Ame rican M ine ralo gist, Volume 82, pages 841-848, 1997
OrthorhombicJahn-Teller distortion and Si-OHin mozartite,CaMn3.O[SiOrOH]: A single-crystalX-ray, FTIR, and structuremodeling study
DlNrnr, Nyrnr-nnrl CunrsrrN.q,Horrvlq.Nxrt TnoMas AnrrnnusrERrr Manrnr KuNzr2.Lxn Eucnlr Lrsownzrv3
Ilaboratorium fiir chemischeund mineralogischeKristallographie, Universitiit Bern, Freiestrasse3, CH-3012 Bern, Switzerland '?HighPressure Group, EuropeanSynchrotron Radiation Facility, BP 220, Avenue des Martyrs, F-38043 Grenoble, Cedex, France 3lnstitut fiir Mineralogie und Kristallographie,Universitiit Wien-Geozentrum,Althanstrasse 14, A-1090 Wien, Austria
Ansrn,q.cr The structureof mozartite,CaMn.*O[SiO,OH], was refinedin spacegrolp P2r2,2, from X-ray single-crystaldata collected at 100 K (R : 2.45Vo,R-: 2.62Vo),300 K (R : 2.60Vo, R. : 2.687o),and 500 K (R : 2.79Vo,R- : 2.8l%o).The Mn3*Ouoctahedron shows approximately orthorhombic geometry, which is explained by a combination of a tetrag- onally compressedJahn-Teller effect with lattice-inducedstress. Comparison with isostruc- tural vuagnatite,CaAl(OH)SiO*, which shows no distortions due to electronic effects, in- dicates that the distorted octahedral geometry in mozartite causesshifts in valence sums of the O atoms that are hydrogen bonded. As a result, the OH group in mozrrtite is located at the isolated SiO, apex and is not linked to the octahedronas reported for isostructural minerals. Isostructural minerals of the adelite group with tetrahedralAs5* and Vt* exhibit a different Jahn-Teller distortion with tetragonally elongated geometry for octahedral Cu2 Oo. The O-H . . . O distanceof <2.5 A in mozartiteis one of the shortesthydrogen bonded O. . .O distancesin minerals and leads to a diffuse FTIR absorptionpeak (stretching mode) polarizedparallel to b between1300 and 1700 cm-l.
INrnonucrroN higher valencestate ofthe tetrahedralcation leadsto elec- forming the tetra- The structureof mozartite, caMn3ro[SioroH], was trostatically more saturated O atoms hedron their affinity for protonization becomesneg- first studiedby Basso et al. (1993) using crystals from and ligible. The divalent octahedralcation, on the other hand, the Cerchiaramine, Northern Apennines,Italy. Mozartite yields a formal undersaturationof one of its next O at- is isostructuralwith vuagnatite,CaAl(OH)SiOo, (McNear oms, favoring the formation of an OH group. et al. 1976) and four members of the adelite group: van- Our attention was focused on mozaftite when we de- adate calciovolborthite, CaCu(OH)VOo (Basso et al. tected unreasonablevalence sums for O atoms coordi- 1989); arsenatesaustinite, CaZn(OH)AsOo (Giuseppetti nating Si and Mn (Basso et al. 1993). The original ge- and Tadini 1988);conichalcite, CaCu(OH)AsOo (Qurashi ometry describedby Bassoet al. (1993) is characterized and Barnes 1963); and nickelaustinite, Ca(Ni,Zn) by thedistances O5-H : 1.05A andH . . .02 : 1.45A. (OH)AsOo(Cesbron et al. 1987).These minerals, with the The 05 atom connectstwo Mn3*Ouoctahedra and bonds general formula CaM(OH)TO., crystallize in the acentric to one Ca atom. Another O atom, 02, is part of a SiO. spacegroup P2r2r2r.The structurehas the characteristic tetrahedron and bonds to two Ca atoms. This arrange- motif referred "backbone" (Shen to as and Moore 1982), ment, according to the bond valence concept of Brown consisting of a linear chain of edge-sharing octahedra and Altermatt (1985), results in the following valence with attachedsingle tetrahedra(Gottardi 1967) developed sums(excluding H): 1.376valence units (v.u.) for 02 and along a twofold screw axis (Fig. l). 1.878for 05. This suggeststhat the OH group is at02 The silicates mozartite and vuagnatite differ from the rather than at 05 as hitherto assumed. arsenates and vanadate of the adelite group by their In addition to the proton position, the Jahn-Tellerdis- charge distribution. In vuagnatite and mozartite the torlion of the Mn3*Ouoctahedron is discussedin this paper. edge-sharing octahedral chains are formed by trivalent To elucidatethe influenceof lattice stresson the orientation cations (Al, Mn3* ), whereasin the adelite group divalent of the Jahn-Teller distortion, network calculations were cations(Cu, Ni, Zn) form the chains.The deficit of pos- performed. Single-crystal X-ray data were collected at itive chargesis stoichiometricly balancedby the valence three temperatures(100, 300, and 500 K) to test the tem- of the tetrahedral cations (Ass+, \,rs+) in the minerals of perature dependenceof the Jahn-Teller distortion. The the adelite group, whereastetrahedral Sia* occursin vuag- ihort hydrogenbonded O-H. ..O distanceof <2.50 A natite and mozaftite. In the adelite group of minerals, this initiated an IR spectroscopicinvestigation. ooo3404xt97l0910-0841$05.00 841 842 NYFELER ET AL.: MOZARTITE
three data sets.The positions of all atoms except H were refined using initially isotropic, then anisotropic, dis- placementparameters (ADP). After refinement of all Mn, Ca, and O atoms, difference-Fourier maps showed a dif- fuse positive electron-densitycloud between 02 and 05, indicating the presence of an H atom. This H position was restrained to have a 1.25 A distance to botli neigh- boring O atoms with a weight factor of 0. 1. Nevertheless, a subsequentrefinement yielded a proton position close to 02 hydrogen bonded to 05. In the final refinements the O2-H bond was restrained to 0.95 A with a weight factor of 0.03, allowing the H atom position to be deter- mined accurately. The reflections were weighted as lllo2, i gPl, where g was fixed at 0.0001. Observed and calculated structure factors are given in Table lt. Bond valences were calculated using the parameters recom- Frcuns 1. Simplifiedrepresentation (a-c plane)of the moz- mendedby Brese and O'Keeffe (1991). artitestructure, composed of linearchains of edge-sharingocta- hedrawith attachedsingle tetrahedradeveloped along a twofold FTIR spectroscopy screwaxis SiOoand MnOu are shown as polyhedra. The spectra were measured on a Nicolet 60SX FTIR spectrometerusing a glowbar light source, a KBr beam ExpnnrlrnnrAr, METHoD splitter, and a liquid-nitrogen cooled MCT detector.For the low-temperature spectra (84 ! 2 K) a commercial Sample description and single-crystalX-ray data cryo-cell (MMR Technologies) with KBr windows was collection used. Polarized single-crystal spectrawere measuredus- Mozartite crystals from the N'chwaning mine, Kalahari ing a gold wire grid polarizer on an AgBr substrateand manganesefield, Republic of South Africa, have the com- an elliptical sample apertureof 150 x 130 pm,. The dou- position Caono.(MnonurFeoorrMgoo,,)Sior6eOl(OH)(Jens bly polished mozartite crystal applied for the measure- Gutzmer, electron microprobe analysis, personal com- ment was 150 x 130 pm'zwide and 61-11;pm thick. A munication). At this locality, mozartite is associatedwith detailed description of the preparationof the small crystal andradite and hausmannite (the latter intergrown with a plate for FTIR spectroscopyis given in an Appendix'. fibrous serpentinephase) grown on calcite and barite. The euhedral,red-brown crystals average0.2 mm in diameter. Network calculations Twinning or intergrowth is common, thus a careful search Bond-valence network calculations [program STRU- for a suitable single crystal was required. MO (Brown 1992)f were performed to atalyze the influ- A single crystal 150 x 175 x 125 pm3 in dimension ence of the crystal structure on the orientation of the was used for the acquisition of the X-ray diffraction data. Jahn-Teller distortion. Octahedral Mn3* possesses3do The crystal was mounted on a silica-glassneedle using a high-spin configuration, which causes(without the influ- high-temperature resistant epoxy resin, and the same ence of an external force field) either an elongated or crystal was used for the collection of three data sets at compressedtetragonal Jahn-Teller distortion. In the bond 100, 300, and 500 K. X-ray intensities were measuredon valence model, a crystal structureis viewed as a network a CAD4 single-crystal diffractometer with graphite-mon- of atoms in which cations are bonded to anions and vice ochromatized MoKct radiation up to 0 : 40'using the versa.Analogous to the atomic valence(v,) characterizing 1.5" o * 0.3tan0 scanmode. atoms, bond valences (vu) are defined to characterizethe During the data collection at 500 K the crystal was chemical bonds. The ideal bond valence is uniquely de- heatedby a regulated(*5 K) hot-air blower. Foi the mea- fined for any given structural topology by two network surementat 100 K, a conventionalliquid nitrogen cooling equations(Brown 1989;Brown 1992): device with an accuracyof -r2 K was used. At each tem- : perature, 12 reflections were used to determine the cell V, sum(j) vu (bond valence sum rule) (1) parameters. sum(loop) u,, : 0 (equal valence rule) (2) An empirical absorption correction was applied using the V-scan method for selectedreflections. Data reduc- In addition, bond valencesare uniquely linked to bond tion, including background,Lorentz, and polarization cor- rections, was performed with the SDP program system IFor a copy of Tables 1 and 6, and the Appendix order Doc- (Enraf-Nonius 1983). The program SHELXTL (Siemens ument AM-97-643 from the BusinessOffice, Mineralogical So- ciety of America, 1015 1990) was used to solve and refine the structure.Scatter- EighteenthStreet NW Suite 601, Wash- ington, DC 20036,U S.A. Pleaseremit in advance.Deposit ing factors $5.00 for neutral atoms were used. Atomic positions items may also be available on the American Minerologist web were refined using >1100 reflections with 1 > 3o, for all site, refer to inside back cover of a current issuefor webiddress. NYFELER ET AL.: MOZARTITE 843
TneLe2. Cell parameters(A), numberof reflections,and R valuesof mozartite(nee) at 100, 300, and 5OOK
leflections c V (/ > 3o) Parameters R (%) R* (%)
100K 5 837(1) 7 211(1) 8.6e3(1) 365 89 1537 71 2.45 2.62 300 K 5 842(1) 7.228(1) I 704(1) 367.53 tJ/o 71 zou 2.68 500 K 5 846(1) 7.248(1) 8.726(1) 369 74 1189 71 279 2.81 A 0 009 0.037 0 033
Nofe: A representsthe differenceof cell parametersbetween 500 and 100 K. F:(:14"" F"..lyEF""") & : {[I(weightlf," F".,"lf]/[I(weight.F:b")]]] lengths for any given cation-anionpair by the empirically all three temperaturesinvestigated are given in Table 2. derived relationship: The thermal expansion is more pronounced for b and c than for a. Atomic coordinatesand displacementparam- sumf) v, : sum(/) exp[(R, - dr)l0.37l (3) eters are given in Table 3, cation-oxygen distances are where d,, are the actual bond lengths and R, are cation- shown in Table 4. Of the anisotropically refined atoms, anion pair-specific constantstabulated by Brown and Al- the 02 atom has the highest and most strongly anisotropic termatt(1985) and Breeseand O'Keeffe (1991).The con- displacement parameters.In Table 5 the ADP contribu- cept of the bond-valencemodel is that each atom tends tions parallel (par) and perpendicular (perp) to the Si-O to distribute its valence as equally as possible among the bonds are distinguished,providing insight into the nature bonds that it forms. The application of Equations 1-3 to of the disorder of the tetrahedron.Two additional empir- a given bond topology allows the comparison between ical values are calculated: ideal bond lengths predicted for a crystal structure free : - of steric and electronic distortions on one hand and the AU(par) U2(par) Ul(par) (5) actual observedbond lengths on the other hand. Because AU(perp) : U2(perp) - Ul(perp) (6) the bond-valencemodel does not take into account steric and electronic distortions, it serves as a useful tool for where U2 and Ul are the respectivemean-square displace- finding and characterizingsuch effects (Kunz and Brown ment amplitudes of O toward Si and of Si toward O. The 1995). In addition, such effects can also be incorporated differences of mean-squaredisplacement amplitudes reflect into the network equations by applying empirically de- the rigidity of the tetrahedron@artelmehs et al. 1995; Dun- rived weights to the bond valences: itz et al. 1988; Kunz and Armbruster 1990). Furthermore, the averagevalues for AU(par) and AU(perp), the latter ex- v,, = V,, I c,, (4) cluding the values for 02, distinguish differences in the na- where Vu is the observedvalue and cu correspondsto the ture of librational motion of 02 (H-bearing) and the re- weight applied to a particular bond. By this procedure a maining Si-coordinating O atoms.In Figure 2, theseaverage weight <1 weakensa bond and thus tends to lengthen it, AU values are plotted vs. temperature.The AU(par) remains whereascu ) 1 strengthensand thereforeshortens a bond. constant as a function of temperature; AU(perp), however, Valence weights (Kunz and Brown 1995) of 0.6 and increases.With increasing temperahrre02 shows both the 5.0 for H. . .05 and H-O2, respectively,were chosento highest U2(prp) and the highest Au(perp). Bond valences give an optimal bond length modeling of the O-H. . .O and valence sums, excluding H, are shown in Thble 6t. All environment. Weighting O-H bonds in the network cal- cations and anions yield satisfuing valence sums,except that culations is essential,since the equal-valencerule forces 02 has a lower value than the other O atoms. The average the O-H . . . O bonds to be equal, thereforenot taking into Ca-O distance of the eiehtfold-coordinated Ca increases account the intrinsic asymrnetry of O-H. . .O configu- from 2.418 A at 100 K-to 2.49I A at 500 K. A slight rations (Brown 1976; 1992). In addition, a calculation increase was also observed for the Mn3*O6 octahedron, was performed with weights according to the when the averagelVln-O distance increasesfrom 2.017 A at O5-H . . .O2 nrangement suggestedby Basso et al. 100 K to 2.023 A at 500 K. The short Mn-O5 bonds do (1993). Furthermore, a valence-least-squaresrefinement not increasewith temperatue. Within this temperaturerange was performed to obtain an estimate of H coordinates. the Si-O distancesremain unchanged. This method is equivalent to a distance-least-squaresre- For the localization of H, the procedure described in finement (Baerlocheret al. 1917) except that valencesare the experimental section yielded physically reasonable used as the target values instead of bond lengths. In this bond distancesand angles.The resulting O2-H, H' . ' 05, refinement only H coordinates were varied and the net- and 02. . . 05 distancesare given in Table 4. The data work-derived bond valenceswere used as tarset valences. generatedfrom the two network calculations are listed in Table 6. Using network-derived valences from the Rnsur-rs Oz-H. . . 05 model a valence least-squaresrefinement Cell parameters(and their difference from 100 to 500 converged at a H position (cited H"",. in Table 3) 0.98 A K), number of reflections, and R values from XRD for from 02 and 1.507 A from 05, which agreeswithin three 844 NYFELER ET AL.: MOZARTITE
Taele 3. Atomiccoordinates, anisotropic displacement parameters, and 4q valuesfor mozartiteat 100, 300, and 500 K xla ytD u,, U"" U"" uQ u., U"" u.. 100 K 0 02238(8) 0 37501(7) 0 67430(5) 0.0040(2) 0 0035(2) 0.0034(2) 0 0001(1 ) 0 0002(2) 0 0008(1) 0 00360(9) Mn 074729(8) 0.25758(5) 0 99647(4) 0 00218(9) 0.00282(s) 0 0030(1) -0.00030(8) 0.0004(2) -0 0001(1) 0 00266(s) Si 0.5104(1) 0.63076(9) 0 81680(7) 0.0023(3) ooo27(2) 0 0026(3) 0 0006(2) -0 0002(2) 0.0005(2) 0.0025(1) o1 0.4894(3) 0 4536(2) 0 9338(2) 0.0033(6) 0 0040(6) 0.0038(6) 0.0005(6) 0.0002(6) 0 0001(4) 0.0037(3) 02 0.0741(3) 0 7006(2) 0 0619(2) 0.0061(7) 0 0034(6) 0.00s1(7) -0.0002(5) -0.0009(5) -0.0015(5) 0.0049(4) o3 0.2708(3) 0 6578(2) o 7220(21 0.0025(6) 0.0042(6) 0.0034(6) 0.0008(s) -0.0002(5) 0.0008(5) 0.0034(4) o4 0.737s(3) 06217(2) o 7067(2) 0.0036(5) 0 0056(6) 0.0034(6) 0.0011(6) 0.0001(5) 0.0006(5) 0.0042(3) U5 0.0032(3) 0 3e51(2) 0 93s8(2) 0 0031(6) 0.0038(6) 0 0047(6) -0.0004(6) -0.0008(6) 0.0003(5) 0.0039(s) H 0 030(7) 0 583(4) 0 033(s) 0.04(1) 300 K 0.0219(1) 0 37462(8) 0.67420(6) 0 0090(2) o.oo72(2\ 0 0059(2) 0 0006(2) 0 0000(2) 0 0016(2) 0 0073(1) Mn 07473(1\ O25740(61 0.99658(6) 0 0040(1) 0 0059(1) 0 0049(1) -0 0003(1) 0 0001(2) 0 0004(2) 0 00494(6) 5l 0 5102(1) 0.6301(1) 0 81659(8) 0.0037(3) 0 0054(3) 0.0050(3) 0.0003(3) 0 0000(3) 0 0000(3) ooo47(21 o1 0 48e8(4) 0.4535(2) 0 s332(2) 0.0064(7) 0.0070(7) 0 0066(7) 0 0003(8) 0.0005(8) 0.0006(5) 0.0067(4) 02 0 0716(4) 0.7020(3) 0.0624(3) 0.013(1) 0 0061(7) 0.0092(9) 0.0004(7) 0 0010(7) 0 0018(7) 0.0095(5) o3 02714(3) 0.6573(3) o 7222(21 0.0045(8) 0 0076(7) 0.007(8) 0.0004(6) -0.0003(7) 0 0019(6) 0.0065(5) o4 0.7373(3) 0.6223(3) 0 7068(2) 0.0057(7) 0.008e(8) 0.0063(7) 0.0020(8) 0.0007(7) 0.0008(6) 0.0070(4) o5 0.0028(3) 03944(2) 0 e391(2) 0.0042(6) 0 0058(7) 0.0067(7) 0.0004(8) -0.0012(7) 0.0014(5) 0.0055(4) n 0.03(1) 0 583(5) 0 042(6) o o7(2) H 0 041 0 573 0.o27 500 K 0 0210(2) 0 3746(1) 0.67425(8) 0 0156(4) 0.0125(3) 0 0085(3) 0.0008(3) 0.0004(3) 0.001e(3) 0.0122(2) Mn o.7477(2) 0.256ee(8) 0.99658(6) 0 0058(1) 0.0083(1) 0 0059(1) 0 0005(1) 0 0007(3) 0.0004(2) 0 00668(8) 5l 0 5096(2) 0 6290(1) 0.8163(1 ) o oo72(4) 0.0082(4) 0 006e(4) -0 0006(4) 0 0002(5) 0 0006(4) 0 0074(21 o1 0 4907(5) 0 4531(3) 0 9333(3) 0 0106(9) 0.00e5(e) 0 0096(e) 0 001(1) 0 001(1) 0 0017(6) 0 00ss(5) 02 0 0676(6) 0.7026(4) 0.0627(3) 0 023(1) 0 010(1) 0 013(1) 0 0002(9) 0 001(1) 0 0047(9) 0 0153(7) o3 02721(5) 0.65s2(4) 0 7219(3) 0 005(1) 0.016(1) 0 011(1) 0 0008(8) -0 0015(9) 0.000e(s) 0.0105(6) o4 0.7368(5) 0.6221(4) o 7072(3) 0.012(1) 0 015(1) 0.007(1) 0.003(1) 0 0010(9) 0 0014(9) 0.0114(6) o5 0.0027(s) 0 3933(3) 0 9390(2) 0.0071(8) 0 008e(8) 0.010(1) -0.001(1) -0 002(1) 0 0005(7) 0.0087(5) H 0.06(1) 0 57e(5) 0 031(7) 0.08(3)
esd with the H position extracted from our X-ray data DrscussroN sets A from 02 and 1.64(4)A fro.n 05, for 100 [0.91(4) Orthorhombic distortion of the octahedron Kl. Polarized single-crystal FTIR spectra at 84 and 298 K are given in Figure3. The stereochemistry of the sixfold-coordinated Mn3* complex is dominated by Jahn-Teller vibronic coupling. The resulting distortion produces either a tetragonally Tae|-e4. Cation-oxygendistances (A) for mozartiteat 100, elongated or compressedoctahedron with quite different 300,and 500 K electronic ground states(Hitchman 1994). The Mn3*O" octahedron in mozartite shows distorted Cation Oxygen 100K 300 K 500 K Mn-O distances(Table 4), but at a first glance it is not o1 2.430(21 2.439(2) 2.446(2) obviouswhether an elongatedor a compressedJahn-Tell- 02 2.608(2) 2626(2) 2 654(3) o2 2.47s(2) 2.46e{2) 2 469(3) er distortionexists. The Ol-M-Ol octahedraldiagonal is UA o3 2 489(2) 2 493(2) 2 507(3) elongated(4.308 A at room temperature),O5-M-O5 is o3 2 537(2) 2 544(21 2 543(3) (3.746 A), and O3-M-O4 is intermediate o4 2 449(2) 2 460(21 2 462(3) compres-sed o4 2 523(2) 2.s26(2) 2 534(3) (4.068 A). One could argue that such an unusualdistor- Ca UC 2316(2) 2 313(21 2.316(2) tion is causedby static or dynamic disorder of differently Average 2 478 2 484 2.491 oriented, tetragonally compressed and elongated direc- Mn 01b 2.135(2) 2 139(21 2.141(3) lead Mn o1d 2 164(2) 2.16s(2) 2.171(3) tions. However,such a disordershould to substantial Mn O3e 2 034(21 2.040(2) 2.047(3) electronic "smearing" evaluated in terms of AU (Thble Mn 04b 2.026(2) 2.028(2) 2.038(3) 5) along the Mn-O bondingvector. Assuming disorder of Mn UJ 1.859(2) 1 860(2) 1 858(3) Mn o5d 1.884(2) 1 886(2) 1 885(3) a short Mn-O distanceof 1.8 A with a lons Mn-O dis- Average 2.017 2.020 2.O23 tance of 2.2 A would yield a mean-squaredifference dis- Si o1 1 638(2) 1.6s5(2) 1 6s7(21 tanceof 0.04 A'?(Chandrasekhar and Biirgi 1984),which 5l o2 1 652(2) 1 647(21 1 650(3) is on the order of one magnitudehigher than-the observed Si o3 1 634(2) 1 631(2) 1 625(3) Si o4 1.63e(2) 1.636(2) 1 635(3) averageAU valuesof about 0.001-0.004 A']. These AU 'I Average 641 1.637 I OJ/ values are in good. agreementwith those for octahedral H 02 0 e2(3) 0.e1(4) 0.s4(4) Fe3*(AU : 0.001A') in andradite(Armbruster and Geig- H UJ 1 5e(3) 1.64(4\ 1.60(4) er 1993) thus ruling out Jahn-Teller disorder. However, o2 o5 2.480(2) 2.501(3) 2.517(3) the observed orthorhombic geometry of the MnOu poly- NYFELER ET AL.: MOZARTITE 845
Trele 5. Mean-squarevibrational amplitudes parallel and perpendicularto the Si-O and Mn-O bond vectors
2 U1(pat) Lt2(par) U1(perp) A@erp) AU(par) AU(perp) 100 K Si o1 0 0021(2) 0.0037(6) o oo27(21 0 0036(6) 0.0016(6) 0.0009(6) Si 02 0.0029(2) 0.0034(6) 0 0023(2) 0.0055(6) 0 0005(6) 0 0032(6) Si a)e 0.0023(2) 0.0024(6) 0 0026(2) 0.0038(6) 0.0001(6) 0.0012(6) JI o4 0 002s(2) 0 0034(6) 0 0025(2) 0.0046(6) o 0009(6) 0.0021(6) Average o.ooo8(3) (0.0014(3)) Mn o1 0 0030(1) 0.0031(6) 0.002s(1) 0.0040(6) 0.0001(6) 0 001s(6) Mn o1 0 0030(1) 0 0041(6) 0.0025(1) 0.0035(6) 0.0011(6) 0.0010(6) Mn o oo29(1) 0.0029(6) 0 0025(1) 0 0036(6) o.oo00(6) 0 0011 (6) Mn o4 0.0029(1) 0.0034(6) 0.0025(1) 0.0046(6) 0 0005(6) o oo21(6) Mn UJ 0.0020(1) 0 0034(6) 0 0030(1) 0.0041(6) 0 0014(6) 0.0011(6) Mn UJ 0 0020(1) 0 0034(6) 0 0030(1) 0.0041(6) 0 0014(6) 0.0011(6) Average (0.0008(2)) (0 0013(2)) 300 K Si o1 0.0053(3) 0.0061(7) 0.0044(3) 0.0069(7) 0.0008(8) 0 0025(8) Si 0.0053(3) 0.0059(7) 0 0044(3) 0.0110(7) 0 0006(8) 0.0066(8) Si ale 0.0040(3) 0 0049(8) o oo51(3) 0.0074(8) 0.000e(9) 0.0023(e) si o4 0.0041(3) 0 0049(7) 0 0051(3) 0.0079(7) 0 0008(8) 0.0028(8) Average (0.0008(4)) (0.0025(4)) Mn o1 0 00s5(1) 0 0066(7) 0.0046(1) 0.0067(7) 0.0011(8) o 0021(e) Mn o1 0 0053(1) 0.0064(7) 0 0048(1) 0 0069(7) 0.0011(8) 0.0021(8) Mn 0 0048(1) 0.0062(8) o 0050(1) 0 0067(8) o.oo14(9) 0 0017(e) Mn o4 0.0048(1) 0.0062(7) 0 0050(1) o.oo74(7) 0 0014(8) 0.0024(8) -o Mn U3 0.0045(1) 0 0036(7) 0 00s2(1) 0.0066(7) 0009(8) 0.0014(8) Mn UC 0.0044(1) 0 00s3(7) 0.0052(1) 0 00s6(7) 0 0009(8) 0.0004(8) Average (0.0008(3)) (0.001e(3)) 500 K Si o1 0 0071(4) 0.0081(9) 0 0076(4) 0.0110(9) 0 0o1o(10) 0.0034(10) -0.0008(1 Si 02 0.0079(4) 0.0071(1 1) 0.0071(4) 0.0190(11) 2) 0.011 e(12) Si o3 o.oo72(4) 0.0058(10) 0 0076(4) o.o132(1 0) -0 0014(1 1 ) 0.00s6(11) Si o4 0 0071(4) 0 0088(10) 0 0076(4) 0 0125(10) 00017(11) 0.0049(11) Average (0.000s(6)) (0.0046(6)) Mn o1 0.0077(1) 0.0092(9) 0.0061(1) 0 0102(s) 0.0015(9) 0 0041(s) Mn o1 0.0079(1) 0.0096(9) 0.0061(1) o 0100(s) 0.0017(9) o 0039(s) Mn o3 0 0061(1 ) 0.0104(1 0) 0 0071(1 ) 0 0106(10) 0.0043(1o) 0.003s(10) Mn o4 0.0061(1 ) 0.0077(10) 0 0071(1 ) 0.0132(10) o.oo16(10) 0.0061(1 0) Mn o5 0.0059(1) 0.0076(8) 0 0071(1 ) 0.0092(8) 0.0017(8) 0.0021(8) Mn 0.0061(1) 0 0079(8) 0 0071(1 ) 0.0oso(8) 0 0018(8) 0.0019(8) Average (0 0021(4)) (0 0036(4))
Note:02 was excluded from the calculationof the average (see text for explanation) LU : LI2 - U1 (A';.
hedron is not unusual in crystals and well described for indicating that a shortening of these bonds is not favor- sixfold-coordinated Cu2* with similar electronic proper- able. The octahedral diagonals in vuagnatite (O1-M-O1 ties as Mn3* (Gazo et al. 1976; Hitchman 1994; Riley et 3.997 L; O3-M-O4 3.848 A; O5-M-O5 3.680 A) prede- al. 1987). The latter authors state that the Jahn-Tellerac- termine the orthorhombic geometry observed in an en- tive vibration is doubly degenerate,and transition be- hanced fashion in mozartite. This observation is an ex- tween the two tetragonal geometries (compressed and ample where the orientation of the electronic effect elongated) can occur by overcoming only small energy (Jahn-Teller) follows and enhancesthe topologically in- barriers (Hitchman 1994). Lattice forces are able to sta- duced distortion (Kunz and Brown 1995; Hoffmann et al. bilize intermediate states such as the orthorhombic co- 1997). Comparison between the Al octahedronin vuag- ordination geometry of Cu2*Ouand Mn3"Oupolyhedra. natite and the Mn3* octahedronin mozartite indicatesthat The lattice-induced stress in mozartite is caused by in spite of the larger diameter (S^hannon1976) of Mn3* mainly two interactions, namely cation-cation repulsion (1.29 A) relative to Al (1.07 A), only the diagonals and electrostaticinteractions imposed by valence sum re- OI-M-OI and O3-M-O4 adaptto the larger Mn3* size in quirements.The M3+-M3+repulsion in mozartite stretches mozartite(increases of 0.311 and 0.213 A, respectively). the Ol-M-Ol distanceto a greaterextent than in isostruc- The diagonal O5-M-O5 for Mn3* is only 0.066 A longer tural vuagnatite, where Al3* with a smaller ionic radius than the correspondingvalue for Al. This observationin- occupies the M site. This allows a shorter Al-Al distance dicatesthat the orthorhombic distortion of Mn3* is a com- of 2.841 A (McNear et al. 1976), whereas in mozartite bination of a tetragonally compressedJahn-Teller effect Mn3* r-epulsesits next Mn3t neighbor to a distance of and lattice-induced stress. 2.9U A (at room temperature),yielding a long 01-M-O1 A similar enhancing effect was observed in the iso- diagonal. In spite of this long Ol-M-Ol distancein moz- structural vanadate and arsenates. Both austinite, artite, 01 has the highest valence sums of all O atoms, CaZn(OH)AsOo(Giuseppetti and Tadini 1988), and nick- 846 NYFELER ET AL: MOZARTITE
0 012 ----+-au(perp) 02 ----+- 0 010 AU(perp) -=.- AU(par) ()t2o 0 008 a AU € r< o t4 0 006 a q,^ IU
0.004 0.5
0 002 3500 3000 2500 2000 ,1500 1000
0 000 Wavenumber(cm-11 100 200 300 400 600 Frcunn3. Polarizedsingle-crystal FTIR spectraofmozartite r (K) at 298 and 84 K. Frcuns 2. The AU valuesof the tetrahedralO atomsDarallel andperpendicular to the Si-O bondsvs. temperature,reilecting the same mode of octahedral chain connection does not the rigid-bodymotion of the tetrahedron.AU(perp) of 02 is imply the same type of Jahn-Tellerdistortion. shownseparately to point out the additionallibration of the OH group. The H atom position A direct consequenceof the Jahn-Teller distortion of the Mn3*Ouoctahedron in mozartite is the position of the H atom. McNear et aL.(1976) refined the H position in elaustinite, Ca(Ni,Zn)(OH)AsOo (Cesbron et al. 1987), vuagnatite close to 05 with a strong hydrogen bond to with octahedralNi and Zn (not subject to electronic dis- 02. Unlike in vuagnatite, where 05 possessesa valence tortion), have distorted octahedra,with diagonalsof 4.331 A (ot-u-or),4.411 A and3.8tt A (os- sum of 1.57 v.u., the relatively short, Jahn-Teller com- 1o:-u-o+1, pressedM-O5 distancesin mozartite increasethe valence M-o5) for austiniteand 4.30 A fot-vt-Ot). 4.29A (O3- sum of 05 Oonded to two M cations and one Ca atom) M-O4), and 3.93 (O5-M-O5) for nickelaustinite. Coni- to 1.867 v.u., thus 05 in mozartite is not privileged to chalcite, CaCu(OH)AsOo(Qurashi and Barnes 1963), and form an OH group. In general,the OH group in minerals calciovolborthite, CaCu(OH)VO4 (Basso et al. 1989). of the adelite group (Basso et al. 1989; Cesbron et al. contain Cu'?*,which is subject to a Jahn-Tellerdistortion. 1987) is part of the octahedron (at O5). In the original Ionic radii of octahedratCu, (0.73 A), Ni (O.esA), anA structure description of mozartite (Basso et al. 1993) the Zn (0.74 A; a." very similar (Shannon t9i6;. fne en- H position close to 05, attachedto the octahedron,was hanced distortion for Cu2* results in octahedraldiaeonals of 4.13A 4.66A (o:-tr,t-o+), extracted from difference-Fourier maps but not refined. 1ot-vr-ot), ana:Iso A However, as stated above, this proton position in mozar- (O5-M-O5) for conichalcite and 4.12l. A (Ol-M-Ol), tite seemsfairly unlikely considering the 05 valence sum 4.752 A (O3-M-O4), and 3.815 A (Os-tr,t-Os) for cal- of 1.867 v.u. (without accountingfor H). From this point ciovolborthite. The electronic effect lengthens the long of view, the valencesums of 02 (bonded to two Ca atoms O3-M-04 diagonal and shortens the intermediate and one Si atom) may be compared for vuagnatite and Ol-M-Ol octahedralaxis, whereasthe O5-M-O5 axis re- mozartite. All bonds to 02 in mozartite lL647Q) A to mains short. Thus the observed geometry for the Cu2*Ou Si;2.469(2) A to Ca; 2.626(2) A to Cal are significantly octahedronis close to a tetragonally elongateddistortion. lonqe,r than corresponding distances in vuagnatite The relatively short O5-M-O5 diagonal indicates a re- t1.615(2)A to Si; 2.328(2)A to Ca; 2.482(2)A to Cal. stricted spatial flexibility along the octahedralchains, par- Thus the 02 valence sum decreasesfrom 1.589 v.u. allel to a, causedby the tetrahedralinking the octahedral (vuagnatite) to 1.365 v.u. (mozartite). Therefore, the OH 05 apices, as shown in Figure 1. This is also confirmed group in mozartite is at 02 (the apex of the tetrahedron) by the temperaturebehavior of mozartite, where a (ap- and is hydrogen bonded to 05, whereasin vuagnatitethe proximately parallel to O5-M-O5) increasesonly slightly OH group is at 05 and is hydrogen bonded to 02. (0.009 +) as a function of temperature,compared to b The performed network calculations confirm this re- (0.037 A) and c (0.033At fruUl" Z). sult. A comparison between the two models shows that Note that despite the isostructural topology of the sil- predictedbond lengths for an O2-H. . '05 model agree icates and the nonsilicates, the type and the orientation very well with the observed geometry, both the lattice- of the Jahn-Teller distortion are different. This difference stress induced distortion around Mn3t (neglecting the /1r99i't reinforces the statementof Hoffmann et al. that Jahn-Tellereffect) as well as the H-induced distortion of NYFELER ET AL.: MOZARTITE 847 the Si tetrahedron.The O5-H' . '02 model (Bassoet al. 2.48_2.52A betweenO donor and acceptoratoms. The 1993),on the other hand, leadsto significantdiscrepan- shift toward lower band energiesupon cooling is in agree- cies. This model no longer predicts O5-Mn-O5 to be the ment with the slight shortening of the hydrogen bond at shortest octahedral diagonal, and more importantly the 100 K. And third, the band shape of the broad band, bond lengths of the SiO,(O . . . H) tetrahedronare not re- which is superimposedon other absorptions,is typical for producei properly. The longest tetrahedral bond, Si-O2 such stronghydrogen bonds (Novak 1974). U.662 A accordingto Bassoet al. (1993) and 1.647 A Finally, the weak but sharp absorption band in the b in this study at 300 Kl, is predicted to be the shortest spectrum at 3557 cm I must be mentioned. This band is (l.593 A). assigned to an almost free OH group (according to its frequency) that is aligned approximately parallel to the b The SiO.(OH) tetrahedron axis. The slight shift toward higher frequencies upon The tetrahedron in mozartite is quite regular with cooling is in agreementwith only weak or no hydrogen O-Si-O anglesbetween 101.2 and 115.2 (at 100 K), and bonding interaction (Lilz 1995). However, the low inten- the largest and smallest angles are associatedwith 02, sity of this band indicates that this OH group is probably the OH group.Distances between Si and O1, 03, and 04 due to structural defects rather than due to a stoichio- are about 1.635A and very similar between100 and 300 metric component of the mozartite structure. K. The distanceto the OH group Si-O2 (1.650A) is the longest within the tetrahedron.The displacementparam- AcrNowr-nocMENTS eters of the tetrahedralatoms (Table 5) reflect the follow- This study was supportedby the "SchweizerischerNationalfonds zur ing trends:
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