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The Crystal Structure of Tetragonal Leucite

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The Crystal Structure of Tetragonal Leucite American Mineralogist, Volume 61, pages l0B-l 15, 1976 The crystal structureof tetragonalleucite FronENzo Mtzzt, C.N.R. Centro di Studio per la Cristallografia stnttturale, c/o Istituto di Mineralogia dell'Uniuersitd, pauia, Italy EnnreNr.roGALLT, nNn Gt-,luco Gorre,Ror Istituto di Mineralogia e Petrologia dell'Uniuersitit, V[odena, Italy Abstract Low temperatureleucite is knownto havea tetragonalstructure which is similar to thehigh temperaturecubic structure.This tetragonalstructure has never been studied because of the finetwinning, which makes the selection of untwinnedsingle crystals difficult. The best crystal chosenfor this researchis madeup of threeindividuals: one, comprising 79 per centof the total,is merohedricallytwinned with respectto thesecond, and pseudo-merohedricallywith respectto the third.The crystalstructure of tetragonalleucite, a: 13.o9,c: 13.75A, space EroupI4r/a, can be derivedby displacingthe cubic structure (some atoms must be shiftedas muchas l A;. Thereis no (Si,Al)order. The K atomsare coordinated by six oxygensat the averagedistance of 3.01A, sixother being at -3.7 A.ln cubicleucite these values are 3.35 and 3.54A; this is the moststriking difference between the crystalstructures. Introduction the chemical analysis also revealedthe presenceof Leucite K[AlSi,O6] was shown by Wyart (1940)to 0.002 Mg and Ti. have a structure with the same topology as analcime, The unit cell dimensionsand the intensitiesof the as determinedby Taylor (1930).Leucite undergoesa reflections were measured with a Philips rwrroo phase transition, also described by Wyart (1940), singlecrystal automatic diffractometerusing graphite from a cubic phase (space group la3d) stable above monochromatized MoKa radiation at room 630"C, to a tetragonal one (space group l4r/a) stable temperature.The cell dimensionsthus determined-a : : at lower temperature. This polymorphic inversion 13.09(l)and c 13.75(1)A-are in good agree- was investigatedin detail by Faust (1963),who also ment with those determined by the least-squares listed previous studies of the problem. Recently refinementof powder diffraction data and with those Peacor(1968) refined the structureofthe cubic phase quoted by Faust (1963) for a sample from the same of leucite at high ternperature. The same author locality. The space group determined from the pointed out the difficulty of obtaining a suitable systematic absencesis I4r/a, the same as determined single crystal of the tetragonal phase due to the fine by Wyart (1940)and by Ndray-Szab6(1942). The in- twins which alwaysdevelop in tetragonalleucite dur- tensitiesof 1040independent reflections in the range ing its inversion from the high temperature cubic 4o-50" 20 were inspected using 0-20 scans with a phase. symmetrical scan range of 1.20' in 2d from the calculated diffraction angle. The scan rate was 0.04o Experimental 0/sec. For each reflection,repeated scans were made until the preset maximum number (3) or the preset A sample of leucite from Roccamonfina,Caserta, number of counts (1000) was reached. Due to the Italy (specimen n. 16-3-3-24 of the Museo di small dimensions(0.05 X 0.05 X 0.08 mm) of the Mineralogia dell'Universitddi Modena) was selected crystal, 529 weak reflections were skipped by the for this study (seenext section).A chemicalanalysis diffractometer program during the data collection, of this materialyields unit cell contents,calculated on becausetheir intensitiesin counts,/sec the basis of 6 oxygens,ot measuredat the top of each reflection (namely, ,lt) and the mean I 6([ Ko aoorCao o,] Fef, Si, intensities ".N [ .+o,Alo.r"]orO. ); in counts,/sec of both backsround 108 CRYSTAL STRUCTURE OF TETRAGONAL LEUCITE 109 measurements (namely, 15) gave: Irzrt< Ib. merohedricallytwinned individuals,the first compris- These "unobserved" reflections were assignedvalues ing 88 percent,the second8 percent,and the third 4 of half the minimum observedintensity. Three stan- percent; the second fragment, composed of only two dard reflections, monitored at three hour intervals, pseudomerohedricallytwinned individuals, compris- showed no variation in intensity greater than I per- ing 96 and 4 percent respectively,was satisfactory for cent. Processingof the data was carried out in the further measurements.The first individual of the twin manner describedby Daviesand Gatehouse(1973) to was actually affected by merohedric twinning, which the refinement of the crystal yield lF.u"l and olF,o"l. No absorption or extinction was detected during corrections were applied. The atomic scattering fac- structure (see next section). tors used are those listed by Hanson et al (1964). If a twinned crystal of tetragonal leucite is heated above 630oC,it inverts to an untwinned cubic crystal The leucite twins which, upon lowering the temperature, gives a Natural leucite crystalsalways grow as the cubic twinned crystal again with the same proportion of phase (Faust, 1963). When a cubic leucite single- twin domains and with the same twin boundaries as crystal inverts to the tetragonal form, a complex twin before. This "memory" displayed by leucite twins develops.All six planesof the cubic form {110} may was demonstrated by optical and/ or X-ray. methods become twin planes in the tetragonal phase (point by Wyart (1938), Peacor (1968), Sadanaga and group 4/m). Two casesmust be considered: Ozawa (1968), and Korekawa (1969). This I. Merohedric twins develop on the tetragonal "memory" was verified once more during this planes(110) and (T10),the two individualshaving ex- research, on the following three twins: actly parallel crystallographic axes, but with a and b I. Two individuals with common a axis, kept at interchanged. X-ray diffraction of such twins gives 800'C overnight. hkl diffractions of one individual exactly superposed II. Four individuals with common a axis, kept at -630'C with khl diffractions of the other. The two in- 700"C overnight,and then for threehours at dividuals of the twin show simultaneousextinction before cooling to room temperature. under crossednicols. III. Four individuals,the a axis being common for IL Peudomerohedric twins develop on the two individuals at a time, were kept at 700"C for one quenchings to room tetragonal planes ( l0l ), (0 I I ), ( 101), (0 I I ), the two day, with four interspersed fast individuals having parallel a (or b) axes, but the temperature. remaining two axes are not parallel. The two in- The intensities of a small set of diffractions were dividuals of such a twin give close but distinct spots measured at room temperature before and after in X-ray photographs and do not extinguish simul- heating for each twin individual; there was no signifi- taneouslywhen viewed under crossednicols. cant variation of these intensities. In short, if singlecrystal X-ray photographsreveal Determination and refinement of the any splitting of spots, at least a pseudomerohedric crYstal structure twin is presenqif no splitting appears,the presenceof a merohedric twin may still be suspected,but it can The structural study started with the atomic be detected only by studying the intensitiesof the parameters given by Peacor (1968) for the cubic diffractions. phase, properly adapted to the tetragonal space The different orientations of the individuals in group. Least squaresrefinement with a modified ver- leucite pseudomerohedrictwins were described in sion of the program Onnm (Busing, Martin and detail by Sadanaga and Ozawa (1968) and by Levy, 1962) allowed the reduction of R from 0.54 to Korekawa (1969). only 0.32, without possibility of further improve- Since the observation of the pseudomerohedric ment. Direct methods (program MulrnN described twins is properly made using the polarizing by Germain, Main, and Woolfson, l97l) were then microscope,thick sectionswere prepared from our applied, and new starting parameters were obtained. leucite specimen to allow the selection of some ap- In this case,R dropped to 0.1I in four least-squares parently untwinned fragments. Most of these frag- cycles. after a first X-ray test with os- At this stage one point emerged: the largest ments were discarded 's cillation photographs, but two were thus selected. differences between lF o"l and lF"'t"l were Measurementsusing the singlecrystal diffractometer observed for pairs of F1,p1and Fnntwhen lF""r"l's of revealed the first to be composed of three pseudo- the two were markedly different. This was ascribedto ll0 MAZZI, GALLI, AND GOTTARDI Trnlp l. Observedand CalculatedStructure Factors rtlrolls"lhtlrollF"lhklp"ilr"lhklF)lr"lhklpollF"lhklF"llF"lhklr"llr"l 2 9 206 411 L2* 246 -427 6 0 44-t -457 r0 3 90t -877 2 8 lO71 -tO9O r=6 4 9 -AS9 -1463 2 0 485 499 792 t2, 252 -t?6 I 0 1439 12 3. 258 -14,9 4 I 108.9 _II:,O -'-.. 4 0 6854 -6856 6 9 571 531 12* 26.2 3.8 L0 O' 22.4 32,7 14 3 75,2 7O.A 6 3 s7.a sj j ",:, 256.3 3 6 0r 16t -102 9 596 592 t2j 2j .S _t,4 12 0a 24.9 -S77 I 0 1945 1962 t0 9 585 14 tii,i-tii,i -452 o*27 s i"; I0 0 ll45 -1I90 l2 9t 242 ,;. ,;: ,;. l i ll:l il 9 lirz a'!. ii,i2aa '129ii,i i i 13* 26r -s69 I I 386 421 s 4 +.i -i.s ;;. ,i'.; i;.; 12 0 659 -685 I I0 2t24 2274 l3r 269 -194 3 I I774 176l 7 4. 207 _LL6 I 9 A22 -8gI lO or 249 _4r9 14 0" 268 388 3 l0 103 I -103 9 -189 s I -327 13. 280 356 9 4, 2?a -262 3 9) 229 116 12 O* 271 22r s l0 159 8 -160 -1891 2 2 488 468 9 7 I l89l tI 4 723 682 5 9* 239 373 7 t0 191 'i.! 4 2 3974 5883 125 '3 a. 21.3406 9 l0 855 833 i;.
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