83 The Canadian Mineralogist Vol 37, pp. 83-90 (1999)
MODULARAPPROACH IN SOLVINGTHECRYSTALSTRUCTURE OF A SYNTHETIC DIMORPH OF NACAPHITE, Na2CalPO4lF, FROMPOWDER-DIFFRACTION DATA ELENA V. SOKOLOVAeuo YURII K. KABALOV Departmentof Crystallography, Facultyof Geology,MoscowStateUniversitJ,Moscow,I19899,Russia GIOVANNI FERRARISI Dipartimentodi ScienzeMineralogichee Petrologiche,IJniversitddiTorino, ViaValpergaCaluso35,I-10125Torino,Italy JULIUS SCHNEIDER Institutfiir KristallographieundAngewandte Mineralogie,ILniversitcit Miinchen,D-80333Miinchen,Germany
ALEXANDER P. KHOMYAKOV Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, Russian Academy of Sciences,Moscow, 121357, Russia AssrRAcr By comparison with the known strxctures of nacaphite and arctite, the crystal structure of a synthetic dimorph of nacaphite, (Na667Cas33)3POaF, has been solved using a modular approach. The structure model, refined by Rietveld method on the basis of X-ray powder-diffraction data including minor quantities of villiaumite, fluorite and fluorapatite, consists of a stacking of 15 nacaphite-type7 .I x 7 .l x 2.1 A modules (slabs) along the t00ll direction of a rhombohedral R3m cell with a = 7 .0179(7), c = 4056@) A. Nacaphite, on the other hand, is triclinic and is basedon the stacking of only four such slabs.The slabs and their links in nacaphite and its synthetic dimorph are quite different in detail; thus the two compounds are polymorphs and not polyt)?es. The role of nacaphitemodules in building minerals ofhyperagpaitic rocks is discussedin terms ofmerotype and plesiotype senes. Modular relationships with sulphohalite-type minerals also are discussed. Keywords: nacaphite dimorph, crystal structure, powder diffraction, polysomatic series. Solavarns A la suite d'une comparaisonavec les structues connuesde la nacaphiteet de I'arctite, nous avons rdsolu la structurecristalline d'un dimorphe synthdtique de la nacaphite, (Na667Caa33)3PO4F, avec une approche fond6e sur la notion de modules. Le modble structural, affind par la m6thode de Rietveld d partir de donn6esen diffraction X (m6thode des poudres) pr61ev6essur un assemblage comprenant des quantit6smineures de villiaumite, fluorite et fluorapatite, montre en empilement de quinze modules de type nacaphite, chacun de dimensions 7.1 x 7.l x 2.1 A.le long de t0011 d une maille rhombo6driqte R3m iyant pour dimensions a 7.O179('1),c 4O.56(4) A. La nacaphite, par ailleurs, est triclinique, et sa structure montre un empilement de quatre de ces modules L'agencement des atomes dans ces modules et les liaisons intermodulaires difftrent dans les d6tails enffe nacaphite et son dimorphe. Il semble donc que ces deux structuresmontrent une relation polymorphique et non polytypique. Nous discutons du r61ede tels modules de nacaphitedans la charpentede min6raux de roches hyperagpaltiquesen termes de s6riesmdrotypiques et pl6siotypiques, et aussi des relations modulaires avec les min6raux du groupe de la sulphohalite. (Traduit par la R6daction) Mots-clds: dimorphe de la nacaphite, structure cristalline, diffraction X, m6thode des poudres, s6rie polysomatique.
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THE CANADIAN MINERALOGIST
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showed slightly split spots typical of twins. X-ray-diffraction data were collected on a CAD-4 Enraf-Nonius In the alkaline rocks of the silica-undersaturated diffractometer (MoKcr radiation, Department of InorLovozero and Khibina massifs,the main hosts of phos- ganic Chemistry, Moscow State University) from the phorus and fluorine are apatite and villiaumite, rarely best of the tested crystals (dimensions 0.20 x 0.22 x lomonosovite and (Na,Ca)-fluorophosphates.The eco- 0. 18 mm). On the basis of 25 reflections with 15 < 20 < nomic importance of apatite results from its abundance 20" , the following hexagonal cell has been obtained: a as a major mineral of apatite-nepheline orebodies. In 7.029(l), c 40.54(L) A, space grolp R3m or R3m. the Khibina alkaline massif, various (Na,Ca)-fluoroAttempts to solve the crystal structure by direct methphosphatesdo occur, including arctite, (Na5Ca)Ca6Ba ods failed, presumably because the crystals are affected lPO4l6F3,nefedovite, NasCa+[PO+]+F,and nacaphite, by some kind of polysynthetic twinning. As a result, we Na2CalPO4lF(Khomyakov et al. 1980, 1981, 1983). decided to use the Rietveld method. These (Na,Ca)-fluorophosphates occur in association Powder-diffraction data were collected on a focuswith apatite,Ca5@Oa)3(OH,F,C1), aegirine,NaFeSi2O6, ing STOE-STADIP diffractometer, equipped with a thermonatrite, NazCO:.HzO, and villiaumite, NaF, in curved Ge (111) primary monochromator (MoKctl rapegmatites emplaced in the ijolite-urtite units, i.e., in diation, |t = 0.70926 A; Institute fiir Kristallographie, an environment characterizedby high concentrationsof Universitiit Mtinchen) with a minimum full-width at sodium and fluorine. Arctite, nefedovite, and nacaphite half-maximum (FWHM) of 0.08', according to the proalso are of general interest because of their high con- cedure described by Wcilfel (1981). The sample was tents of phosphorus and fluorine, and becausethey ac- contained in a qtartz capillary rotated around its axis to tually replace fluorapatite in the hyperagpaitic rocks. minimize preferred orientation. Diffracted intensities In order to prove the important role of fluorine and were collected in a stepwiseoverlapping mode by a linsodium in the processesofformation ofapatite and as- ear position-sensitive detectorwith about 5" acceptance sociated(Na,Ca)-fluorophosphateminerals (Dawson & angle and 0.02o channel (Witlfel 1983); the yield was of Hill 1998, Khomyakov 1995), experimental studies of 2250 datapoints in the range 5
