The Crystal Chemistry of the [M3 11 14] Trimeric Structures: From Hyperagpaitic Complexes to Saline Lakes

Abstract

.94(1)°, V 778.54(7) Å 3 , from the Khibina-Lovozero alkaline complex, Kola Peninsula, Russia, have been solved by direct methods for the holotype single crystals in the space groups P1 (Z = 4) and P1 (Z = 1) and refined to residual R values of 5.6 and 3.1%, respectively, using 2293 and 7303 observed (| F o | > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK␣ X-radiation. Nacaphite is twinned; this twinning of the true nacaphite unitcell emulates the C-centered unit-cell previously described for nacaphite. The chemical formulae of both minerals have been slightly revised. There is a close relation between the structures of nacaphite and quadruphite. The main fragment of the nacaphite structure is a cluster of three octahedra with a common F-F edge. These trimeric clusters link to form chains extending along [100] that are connected into a framework by (PO 4 ) tetrahedra. The structure of quadruphite consists of two structural blocks (TS and AC) stacked along [001]. The Ti silicate (TS) block has a three-layered structure consisting of a central sheet of octahedra and two adjacent Ti-Si sheets. The AC block of alkali cations can be described in terms of nacaphite-like trimeric clusters; as in nacaphite, these trimeric clusters link along [100] to form an infinite chain. Trimeric clusters of the form [M 11-14 ], M = Na, Ca; : O, OH, F, Cl, are common in a group of alkali sulfates, phosphates (and silicates), occurring in sulphohalite, Na 6 (SO 4 ) 2 F Cl, galeite, Na 15 (SO 4 ) 5 F 4 Cl, schairerite, Na 21 (SO 4 ) 7 F 6 Cl, kogarkoite, Na 3 (SO 4 ) F, the synthetic compound (Na 2 Ca) (PO 4 ) F (a dimorph of nacaphite), arctite, (Na 5 Ca) Ca 6 Ba (PO 4 ) 6 La nacaphite est maclée; ces macles de la vrai maille simulent une maille à C centré, attribuée antérieurement à la nacaphite. Les formules chimiques des deux minéraux ont été légèrement corrigées. Une relation étroite existe entre les deux structures. Le fragment principal de la structure de la nacaphite est un groupement de trois octaèdres partageant une arête F-F commune. Ces groupements trimériques sont rattachés en chaînes le long de [100], à leur tour rattachées par des tétraèdres (PO 4 ) pour former une trame. La structure de la quadruphite contient deux blocs structuraux (TS et AC) empilés le long de [001]. Le bloc à silicate de Ti (TS) possède une structure à trois niveaux contenant un feuillet central d'octaèdres et deux feuillets adjacents Ti-Si. On peut décrire le bloc AC contenant des cations alcalins en termes de structures trimériques ressemblant à la nacaphite; tout comme dans la nacaphite, ces groupements trimériques sont liés le long de [100] pour former une chaîne infinie. Des groupements trimériques répondant à la formule [M 11-14 ], M = Na, Ca; : O, OH, F, Cl, sont répandus dans un groupe de sulfates, phosphates (et silicates) alcalins, par exemple

From Structure Topology to Chemical Composition. I. Structural Hierarchy and Stereochemistry in Titanium Disilicate Minerals

“…Sokolova (1997) designated it as a nacaphite block, as it is topologically identical and chemically very close to the crystal structure of nacaphite, Na (NaCa) (PO 4 ) F (Sokolova et al 1989, Sokolova & Hawthorne 2001. Sokolova & Hawthorne (2001) designated this part of the I block as the AC block to emphasize the key role played by alkalis in the structure of these minerals.…”

“…Sokolova (1997) designated it as a nacaphite block, as it is topologically identical and chemically very close to the crystal structure of nacaphite, Na (NaCa) (PO 4 ) F (Sokolova et al 1989, Sokolova & Hawthorne 2001. Sokolova & Hawthorne (2001) designated this part of the I block as the AC block to emphasize the key role played by alkalis in the structure of these minerals. The presence of the trimeric cluster in a series of crystal structures indicates its high stability as a structural motif and testifi es to a highly alkaline environment of formation (Sokolova & Hawthorne 2001).…”

The Crystal Chemistry of Epistolite

“…1b). The sheet of octahedra is a central part of the TS (titanium silicate) block that is a constituent of several Ti-silicate minerals (Sokolova & Hawthorne 2001). Two outer parts of the block are heteropolyhedral sheets of [Si 2 O 7 ] groups and M(1) octahedra (Figs.…”

Synthesis and crystal structure of a new open-framework iron phosphate (NH4)4Fe3(OH)2F2[H3(PO4)4]: Novel linear trimer of corner-sharing Fe(III) octahedra

“…Trimers consisting of three mononuclear or polynuclear octahedra, although those involving other similar polyhedra (e.g., trigonal bipyramids and square pyramids) are also included, are common building units in phosphates and are known to exert important influences on the catalytic and magnetic properties of these materials [6,7,14,32,33]. However, most trimers in both natural and synthetic phosphates polymerize further to form larger clusters (e.g., tetramer, pentamer, heptamer, etc.)…”

“…However, most trimers in both natural and synthetic phosphates polymerize further to form larger clusters (e.g., tetramer, pentamer, heptamer, etc.) or infinitely extended chains, columns, layers, and frameworks [7,14,[32][33][34]. There are relatively few examples of isolated trimers linked by (PO 4 ) groups (Fig.…”