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THE STRUCTURE of CALCIUM SEIDOZERITE S. M. Skszat and V. I. Simonov

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THE STRUCTURE of CALCIUM SEIDOZERITE S. M. Skszat and V. I. Simonov SOVIET PHYSICS-CRYSTALLOGRAPHY VOL. 10, NO.5 MAR. -APR., 1966 THE STRUCTURE OF CALCIUM SEIDOZERITE S. M. Skszat and V. I. Simonov Nicolaus Copernicus University, Torun, Poland Institute of Crystallography, Academy of Sciences of the USSR Translated from KFistallografiya, Vol. 10, No.5, pp. 591-595, September-October, 1965 Original article submitted February 23, 1965 The structure of a new calcic variety of seidozerite from the Baikal area has been analyzed. The atomic coordinates were refined by the method of least squares. A comparison is drawn between the structure analysis results for the Khibiny and Baikal minerals. In 1958 E. 1. Semenov found and described and the systematic absence rule obeyed confirmed seidozerite, a new Zr, Ti silicate from the Lovo- that Ca seidozerite had the x-ray group 2/mP- /c. zero Massif (Kol'skii Peninsula) [1]. A struc- From a comparison of the Patterson projections of ture analysis of this mineral [2] showed the pres- the Khibiny and Baikal specimens, which confirmed ence of something which was unusual from the crys- the closeness of their structures, it was decided to tallochemical point of view, the isomorphous re- assign the Ca seidozerite the centrosymmetric placement of titanium by 25% zirconium. Not long space group P2/c, and to take its initial atomic co- ago, Portnov discovered a new variety of seidoze- ordinates in the structure analysis to be the same rite in the northern Baikal area [3]. The basic dif- as those of the Khibiny seidozerite [2]. As already ference between the chemical compositions of the pointed out, we considered that the most important Kol'skii and Baikal minerals was that the latter had question was the distribution of the cations among a considerably higher Ca content, with a corre- the independent crystallographic positions. The sponding reduction in Ti. It would be difficult to cationic compositions of the seidozerite samples, class this as isomorphic replacement, because of calculated on two Si atoms, were Na1.79CaO.19Mgo.1'( the great difference between the ionic radii. The . MnO.23Feo.19A1o.10Zro.72Tio.63NbO.02for the Khibiny purpose of the present investigation was to resolve specimen (Semenov [1]), and Nat.8t CaO.64MnO.28. the question of isomorphic replacement in calcium .Feo 21Zro 70Tio 39for the Baikal specimen (Portnov seidozerite, and to refine its structure. This last [3]): • • ! appeared desirable, since the structure of seido- These atoms were distributed among six crys- zerite had been interpreted from two projections tallographic positions, of which two were general, containing overlapping atoms. and four were multiple, on the twofold rotation axis. The unit cell constants for Ca seidozerite were Working from the chemical analysis, the peak a = 5.54 ± 0.02, b = 7.10 ± 0.03, c = 18.36 ± 0.10 A, heights on the electron density maps, and Hie in- and /3 = 102°40', calculated from rotation photo- teratomic distances, successive approximations graphs (RKV-86 camera, Mo radiation), which is led to the following groups of cations for the Khibiny in good agreement with the constants of Khibiny seidozerite (note that a change in the distribution seidozerite, a = 5.53 ± 0.02, b = 7.10 ± 0.03, c = of catiops affected the coordinates of the atoms): 18.30 ± 0.10 A, and /3= 102°43'. The experimental (0.72Zr + 0.28Ti), (0.23Mn + 0.17Mg + 0.10Fe), material for the reflection intensity studies, col- (0.35Ti + 0.02Nb + 0.09Fe + O.04AI),and (0.90Na + lected on six Weissenberg layer line photographs 0.10Ca) for one general and two special positions. (equi-inclination x-ray goniometer, Mo radiation), This arrangement of cations gave good agreement consisted of 135 independent, nonzero reflections for the peak heights and satisfactory interatomic oftypehOl, 160-hIl, 157-h2l, 101-0kl, 184-1kl, distances for the lowest value of the reliability 248-2kl. The symmetry of the x-ray photographs, factor. Particular attention was paid to the deter- 505 506 S. M. SKSZAT AND V. 1. SIMONOV mination of the kinds of cations which randomly TABLE 1. Coordinates and Individual Isotropic Temperature occupied the same crystallographic position as the Factors for the Principal Atoms in Calcium Seidozerite zirconium. Because of the marked difference be- Atom x/a II/b _/0 B, A' tween the ionic radii of Zr+4(r == 0.82 A) and Ti+4 (r == 0.64 A), there was little a priori probability Zr 0.198(2) 0.118(9) 0.0739 1.4 of isomorphous introduction of Ti into positions Ti 0.0000 0.109(3) 0.2500 1.0 Ca (Mn) 0.5000 0.348(8) 0.2500 1.0 occupied by Zr , However, the experimental evi- Na, (Ga) 0.202(1) 0.611 (6) 0.0693 1.6 dence (the R-factor, the peak heights, and the in- Nan 0.0000 0.609(6) 0.2500 1.1 Nam 0.5000 0.850(0) 0.2500 1.5 teratomic distances) meant that the above associa- Sir 0.723(1 ) 0.385(3) 0.1042 0.7 tion of cations had to be accepted. A crystallo- Sin 0.722(1) 0.839(8) 0.1053 0.9 0, 0.736 0.611 0.108 1.5 chemical interpretation of these results has been On 0.440 0.318 0.065 1.4 OIII 0.440 0.903 0.076 0.9 provided by Belov [4], who put forward the idea of OrV 0.919 0.314 0.056 0.9 1.0 the governing role of the manganese in the replace- Ov 0.900 0.909 0.051 Gv r 0.787 0.306 0.188 0.9 ment of zirconium by titanium. Oyn 0.816 0.919 0.187 1.2 1.6 The structure of the calcium seidozerite was O'vm 0.241 0.127 0.183 (F,OH) 0.302 0.570 0.191 0.7 refined by the least-squares method. The parti- cular features of the computing program used [5] with the final coordinates of the principal atoms. meant that the coefficients for bringing different These corrections carry within themselves the sys- layer lines to a common scale could only be re- tematic errors through neglecting absorption, and fined for one system of layer lines. Wewere there- the chance errors arising in the visual estimation fore unable to relate the Okl, 1kl, 2kl reflections of intensities. with the hOl, hIl, h2l reflections, these layer lines The reliability factors were about one percent being recorded on rotating the crystal about the higher if calculated without taking the individual axes a and b respectively. We first refined the temperature factors into account (15.6 and 11.0%). y- and z-coordinates of the basic atoms (keeping The most probable distribution of cations, the x-coordinates "frozen"), using 533 independent based on the chemical analysis [3],which was in nonzero reflections of the types Okl, 1kl, 2kl. The accord with the peaks found on the electron density y-coordinates obtained in this manner were then maps and which gave the reliability factors men- fixed during the refining of the x- and z-coordi- tioned above, had the form Zr(0.70Zr + 0.20Mn.+ nates, from 452 reflections of the types hOl, hIl, 0.10Fe), ~ Ti(0.39Ti + O.lIFe), ~ Ca(0.42Ca + h2l. These two cycles were repeated alternately 0.08Mn), NaI(0.78Na + 0.22Ca), ~Nan(0.50Na), several times. The procedure used enabled us to ~ Nam(0.50Na). The Baikal seidozerite lacked the compare the refined z-coordinates obtained from titanium which was isomorphous1y present in the two different sets of layer lines, and to estimate zirconium positions in the Khibiny seidozerite. the real accuracy of the results. The maximum The amount of titanium present in octahedral co- spread of the z-coordinates of the cations was found ordination, on the twofold axes, was practically to be 0.005 A. The corresponding average value the same in the Khibiny and Baikal minerals. The for the anion z-coordinates was 0.013 A, with a relatively small amount of Fe and Mn present, and maximum difference of 0.038 A. The linear x-ray the closeness of these elements in the periodic absorption coefficient (i.. == 0.71 A) in the seidoze- table, made it difficult to decide their distribution rite was J-l == 35 em -1. When F2hkl was calculated unambiguously, among the crystallographic posi- from the experimental intensities without taking tions in Ca seidozerite. A further difficulty arose the absorption into account, an error was intro- through the variety of valence states typically duced into the structure amplitudes which was shown by Fe and Mn, Table 2 lists cation -anion greatest for reflections of type Okl, 1kl, 2kl, since interatomic distances in the Ca seidozerite struc- the crystal fragment used, with roughly equal di- ture. These distances support the distribution of mensions along a and c, was elongated along b. cations suggested above, and are not abnormal [6]. This was reflected in the final reliability factors, The only discrepancy involves the Ca(Mn) octa- which had the values of 14.6 and 9.97% for all non- hedron, which will be discussed further below. The zero reflections of the Okl, 1kl, 2kl and hOl, hIl, edges of the SiI- and Sin-tetrahedra have lengths h2l groups respectively. In the last stages of the of 2.59-2.69 A and 2.57-2.68 A. The distances be- structure refinement, the least-squares method was tween oxygens in the Ti octahedra are 2.72-2.89A. used to obtain the individual isotopic temperature The 0V-Ov* edge shared between two Zr octa- corrections, which are listed in Table 1 together hedra is shortened, down to 2.70 A.
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