The crystal structure of magnesium astrophyllite, ideally K 2 Na [Fe 4 Mg 2 Na] [5] Ti 2 Si 8 O 26 (OH) 4 , is monoclinic, sp. gr. C 2/ m , a = 5.3327(2), b = 23.1535(9), c = 10.3775(4) A, β = 99.615(1)°, V = 1263.3(1) A 3 , Z = 2, D calc . 3.158 g cm −3 from Mt. Yukspor, Khibina alkaline massif, Kola Peninsula, Russia, has been refined to R 1 3.2% on the basis of 1861 unique reflections ( F o > 4σ F ) collected on a Bruker AXS SMART APEX diffractometer with a CCD detector and Mo K α-radiation. Electron microprobe analysis gave (wt.%): SiO 2 39.72, Al 2 O 3 0.24, Nb 2 O 5 0.64, TiO 2 13.11, FeO 18.86, MgO 6.72, MnO 4.21, CaO 0.65, BaO 0.13, Na 2 O 4.22, K 2 O 7.66, F 0.29, H 2 O 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: (K 1.97 Ba 0.01 ) ∑=1.98 (Na 0.65 Ca 0.14 ) ∑=0.79 (Fe 3.18 2+ Mg 2.02 Na 1.00 Mn 0.72 ) ∑=6.92 (Ti 1.99 Nb 0.06 ) ∑=2.05 [(Si 8.01 Al 0.06 ) ∑=8.07 O 24 ] O 2 (OH) 4.03 F 0.19 , 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 Na 1.00 , (Fe 1.25 2+ Mn0. 75 ), (Fe 1.20 2+ Mg0. 80 ) and (Mg 1.25 Fe 0.75 2+ ), with 〈 M (1,2,3,4)-O〉 = 2.372, 2.166, 2.113 and 2.094 A, respectively. In the H sheet, there is one [5]-coordinated D site, occupied by (Ti 1.97 Nb 0.03 ) with 〈 D –O〉 = 1.907 A, and there are two tetrahedrally coordinated T sites occupied primarily by Si with minor Al, with 〈 T -O〉 = 1.625 A. There are two interstitial sites, a [10]-coordinated A site and a [8]-coordinated B site occupied by (K 1.96 Ba 0.01 □ 0.03 ) and (Na 0.65 Ca 0.14 □ 0.21 ), respectively, with 〈 A -O〉 = 3.120 and 〈 B –O〉 = 2.536 A. In the structure of magnesium astrophyllite, M(1–4) octahedra form a close-packed O sheet. (SiO 4 ) tetrahedra form (T 4 O 12 ) astrophyllite ribbons along a . There are two types of (T 2 O 7 ) 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 (TO 4 ) 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 (T 2 O 7 ) 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 (T 2 O 7 ) 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 K 2 Ca [Fe 4 Mg 2 Na] [6] Ti 2 Si 8 O 26 (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.