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THE CRYSTAL CHEMISTRY OF THE [M_311–14] TRIMERIC STRUCTURES: FROM HYPERAGPAITIC COMPLEXES TO SALINE LAKES

Elena Sokolova and Frank C. Hawthorne

Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

E-mail address: sokolova{at}ms.umanitoba.ca

The crystal structures of nacaphite, Na (Na,Ca)₂ (PO₄) F, a 5.3232(2), b 12.2103(4), c 7.0961(2) Å, 90.002(1), ß 89.998(1), 89.965(1)°, V 461.23(5) ų, and quadruphite, Na₁₄ Ca₂ Ti₄ [Si₂O₇]₂ (PO₄)₄ O₄ F₂, a 5.4206(2), b 7.0846(2), c 20.3641(7), 86.89(1), ß 94.42(1), 89.94(1)°, V 778.54(7) ų, from the Khibina–Lovozero alkaline complex, Kola Peninsula, Russia, have been solved by direct methods for the holotype single crystals in the space groups P (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 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_11–14], M = Na, Ca; : O, OH, F, Cl, are common in a group of alkali sulfates, phosphates (and silicates), occurring in sulphohalite, Na₆ (SO₄)₂ F Cl, galeite, Na₁₅ (SO₄)₅ F₄ Cl, schairerite, Na₂₁ (SO₄)₇ F₆ Cl, kogarkoite, Na₃ (SO₄) F, the synthetic compound (Na₂Ca) (PO₄) F (a dimorph of nacaphite), arctite, (Na₅Ca) Ca₆ Ba (PO₄)₆ F₃, the synthetic compound Na₂ Fe (PO₄) (OH), polyphite, Na₁₇ Ca₃ Mg (Ti,Mn)₄ [Si₂O₇]₂ (PO₄)₆ O₂ F₆, and sobolevite, Na₁₁ (Na,Ca)₄ (Mg,Mn)Ti₄ [Si₂O₇]₂ (PO₄)₄ O₃ F₃. 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.

Keywords: nacaphite, quadruphite, crystal structure, Khibina–Lovozero, Kola Peninsula, Russia.

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