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Refinement of the Crystal Structure of Ramsayite (Lorenzenite) Rurro Krvnxas

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Refinement of the Crystal Structure of Ramsayite (Lorenzenite) Rurro Krvnxas American Mineralogist, Volume 72, pages 173-177, 1987 Refinementof the crystal structure of ramsayite (lorenzenite) M,q.RKru R. SuNonBnc Department of Inorganic Chemistry, University of Helsinki, Vuorikatu 20, SF-00100 Helsinki 10, Finland Ml.nrtr LnHrrNnN Department of Geology, University of Helsinki, Snellmaninkatu 5, SF-00170 Helsinki 17, Finland Rurro KrvnxAs Department of Inorganic Chemistry, University of Helsinki, Vuorikatu 20, SF-00100 Helsinki 10, Finland Ansrn-c.cr The lattice parameters and crystal structure of ramsayite, NarTirSirOr, have been re- determined from a specimenfrom the Kola Peninsula.The spacegroup is Pbcn v/rth Z : 4; a : 8.7128(10),b : 5.2327(5),and c : 14.481(2)A. ttre anisotropicrefinement of the data using 778 reflections produced an R value of 0.026. The coordination polyhedron around Ti is a markedly distorted octahedron with no center of symmetry. Around Na there are seven oxygen atoms (Na-O distancesin the range 2.307-2.602 A); these poly- hedra form dimers by sharing three oxygens. The SiOo tetrahedra form pyroxene-type chainsin the direction ofthe b axis by sharingtwo oxygenswith the neighboringtetrahedra. The two remaining oxygensare sharedby two Ti atoms. Two vicinal TiOu polyhedra also share two oxygens(common edge),and the third facial oxygen is shared with the third TiOu polyhedron. These pairs of polyhedra lie approximately parallel to the a-b and a-c planes,respectively. INrnooucrroN ExpnnIltnNTAL DETAILS The two mineral namesramsayite and lorenzeniterefer The material used for study was from the collections of the to the samespecies (Kraus and Mussgnug,1941; Sahama, Mineralogical Museum, University of Helsinki. According to the 1947; Bogglld, 1953).Lorenzenite was found on an ex- original Russian label, the specimen(no. 4937) was collectedin pedition to southern Greenlandin 1897 (Flink, 1901), 1922 by the A. E. Fersman expedition and originates from the occurringin a nephelinesyenite. Chemical analysis showed Lujavr-Urt (:tovozero) alkali massif. Ramsayitefrom the same by Sahama(1947). that it contained ll.92o/o ZrOr. Later o\ in 1922, A. E. specimenwas chemically analyzed The specimenis a coarse-grainednepheline syenite (lujavrite) Fersman's expedition found a related mineral in the and has, in addition to greenishnepheline and microcline, red- nephelinesyenite of the Kola Peninsula.No Zr was found dish-brown eudialite and dark brown ramsayite as its major in this species(Kostyleva, 1937). Because there are very components. The most common accessorymineral is aegirine marked similarities in the crystal forms as well as in the (acmite). The subhedral ramsayite and eudialite crystals range chemical composition of the two minerals, their chemical up to 3 cm in diameter. The reader is referred to Kostyleva composition was redetermined by Sahama (1941). He (1937)and Masov et al. (1966)for thoroughdescriptions ofthe found less than 0.2o/oZrO, in both minerals; moreover, properties of the Lovozero ramsayite. ramsayite and lorenzenite specimens showed similar The powder-diffraction data of Lovozero ramsayite were re- (1974). compositions. Becausethere is a wide variation in the corded and treated as describedin detail by Lehtinen In this connection, it may be mentioned that the diffractogram of parameters reported for the unit cell (Table 1) and be- lorenzenite powder (Sahama, 1947) from Narsarsuk (Narssars- causethe R value (0.128) ofan earlier X-ray study of suk), Greenland, is virtually identical with that of the Lovozero poor ramsayitewas rather (Ch'in-Hanget al., 1969),we ramsayite. The observed and calculated d values are given in undertook redeterminations of both unit-cell parameters Table 2.r and the crystal structure. The single-crystalstudy wasperformed with a Nicolet P3 four- Besides a variation in cell parameters, there is also circle diftactometer. The unit-cell parameterswere obtained from variation in the reported spacegroups (Table l). In par- l8 independent high-angle reflections by using least-squares ticular, Shurtz (1955) showed that the spacegroup ob- methods. The details of the intensity measurementsare given in tained by Kraus and Mussgnugis wrong. Moreover, the Table 3. During data collection, one standard reflection was quality of indexingon the JCPDScard l8-1262 doesnot t Tables 2 and 4, order Document AM- seem to be satisfactory. About half the reflections are To receive copies of 87-325 from the BusinessOfrce, Mineralogical Societyof Amer- unindexed, and the number of the spacegroup on the ica, 1625 I Street,N.W., Washington, D.C. 20006. Pleaseremit card should be 60 rather than 50. $5.00 in advancefor the microfiche. 0003-o04x/87/0l 02{ r 73$02.00 173 174 SUNDBERGET AL.: REFINEMENTOF THE CRYSTALSTRUCTURE OF RAMSAYITE(LORENZENITE) Table1. Latticeparameters and spacegroups reported for ramsayite(lorenzenite) S.g. Ref. 14.51 8.73 5.22 Pbcn Krausand Mussgnug(1941)" 14.51 8.76 5.23 Pbcn Krausand Mussgnug(1941)o 14.26 8.57 5.09 Pnca Belovand Belyaev(1949) 8.66 ( 1n 14.42 Pbcn Shurtz(1955)" 14.518(3) 8.e76(3) 5.081(5) Pnca ch'in-Hanget al (1969) 14.52 8.98 5.09 Pnca Povarennykh(1972) 8.66 14.42 5.18 Pcnb Robertset al. (1974) 14.54 8.75 J.ZJ Pnca JCPDS,card 18-1262(1980) 8.707(3) 5.234(4) 14.492(3) Pbcn This workd 8.7128(1 0) 5.2327(5) 14.487(2\ Pbcn This work" b dpowder-diffraction " Ramsayite; lorenzenite;"synthetic lorenzenite; method; "single-crystal method. measuredafter every 39 reflections.The intensitiesof the former There is, however, one exception: The first coordinate of showed only statistical variation. The data were corrected for our 03 is 0.3336,whereas the correspondingearlier value Lorentz-polarization effectsand also for absorption, since the ry' is 0.431.The presentrefined atomic coordinatesand tem- scansshowed significant variation. The refinement ofthe struc- peraturefactors are shown in Table 5. The structure (Fig. ture was carried out using the x-RAy 76program (Stewart, 1976) l) consistsof pyroxene-type [SirOu]- chains running ap- and a Univac 1108 computer. The atomic coordinatesreported proximately in the direction of the b axis and having 05 earlier (Ch'in-Hang et al., 1969)were transformedto spacegroup 04 of the silicate chain is Pbcn and then used as the starting parameters.After anisotropic as the bridging oxygen atom. refinement, the final R value was 0.026 and R* was 0.035 (l/o'? bonded to one Ti only, whereas03 of the chain connects weights).Anomalous dispersion corrections for Ti, Na, and Si two neighboring Ti atoms. The TiOu octahedralie in two were obtained from Ibers and Hamilton (1974). The observed adjacent layers, alternating in each layer with chains of and calculatedstructure factors are given in Table 4 (seefootnote NaO, polyhedra. This alTangement generatesa close- l). packing system of unique type (the ramsayite type). The bond lengths and anglesare shown in Table 6. Rnsur-rs AND DrscussroN Lattice parameters TiOu polyhedron The lattice parametersobtained are shown in Table I The TiOu polyhedron is a considerably distorted octa- togetherwith valuesreported earlier. The precision ofthe hedron, with six nonequivalent Ti-O bond lengths(in the parametersfrom the literature was, in most cases,difficult range 1.820to 2.181 A) and with Ti lying off-center.Al- to evaluate becauseno standard deviations were given. though one of the coordinatesfor 03 differs significantly However, the lattice parametersreported by Ch'in-Hang from the value reportedby Ch'in-Hang et al. (1969),the et al. (1969) deviate significantlyfrom our values.It seems averageof the bond lengths (1.99 A) is identical to the probable that the poor R value they obtained is due to correspondingaverage of the earlier determination. The inaccurate lattice parameters and the film method they averagevalue falls halfway between the two values cor- used. respondingto the Ti4+-O and Ti3+-O bond lengths, cal- culated from the sum ofthe effectiveionic radii, 1.96 and Outline of the structure 2.02 A, respectively(Shannon, 1976). However, it seems The outline of the structure obtained in this study is that even when Ti has the oxidation number +4, there similar to that reported earlier (Ch'in-Hang et al., 1969). is considerable variation in the bond lengths. In BauTi,rOoo,among I I TiO6 polyhedra, the Ti-O dis- Table3. Crystal,intensity collection, and refinement tances tend to have greater values when Ti does not lie data at the center of symmetry (Hofmeister and Tillmanns, 1984).Moreover, the chemicalanalysis (Sahama, 1947) Formula NarTi2SirO, to assumethat the valence state of Ti is Tia+. Formulaweight 341.95 leads us F(000) 664 The off-center distortion is well known in the poly- z 4 morphs of TiOr, where variations in temperature and Density(g.cm{) Observed 3.45(5) pressure produce significant changesboth in symmetry Calculated 3.44 around Ti(I$ and in the bonding parameters (Simons Space group Pbcn (4i) and Dachille, 1967; Horn et al., 1972;'Meagher andLag- Crystal dimensions(mm) 0.11x0.13x0.20 Radiation MoKa (X : 0.71069A) er, 1979). It has been proposed that the ferroelectricity 2, limits 5'< 2d < 60' and the accompanying distortion of the TiOu coordina- Scan rate 0.9'to 29.3'/min polyhedron in perovskite-type crystals may have their Abs. coefficient(cm-') 30.3 tion Number of independentreflec- origin in a second-order Jahn-Teller effect (Bersuker, tions measured 1167 1966). > Uniquedata used [Fo 6"(F.)] 778 In one direction the TiOu pseudo-octahedraform a di- SUNDBERG ET AL.: REFINEMENT OF THE CRYSTAL STRUCTURE OF RAMSAYITE GORENZENITD 175 Table5. The atomiccoordinates
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