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The Crystal Structure of Synthetic Soda Melilite, Canaaisi207 1. Introduction

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The Crystal Structure of Synthetic Soda Melilite, Canaaisi207 1. Introduction Zeitschrift fUr Kristallographie, Bd. 131, S. 314--321 (1970) The crystal structure of synthetic soda melilite, CaNaAISi207 By S. JOHN LOUISNATHAN Department of the Geophysical Sciences, The University of Chicago, Chicago (Received 25 August 1969) Auszug Die Kristallstruktur von synthetischem N atriummelilith wurde aus Dif- fraktometerdaten bis R = 5,10/0 verfeinert. Die Verbindung hat die gleiche Struktur wie die iibrigen, natiirlich vorkommenden Melilithe. Die Aluminium- und Siliciumatome sind in der Struktur geordnet, die Calcium- und Natrium- atome statistisch verteilt. Abstract The crystal structure of synthetic soda melilite was refined to R = 5.10/0 from three-dimensional diffractometer data. Soda melilite is isostructural with the other naturally-occuring melilites. Aluminum and Si atoms are ordered in the structure while Ca and N a atoms are disordered. 1. Introduction The literature on the melilite group of minerals with a discussion of the various proposed end-members was reviewed by CHRISTIE (1961 a and b; 1962). After prolonged discussion it is now generally accepted that CaNaAlSi207, called soda melilite, can be established as an end member, partly represented in some natural melilites. YODER (1964) showed that pure soda melilite is stable only at pressures in excess of 4 kbar. Its optical properties were determined by SCHAIRER, YODER and TILLEY (1965). The crystal-structure determination of soda melilite was under- taken as a part of a systematic structural study of the melilite group. The general composition of a melilite can be written as (Ca,Na)2 (Mg,Al)(Al,Si)207. Order-disorder of Mg,Al and Si among the tetra- hedral sites and diadochy of Ca, N a etc. in the 8-coordination sites are of principal interest in this group of minerals as eXplained by SMITH (1953). The crystal structure of synthetic soda melilite 315 2. Experiments The specimen was prepared by YODER from glass at 1150°C, 10kbar, 70 hours. The product is more than 95% soda melilite with some glass and rare wollastonite needles. Traces of platinum from the sealing tube were also present. A diffractometer powder pattern using Ni-filtered Cu radiation, and a photographic powder pattern using Mn-filtered Fe radiation were made using spectroscopically pure silicon as internal standard. Of the 56 lines, four weak lines agreed with four of the stronger lines of wollastonite, and two weak lines with two of the stronger lines of platinum. The remaining 50 lines (Table 1) were readily indexed on a tetragonal cell with cell dimensions determined by the least-squares technique: a = 7.6344 ::I::0.0006 A and c = 5.0513 ::I::0.0006 A. Table 1. Powder pattern of soda melilite, CaNaAISi,O, , , , .kI dobs deale hkI , Jobs dcalc hkI dobs dcalc hkI dobs d calc , , 001 89 5.058 X 5.051 K 311 2.178 X 2.178N. 303 6 ,4041 X 1.4038 R 602 " 1.1361 X 1.1361". X q.213 101 11 "'.218 212 2.027 2.030 313 11 1.3814 1.3810 503 1.1310 1. 131 1 111 , 3.688 1.908 521 8 1.3646 1.3650 1.1190 1.1189 3.691 '00 1.9°9 3" 210 7 }.414: 222 "'96 1.844 512 22 1.2875 1.2879 m 1.0850 1.0845 3."'5 }.045 1.8"'. 3" 201 JJ 3.045 312 37 1.744 1.745 00' 17 1.2628 1.2627 334 ~1.0340 1.0337 211 100, 2.827 2.829 420 J '.707 1.707 522 vw 1.236} 1.2363 721 1.0266 1.0268 220 2.699 2.699 OOJ 11 1.684 1.684 3JJ * ~1.229' 1.2286 60J 1.01118 1.0151 002 J9 2.526 2.525 10J 8 1.644 1.641t 611 1.2187 1.2181 130 1.0028 1.0025 J10 17 2.415 2.41'1 '21 6 1.618 1.617 42J 1.1978 1.1987 115 0.9931 0.9930 1.1843 0.9810 102 62 2.398 2.398 113 17, 1.606 1.607 2" 1.1837 623 0.9809 221 17 2.383 2.381 203 1.539 1.540 621 1.1749 1.1741 0.9731 0.9731 112 26 2.287 2.288 21J 1.5097 1.5100 1. 1608 1.1604 5" J01 22 2.271 2.273 22J 8 1.4281 1.4285 5" 1.1442 1.1438 " '" "All lines above this were obtained frottl Cu-Ni diffractometer pattern, this line and all the following lines were obtained from Fe-Mn photographic powder pattern Precession and Weissenberg photographs of a [001] cleavage frag- ment of 0.06 X 0.1 X 0.2 mm dimensions were made yielding space- group extinctions compatible with P421m. Four hundred and ninety one independent intensities, representing the layers hOl to h6l, were measured on a manual scintillation-counter diffractometer of Weis- senberg geometry, using Zr-filtered MoKiX radiation. The observed intensities were corrected for Lorentz, polarization, transmission, and absorption (fl = 16.4 em-I) effects. 3. Structure determination and refinement It was assumed that soda melilite is isostructural with the other melilites, with Al and Si ordered in the tetrahedral sites Ah.o in Tl.2 and Sh.o in Ta_6 following the notation of LOUISNATHAN(1970) * and Ca and Na disordered (CaO.5NaO.5) in the 8-coordination sites. Using the appropriate half-ionized scattering functions, calculation of structure amplitudes with the atomic coordinates obtained for * Tl.2 tetrahedra are at 4, and T3-6 tetrahedra are at m point symmetry. 316 S. JOHN LOUISNATHAN gehlenite (LOUISNATHAN, 1970) gave an initial agreement factor of R = 0.24. The structural parameters were refined by the least-squares technique using the SORFLS program [a local modified version ofthe ORFLS program of BUSING, MARTIN and LEVY (1962)]. Six cycles of refinement varying the scale factor, positional and isotropic thermaI- vibrational parameters of the atoms brought R to 0.14. At this stage a calculation of interatomic distances showed that the T1-O(3) distance was 1.75:1:: 0.02 A and the mean T3-O distance 1.61 :1::0.01A, indicating clear separation of Al into Tl from Si into T2. ]1-'urther re- finements used a weighting scheme based on the ratio of peak to background intensity, with where I is the number of counts obtained for the peak without the background, B the number of background counts, with x and y taken as unity. Such a weighting scheme assigned weights between 1.0-2.0 for reflections with FObSin the range of 60-20 (scale factor being close to unity) and weights between 2 and 10 reflections with FobS in the range of 20 to nearly zero. Such a weighting scheme appeared to be justifiable, since the quantity g = {E[w(Fobs-Fcalc)2]j(No-Nv)}t, (where No is the number of observations, Nv the number of para- meters varied) was always close to unity (HAMILTON, 1964). Different scale factors for each k layer, anisotropic temperature factors for all atoms, and anomalous-scattering corrections with f' = f" = 0.1 for the tetrahedral sites and f' = 0.1 and f" = 0.25 for the 8-coordination sites were introduced. Refinement was continued varying also the site-occupancies of the cation sites. After six cycles of refinement R was 0.10 and weighted R (R') 0.07. Further cycles involved two parallel sets of refinements one on hkl set and the other on hkl, in order to minimize the polar dispersion errors in interatomic distances. After four cycles of adjustment the refinements converged giving the following agreement factors: set R R' g hkl 0.081 0.052 0.9 hkl 0.080 0.051 0.9 The differences in the positional coordinates of atoms between the two polarities were within the estimated errors in these quantities. The crystal structure of synthetic soda melilite 317 Table 2. The observed and calculated structure amplitudes in the synthetic soda melilite The table contains 493 reflections. The overalI scale factor is 1.16 x 10-' b k 1 F, F, h k 1 F, F , h k 1 F, F , h k I F, F, .h k 1 F, F, , 002 40g8 3974 10 0 7 1353 1233 2 2 JI 3461 3593 ~38 1216 1224 8 6 1567 1343 3 4030 4025 111 3451 3369 5 3175 3461 9 136 7 1li93 1308 , 6568 6656 2690 6 1607 111.25 0 2004"3 2121t 2 2743 .5 '3 , 8 861 698 5 1357 717 ,3 3350 3424 7 1974 1964 1 980 1031 9 0 1287 1343 6 3113 3191 553 333 8 903 803 2 1832 2110 1 1480 1523 1676 1566 2762 2828 1410 1069 1206 7 5 9 H52 3, 2 77' 632 8 '00 211 6 1511t 1454 3 2 0 101 1647 1604 3 239 2096 1867 2526 2626 1 2199"6 206} 961 109} , 5" 9 7 5 751 '59 10 583 176 8 6'7 2 2558 2502 6 1833 1776 5 1030 796 1 0 2 4179 4278 9 1220 12415" 3 21t37 2639 7 1395 1159 6 691 782 , "5 ,3 2079 1885 10 9'2 788 569 8 981 838 7 105 1124 1318 1 0 2734 2820 5 788 703 9 699 10 4 0 28975" }109 5 160 1 71(56 6 11194 1424 6 3 0 1745 1631'" 1 1!t04 1454 '67 241,.1 7'-30 6 2344 2 3571 7 73' 1 1007 1178 2 1801 1992 1216 1028 4605 43613"" 8 708 627"5 2 1540 1056 7 3, 1737 3, 79' 8 1175 "75 657 735 , 9 987 ,3 870 780 1541 1520 9 398 5 1825 1921 2 0 5552 5396'9' 1240 1357 5 226 10 531 39' 6 2077 1969 1 53' 3" 19J".5 1939 5 96 6 813 668 2 0 0 59', 506 7 760 2 2362 6 2013'" 19'9 7 776 217 1 Hlt3 8 201t6 2"'18 J"."'39 lJ".34 15J".3'" 3 2053 7 69' '53 5 5 0 1541 563 2 951 803 9 768 , 2172 2183 8 986 1017 1 729 721 3 2112 2192 10 1217 1086'5' 5 1523 1604 9 88' 697 2 2027 "2059 , 262 5167 6 1708 "6 3 1 0 5159 1499 7 3 0 5"'83 6374 3, 3976 3910 5 2520 2J".54 1 173% 1621 7 1355 1273 1 1995 2031 1704 1889 6 825 686 2 675ft 6776 8 665 522 2 3856 3812 5 2131 2269 7 1585 15"2 3 3389 3250 9 803 630 3 108'" 6 793 68.
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