Synthesis and crystal-chemistry of Na(NaMg)Mg5Si8O22(OH)2, a P21/m amphibole

Abstract In the present work, we characterize the amphibole Na(NaMg)Mg5Si8O22(OH)2 synthesized at 0.4 GPa and 750, 800, and 850 °C, and 0.5 GPa, 900 °C. Experiments at 800 and 900 °C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)Mg5Si8O22(OH)2 has P21/m symmetry at room T. The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)Mg5Si8O22(OH)2 has two well-defined absorption bands at 3742 and 3715 cm-1 which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower frequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 cm-1 band is consistent with a stronger interaction of the H2 atom with ANa, which is confirmed by structure refinement. Increasing T of synthesis causes a progressive departure from the ideal stoichiometry via the A⃞1BMg1ANa-1BNa-1 substitution, as confirmed by EMPA, structure refinement, and FTIR spectroscopy.

[1]  C. Henderson.  The preparation of silicate compositions by a gelling method , 2006 .

[2]  P. Witte,et al.  Synthetische Amphibole mit OH-Überschuß im System Na2O−MgO−SiO2−H2O , 1969, Naturwissenschaften.

[3]  F. Cámara,et al.  The P21/m↔ C2/m phase transition in synthetic amphibole Na NaMg Mg5 Si8 O22 (OH)2: thermodynamic and crystal-chemical evaluation , 2003 .

[4]  G. Pedrazzi,et al.  BNa–BLi solid-solution in A-site-vacant amphiboles: synthesis and cation ordering along the ferri-clinoferroholmquistite–riebeckite join , 2003 .

[5]  R. Downs,et al.  The American Mineralogist crystal structure database , 2003 .

[6]  M. Wilke,et al.  The oxidation state of iron in silicic melt at 500 MPa water pressure , 2002 .

[7]  F. Hawthorne,et al.  Constraints on F vs. OH incorporation in synthetic [6]Al-bearing monoclinic amphiboles , 2001 .

[8]  M. Carpenter,et al.  Phase transition and mixing behaviour of the cummingtonite–grunerite solid solution , 2001 .

[9]  F. Hawthorne,et al.  The OH-F substitution in synthetic pargasite at 1.5 kbar, 850 °C , 2000 .

[10]  F. Hawthorne,et al.  Near-infrared study of short-range disorder of OH and F in monoclinic amphiboles , 1999 .

[11]  D. Jenkins,et al.  A Rietveld and infrared study of synthetic amphiboles along the potassium-richterite–tremolite join , 1997 .

[12]  W. Heinrich,et al.  Tremolite synthesis from CaCl 2 -bearing aqueous solutions , 1996 .

[13]  J. Klinowski,et al.  A MAS NMR study of a monoclinic/triclinic phase transition in an amphibole with excess OH; Na 3 Mg 5 Si 8 O 21 (OH) 3 , 1996 .

[14]  F. Hawthorne,et al.  Short-range disorder of Si and Ti in the tetrahedral double-chain unit of synthetic Ti-bearing potassium-richterite , 1996 .

[15]  F. Hawthorne,et al.  (super [6]) Al disorder in amphiboles from mantle peridotites , 1995 .

[16]  F. Hawthorne,et al.  Synthesis and crystal structure refinement of synthetic fluor-pargasite , 1995 .

[17]  F. Hawthorne,et al.  Amphibole synthesis at low pressure : what grows and what doesn't , 1991 .

[18]  J. Robert,et al.  The infrared OH-stretching region of synthetic richterites in the system Na20-K20-CaO-MgO-Si02-H20-HF;The infrared OH-stretching region of synthetic richterites in the system Na20-K20-CaO-MgO-Si02-H20-HF , 1989 .

[19]  D. Jenkins.  Synthesis and characterization of tremolite in the system H 2 O-CaO-MgO-SiO 2 , 1987 .

[20]  F. Hawthorne.  The crystal chemistry of the amphiboles; Crystal structure refinement of amphiboles , 1983 .

[21]  R. Young,et al.  A new computer program for Rietveld analysis of X-ray powder diffraction patterns , 1981 .

[22]  R. D. Shannon.  Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides , 1976 .

[23]  W. Maresch,et al.  Synthesis, lattice constants and OH-valence vibrations of an orthorhombic amphibole with excess OH in the system Li2O-MgO-SiO2-H2O , 1976 .

[24]  R. Strens.  The Common Chain, Ribbon, and Ring Silicates , 1974 .

[25]  V. Farmer,et al.  The effect of “A” site occupancy upon the hydroxyl stretching frequency in clinoamphiboles , 1973 .

[26]  I. F. Chang,et al.  Application of a Modified Random-Element-Isodisplacement Model to Long-Wavelength Optic Phonons of Mixed Crystals , 1968 .

[27]  H. S. Young,et al.  The Hydrothermal Synthesis of Sodium Amphiboles , 1964 .