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Download the Scanned American Mineralogist, Volume 67, pages 604409, 1982 The crystal structure of cascandite,CaScSi3Os(OH) Mancsrro MeLr-rNr AND STEFANoMERLTNo Istituto di Mineralogia e Petrografia, Universitd di Pisa and C.N.R., C.S. Geologia Strutturale e Dinamica dell'Appennino via S. Maria 53, 56100Pisa, Italy Abstract A new pyroxenoid mineral, cascandite,with the ideal formula cascSi3os(oH), is = j.076(0A, triclinic with a 9.791(3)4,b: fi.a2}e)A, c = a : 98.91(8f,F: 1O2.63(8).,y = 84.17(8)";Z : 4 for the CT unit cell setting. The crystal structure of cascanditeis similar to those of pectolite and seranditeand is composedof two main structural units: octahedral chains and three-repeattetrahedral chains. Cascandite,unlike pectolite and serandite,has only two octahedralcations per formula unit, and the octahedralchains are formed by two strands of edge-sharingoctahedra occupied by calcium and scandium cations. Cascanditeis a member of the hydrous pyroxenoid series,whose peculiaritiesare related to the presenceof intrachain hydrogen bonding. Introduction obtained starting from the atomic coordinates of Cascandite, a new scandium silicate, was found wollastonite based on PT setting, as given by in a geode from the granite of Baveno, Italy, Buerger and Prewitt (1961).However the E statis- together with quartz, orthoclase, albite and jervi- tics indicated a non-centrosymmetricstructure, and site, another new scandium silicate with ideal for- thus we removed the symmetry constraints on the mula NaScSi2O6.The descriptionsof these new octahedral cations. We assumedthat one of the six minerals are given by Mellini et al. (1982). The octahedral sites in the unit cell of the trial structure chemical, physical and crystallographic properties was vacant and the site that had been centrosymme- indicate that cascandite is a three-repeat pyroxe- trically related was occupied by a scandiumcation, noid. as suggested by the preliminary analytical data. The present structural study was undertaken to Four refinement cycles were calculated, each fol- give a better description of the new scandium lowed by a Fourier synthesis.The behavior of the mineral and to acquire a deeper insight into the thermal parameters and the heights of the various crystal chemistry of pyroxenoids. peaks in the Fourier maps strongly indicated that one more octahedral site had to be considered Structure determination vacant and that two silicon atoms had to be shifted A small crystal of cascandite was selected and from their positions in the trial structure. The new examined first by X-ray film methods. Triclinic cell structural arrangementsmoothly refined to an R, : parameters(Table 1) were obtainedby least squares > | lF"l - lF.l lDlF"l: 0.08. refinement using 25 reflections centeredon a Philips We then realized that the arrangement was in- PW 1100single crystal diffractometer with graphite deed a centrosymmetric one, closely related to that monochromatized MoKa radiation. A total of 1526 in minerals of the pectolite-serandite series. The non-zero reflections were measuredfrom 3 to 30' 0, false indication of the absenceof a symmetry center using a 0120scan and scan width 1.20". The data most probably resulted from not having taken into were corrected for Lorentz and polarization fac- account the "unobserved" reflections. The refine- tors. No absorptibncorrection was made because ment was then continued in the PT spacegroup. The of the small dimensions of the crystal. introduction of anisotropic thermal parametersfor Only qualitative energy dispersive analytical data all atomsled to Rr:0.042 and R2 : (2w I lFot- zDw were available when the structure determination lF.l | lFol2)1/2: 0.046, where u/ was the recip- was undertaken. The solution of the structure was rocal of the variance on F" as estimated from 0003-004)v82l0506-o6M$02.00 6& MELLINI AND MERLINO: CASCANDITE Table 1. Unit cell parameters of cascandite, serandite and these different topologies appearin the structures of schizolite wollastonite, bustamite and minerals of the pecto- + lite-serandite series. With regard to the general Cascand i te Serandite * schizolite cascSi,0U (0R) {nrNaSirOU(Ofl) Ca}{nNaSi308(OH) topological features, cascanditeis closely related to the pectolite-seranditegroup. This is clearly shown a 7.503(6),i s.zsr (o) i 9.909(9) A 10.o59(4) A by the comparison of Figure I in the present paper b 1.016(6) 10 42o(9) L0.661(9' 10.88o(8) with Figure 3 of Ohashi and Finger (1978). c 6.777(6) 1.o76(6) 6.913(4) 6.978(6) The bond lengths in the various tetrahedral and o 92.23(8)' 98.91(8) " 99.10(6)" 98.84(7)' octahedral polyhedra, together with their standard 0 93.s8(8) to2.63 (8) 1OO.51(6) 1O0.58(5) deviations, are given in Table 4. Two longer Ml- Y 1o4.49(8) 84. 17(8) 82,49 (7) 82.64(5) OC3 and M1-OC3*distances are reportedin brack- Space group Pl ct c1 c1 ets and were taken into account in calculating the valence bond balance. The mean bond lengths in * Values for serandite and schizolite are fron ohashi and FinBet Ml and M2 octahedra are given also in Table 5 (L978). together with the mean bond lengths in Ml, M2 and (Ohashi and counting statistics. Observed and calculated struc- A sites in schizolite and serandite the cation ture factors are given in Table 2.1 Finger, 1978).The same table compares present refinement with The entire refinement was carried out in the occupancies used in the Finger (1978) for conventional PT setting of the unit cell, whereasthe those obtained by Ohashi and site, occupied by crystal structure will be described and discussed schizolite and serandite. The M2 than the with reference to the C-centered cell first intro- scandium cations, is definitely smaller pectolite-schizo- duced by Narita (1973;quoted by Morimoto,1974) corresponding site in the entire difference in and used also by Ohashi and Finger (1978)in their lite-serandite series. Furthermore the M2 sites results discussion of the three-repeat pyroxenoid crystal size between the octahedralMl and The tetrahedral chemistry. Table 1 compares the C-centered unit in distortion of tetrahedral chains. of three tetra- cell parameters in this setting with the correspond- triplet, namely the C-shapedcluster arrangement ing parameters given by Ohashi and Finger (1978) hedra which provides a nearly square pectolite, deformed in for the seranditeand schizolite specimensstudied in of oxygens in is slightly arrangement their paper. The transformation matrix from the cascandite towards a diamond-shaped Moreover the A site, which in the conventionalPT cell to the CT cell is [101/10T/010].of oxygens. group is occupiedby The final atomic positional and thermal parameters pectolite-serandite of minerals and in this for cascanditein the CT setting are given in Table 3. sodium, appears vacant in cascandite respect cascandite is unique in the entire three- Description and discussionof the structure repeat pyroxenoid family. analysis clearly indi- The crystal structure of cascandite (Fig. 1) may The results of the structure for cascan- be described in terms of two main structural units: cate the ideal formula CaScSi3Oa(OH) presence the hydrogen atom, which octahedral double chains, formed by two strandsof dite. The of by the close relationship edge-sharing octahedra, and tetrahedral single was strongly suggested and the minerals of the pecto- chains with a repeat period of three tetrahedra. between cascandite was confirmed by the results of Each octahedral chain is connected to six tetrahe- lite-serandite series, (Table 6) and ascertained dral chains by corner sharing, whereas each tetra- the valence bond balance Apart from the hedral chain is connected to three octachedral by a difference Fourier synthesis. to the hydrogen atom, two chains. peak corresponding peaks found in the difference map' Cascanditeis thus classifiedwith the three-repeat additional were position (M2 pyroxenoids. It is well known that three-repeatpy- one corresponding to the scandium to the position of the sodium roxenoids develop three distinct stacking schemes site) and the other (A pectolite, schizolite and seran- of tetrahedral and octahedral structural layers: cations site) in dite. The chemical analysis (Mellini et al', 1982) showed the presence of significant quantities of I Document AM-82-200 To receive a copy of Table 2, order manganeseand iron and indicated the presenceof Mineralogical Society of America, from the Business Office, the chemical 2000 Florida Avenue, N.W., Washington, D.C. 20009. Please more than two octahedral cations in remit $1.00 in advance for the microfiche. formula of cascandite.On the basisof thesechemi- 606 MELLINI AND MERLINO: CASCANDITE -(h2 Table 3. Atomic positionalcoordinatesand anisotropic temperaturefactors (Xl0a) in the form: exp Ft * k2 hz + 12Be + zhk pn + 2hl BB + zkl fur).The coordinatesof the H and A sites were obtained from the difference Fourier synthesis. 9tt -22 9tz -23 JJ IJ Ml 0.Oo27(1) o.6629(L) 0.9128(1) 37(1) 24(r) 36(2) 3(1) 13(r) 1o(1) -o.0o19(1) t42 o.642r(r) 0.4062(1) 10(1) 13(1) 10(1) -4(1) o(1) 4(1) si1 0.2163(1) 0.0402(1) o.0886(2) 19(1) 2o(1) 27(2) -r (1) 4(1) 8(1) si2 o.2Lo2(r) o.0666(1) o- 5229 (2) 18(1) 18(1) 26(2) -2(1) 3(1) 10(1) si3 0.2o5s(1) o.8366(1) o.7399 (2) 20(1) 18(1) 28(2) -6(1) 2(7) 6(1) 0A1 o.1160(3) o.4490(3) o.8729 (s) le (3) 26(3) 42(6) -4(2) 1(3) 10(3) oA2 0.1176(3) o.4462(3) o.4203(5) 18(3) 25(3) 40(6) -1()\ 2(4) 8(3) (4) oA3 o.r27 9 o.6930(4) o.24L6(s) 24(3) 38(3) 47(6) -6 (3) 3(4) 13(4) - oBl o.L295(4) o.1s68(3) o.0262(5) 46(4) 2s<3) s2(7) 10(3) 16(4) 19(4) oB2 o.r479(4) o.
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