Further developments in the structure topology of the astrophyllite-group minerals

The structure topology and crystal chemistry have been considered for ten astrophyllite-group minerals that contain the HOH layer, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated D polyhedra and the astrophyllite (T (sub 4) O (sub 12) ) ribbons. The HOH layer is characterized by a planar cell with a approximately 5.4, b approximately 11.9 Aa and a^b approximately 103 degrees . The ideal composition of the O sheet is Fe (super 2+) (sub 7) (astrophyllite) or Mn (super 2+) (sub 7) (kupletskite). All structures consist of an HOH layer and an I (intermediate) block that consists of atoms between two HOH layers. In the astrophyllite group, there are two types of structures based on the type of linkage of HOH layers: (1) HOH layers link directly where they share common vertices of D octahedra, and (2) HOH layers do not link directly via polyhedra of the H sheets. The type-1 structure occurs in astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite and kupletskite-(Cs); the type-2 structure occurs in magnesioastrophyllite, sveinbergeite and devitoite. The general formulae for the eight astrophyllite-group minerals (astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite, kupletskite-(Cs), magnesioastrophyllite) and for the extended astrophyllite group including devitoite and sveinbergeite are A (sub 2) BC (sub 7) D (sub 2) T (sub 8) O (sub 26) (OH) (sub 4) X (sub 0-) and A (sub 2p) B (sub r) C (sub 7) D (sub 2) (T (sub 4) O (sub 12) ) (sub 2) IX (super O) (sub D2) X (super O) (sub A4) X (super P) (sub Dn) , respectively, where C and D are cations of the O and H sheets, C = (super [6]) (Fe (super 2+) , Mn, Fe (super 3+) , Na, Mg, Zn) at the M(1-4) sites; D = (super [6,5]) (Ti, Nb, Zr, Fe (super 3+) ); T = Si, minor Al; A (sub 2p) B (sub r) I is the composition of the I block where p = 1,2; r = 1,2; A = K, Cs, Li, Ba, H (sub 2) O, <; B = Na, Ca, Ba, H (sub 2) O, <; I represents the composition of the central part of the I block, excluding peripheral layers of the form A (sub 2) B; X = O, OH, F and H (sub 2) O; n = 0, 1, 2. Two topological issues have been considered: (1) the pattern of sizes of the M octahedra in the O sheet, M(1) > M(2) > M(3) > M(4) and (2) different topologies of the HOH layer in magnesioastrophyllite and all other structures of the astrophyllite group.