The crystal chemistry of the [M3φ11-14] trimeric structures:: From hyperagpaitic complexes to saline lakes

The crystal structures of nacaphite, Na (Na,Ca)(2) (PO4) F, a 5,3232(2), b 12.2103(4), c 7.0961(2) Angstrom alpha 90,002(1), beta 89.998(1), gamma 89.965(1)degrees, V 461.23(5) Angstrom (3), and quadruphite, Na-14 Ca-2 Ti-4 [Si2O7](2) (PO4)(4) O-4 F-2, a 5.4206(2), b 7.0846(2), c 20.3641(7), cl 86.89(l), beta 94.42(l), gamma 89.94(1)degrees, V 778.54(7) Angstrom (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 Pt (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\ > 4 sigmaF) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK alpha X-radiation. Nacaphite is twinned; this twinning of the true nacaphite unit-cell 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 (PO4) 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 phi (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 (SO4)(2) FC1, galeite, Nali (SO4)5 F-4 Cl. schairerite, Na-21 (SO4)7 F-6 Cl, kogarkoite, Na-3 (SO4) F, the synthetic compound (Na2Ca) (PO4) F (a dimorph of nacaphite), arctite, (Na5Ca) Ca-6 Ba (PO4), F, the synthetic compound Na-2 Fe (PO4) (OH), polyphite, Na-17 Ca-3 Mg (Ti,Mn)(4) [Si2O7](2) (PO4)(6) O-2 F-6 and sobolevite, Na-11 (Na,Ca)4 (Mg,Mn)Ti4 [Si2O7](2) (PO4)4 O-3 F, Bond-valence considerations suggest that Si analogues of some of these structures should be stable, and several phases of tricalcium silicate, Ca3 (SiO4) O, contain such trimeric clusters.