Si with minor Al, with 〈T-O〉 = 1.625 Å. There are two interstitial sites, a [10]-coordinated A site and a [8]-coordinated B site occupied by (K1.96Ba0.01□0.03) and (Na0.65Ca0.14□0.21), respectively, with 〈A-O〉 = 3.120 and 〈B–O〉 = 2.536 Å. In the structure of magnesium astrophyllite, M(1–4) octahedra form a close-packed O sheet. (SiO4) tetrahedra form (T4O12) astrophyllite ribbons along a. There are two types of (T2O7) groups, bridging and terminal, in the branched astrophyllite ribbon. Astrophyllite ribbons and [5]-coordinated D polyhedra link through common vertices to form the H sheet. An O sheet and two adjacent H sheets link through common vertices of (TO4) tetrahedra, D polyhedra and M octahedra to form an HOH block parallel to (001). The HOH blocks repeat along [001], and K and Na atoms occur at the interstitial A and B sites. The crystal structure of magnesium astrophyllite is topologically different from any other crystal structure of the astrophyllite group due to the linkage of two H sheets and a central O sheet within one HOH block. In the structure of magnesium astrophyllite, an octahedron of the O sheet shares trans edges with two terminal (T2O7) groups belonging to two H sheets of the HOH block (linkage A). In the other astrophyllite-group minerals, two types of linkage occur: (1) linkage A as in magnesium astrophyllite, and (b) linkage B, where the central octahedron of the O sheet shares an edge with a terminal (T2O7) group of one H sheet and a corner with a D octahedron of the other H sheet. There is the possibility of existence of a Ca end-member, ideally K2 Ca [Fe4 Mg2 Na] [6]Ti2 Si8 O26 (OH)4 F, with the topology of the magnesium astrophyllite structure. Na and Ca end-members may differ in coordination of Ti at the D site.