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Volume 67 
Part 6 
Pages i33-i35  
June 2011  

Received 31 January 2011
Accepted 26 April 2011
Online 5 May 2011

On the symmetry of wulfenite (Pb[MoO4]) from Mezica (Slovenia)

aDepartment of Mineralogy, Eötvös Loránd University, Pázmány P. stny. 1/c, 1117 Budapest, Hungary,bInstitute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri út 59-67, 1025 Budapest, Hungary, and cDepartment for Nanostructured Materials, Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
Correspondence e-mail: coraildiko@gmail.com

Wulfenite [lead(II) molybdate(VI)] is known as a scheelite structure in the I41/a space group. The structure of the unusual `hemimorphic' wulfenite crystals from the Mezica mine was refined in the noncentrosymmetric space group I[\overline{4}] using a Pb/Mo exchange disorder model with the approximate composition Pb0.94Mo0.06[MoO4]. Pb atoms in the 2b positions are substituted by Mo at about 12%. The crystal is shown to be twinned by inversion. Hemimorphism may result from the short-range chemical ordering of the metal atoms at the 2b positions.

Comment

The mineral wulfenite (Pb[MoO4]) is a member of the scheelite group of minerals with the general formula ABO4, where the most abundant cations in the A and B positions are Ca, Sr, Ba and Pb, and Mo and W, respectively. Structure determinations for scheelite using single-crystal X-ray and neutron diffraction techniques indicated the I41/a space group (Zalkin & Templeton, 1964[Zalkin, A. & Templeton, D. H. (1964). J. Chem. Phys. 40, 501-504.]; Kay et al., 1964[Kay, M. I., Frazer, B. C. & Almodovar, I. (1964). J. Chem. Phys. 40, 504.]). Gürmen et al. (1971[Gürmen, E., Daniels, E. & King, J. S. (1971). J. Chem. Phys. 55, 1093-1097.]) studied the structure of scheelite-type minerals, such as SrMoO4, SrWO4, CaMoO4 and BaWO4, using neutron diffraction refinement and compared their O-atom positions. Arakcheeva & Chapuis (2008[Arakcheeva, A. & Chapuis, G. (2008). Acta Cryst. B64, 12-25.]) gave a comprehensive study of scheelite-like structures with both commensurate and incommensurate modulations in the site occupations and with substitutions along the <uv0> directions.

The first isotropic (Leciejewicz, 1965[Leciejewicz, J. (1965). Z. Kristallogr. 121, 158-164.]; neutron diffraction study) and anisotropic (Lugli et al., 1999[Lugli, C., Medici, L. & Saccardo, D. (1999). Neues Jahrb. Mineral. Monatsh. 6, 281-288.]; X-ray diffraction data) structure refinements of wulfenite from various localities resulted in the same symmetry. Recently, Hibbs et al. (2000[Hibbs, D. E., Jury, C. M., Leverett, P., Plimer, I. R. & Williams, P. A. (2000). Mineral. Mag. 64, 1057-1062.]) studied `hemihedral' (with polar symmetry along the c axis) tungstenian wulfenite crystals (Pb[Mo0.64W0.36O4]) from Chillagoe (Australia) and found different W-Mo distributions over the 2a and 2c tetrahedral positions, responsible for the reduced I[\overline{4}] symmetry. Hemihedrism was interpreted as a result of ordered substitution of Mo by W. Their conclusion may be subject to debate, since the difference in the R values resulting from isotropic structure refinement in the space group I41/a (R = 0.040) and from anisotropic refinement in the space group I[\overline{4}] (R = 0.038) is not very significant.

Hurlbut (1955[Hurlbut, C. S. (1955). Am. Mineral. 40, 857-860.]) first described two types of unusual `hemimorphic' wulfenite crystals from the Mezica mine (Slovenia) showing two habits: (i) pyramidal crystals indicating polar character on the [001] axis; and (ii) tabular crystals twinned on {00[\overline{1}]}. Based on their morphology and on etch and piezoelectric tests, he suggested that wulfenite from the Mezica mine crystallizes with tetragonal-pyramidal (4) symmetry. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) microdiffraction studies of these hemimorphic wulfenites from the Doroteja locality in the Mezica mine imply I4 (acentric tetragonal-pyramidal) symmetry (Zavasnik et al., 2010[Zavasnik, J., Recnik, A., Samardzija, Z., Meden, A. & Dódony, I. (2010). Acta Mineral. Petrogr. Abstr. Ser. 6, 727.]). As a result of this lower symmetry, Recnik (2010[Recnik, A. (2010). In Minerals of the Lead and Zinc Ore Deposit Mezica. Ljubljana: Bode Vlg.]) explained a new law of basal inversion twinning, the so-called Doroteja Law, macroscopically observable in wulfenite crystals from this locality. These hemimorphic wulfenite crystals are the subject of this work.

Most of the previous structural studies indicated the space group I41/a for wulfenite. In contrast, many intense reflections in our data set violate the extinction rules of this symmetry, e.g. for 00l the l = 2n (with l values up to ±22) reflections, and several hk0 reflections (hk [not equal to] 2n), are evident. Experimentally, only the I-centring is proven (h + k + l = 2n). The space groups I4, I4/m, I[\overline{4}], Imm2, I4/mmm and I2/m were checked in structure refinements, as well as I41/a. Except for I[\overline{4}], the best R values resulting from the refinements in the above space groups converged to only about R = 0.07-0.08.

In the I[\overline{4}] space group, racemic twinning by merohedry was indicated as early as the isotropic refinement stage. The volume ratio of the twinned pair refined to 0.50 (6). The Pb and Mo cations were both allocated to two Wyckoff sites (Table 1[link]). The Pb sites at the 2b positions proved to be substituted by ~12% Mo, which corresponds to the composition Pb0.94Mo0.06[MoO4]. The final anisotropic refinement converged to acceptably low R values for 30 parameters and no restraints. The resulting Pb:Mo ratio is within the range of the quantified energy-dispersive X-ray spectroscopy (EDS) data, the considered uncertainties and the different sample volumes. Final atomic parameters are listed in Table 1[link].

Besides the explanation of Pb/Mo substitution, the difference between the occupancy of the 2b and 2d Pb positions (and the potentially minor difference between the 2a and 2c Mo positions) could also be regarded as vacancies, which would also be in line with the Pb > Mo content observed from the EDS analyses. However, our X-ray model refinement, based on Pb/Mo substitution at the 2b site, resulted in consistently lower R, wR2 and S values (by about 0.005-0.01%) and also in lower residual densities at the metal sites.

Relevant bond distances for both I41/a Monte Cengio wulfenite (Lugli et al., 1999[Lugli, C., Medici, L. & Saccardo, D. (1999). Neues Jahrb. Mineral. Monatsh. 6, 281-288.]) and I[\overline{4}] Mezica wulfenite are listed for comparison in Table 2[link]. Although the degree of freedom in the space group I[\overline{4}] is higher than that in I41/a, the corresponding atomic distances and bond angles in the MoO4 and PbO8 groups in both the Mezica wulfenite and wulfenites with I41/a symmetry are identical within the known s.u. criteria. The hemimorphism of the Mezica wulfenite (cf. Fig. 1[link]) may be interpreted as a result of ordering.

[Figure 1]
Figure 1
Polyhedral representation of Mezica wulfenite, projected along the b axis.

Experimental

A small single-crystal chip was carefully selected as a good scattering sample for experiments at T = 294 K. EDS analyses were measured on 100-200 nm sized wulfenite crystals using a Technai G2 X-Twin 200 kV analytical TEM. Besides Pb and Mo, the O content was also measured. The measured Pb:Mo ratios were close to 1 [atoms per formula unit (apfu) for Pb = 0.98 (4)]. These values are consistent with the ~12% substitution in the 2b Pb site. A slight correlation was observed between the resolution of the data and the refined substitution at the 2b Pb site (at 0.6 Å resolution the substitution at the 2b site is ~4%, while at 0.83 Å resolution it is ~12%). Such virtual resolution dependence of the composition might be easily interpreted in terms of the clearly deficient scattering model at a higher than usual (0.6 Å) resolution. Here, the dominant scattering of the core electrons and the use of spherical scattering factors yield an over-weighted spherical scattering model. The disorder-dependent smearing of the electron densities, as well as the deformation density due to valence and higher-orbital electron densities, thus logically provide somewhat lower populations than at a lower (0.83 Å) resolution.

Finally, we note that this model describes a twinned and disordered crystal using a discrete substitution site. This disorder model itself is only an approximation, as other Wyckoff sites might also contain minute contamination of, for example, chalcogenide elements.

Crystal data
  • Pb0.94Mo0.06[MoO4]

  • Mr = 360.42

  • Tetragonal, [I \overline 4]

  • a = 5.442 (1) Å

  • c = 12.1177 (14) Å

  • V = 358.87 (5) Å3

  • Z = 4

  • Mo K[alpha] radiation

  • [mu] = 47.59 mm-1

  • T = 294 K

  • 0.10 × 0.10 × 0.05 mm

Data collection
  • Rigaku R-AXIS RAPID diffractometer

  • Absorption correction: numerical (NUMABS; Higashi, 2002[Higashi, T. (2002). NUMABS. Rigaku Corporation, Tokyo, Japan.]) Tmin = 0.064, Tmax = 0.336

  • 6322 measured reflections

  • 335 independent reflections

  • 333 reflections with I > 2[sigma](I)

  • Rint = 0.120

Refinement
  • R[F2 > 2[sigma](F2)] = 0.025

  • wR(F2) = 0.061

  • S = 1.16

  • 335 reflections

  • 30 parameters

  • [Delta][rho]max = 0.71 e Å-3

  • [Delta][rho]min = -0.86 e Å-3

  • Absolute structure: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]), with 156 Friedel pairs

  • Flack parameter: 0.50 (6)

Table 1
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)

  Wyckoff position x y z Uiso*/Ueq
Pb1 2d 0.5000 0.0000 0.2500 0.0213 (13)
Mo1 2c 0.0000 0.5000 0.2500 0.018 (3)
Mo2 2a 0.0000 0.0000 0.0000 0.022 (4)
Pb3# 2b 0.5000 0.5000 0.0000 0.025 (2)
Mo3+ 2b 0.5000 0.5000 0.0000 0.025 (2)
O1 8g 0.2344 (13) -0.1372 (14) 0.0806 (6) 0.0302 (17)
O2 8g 0.2338 (14) 0.3648 (14) 0.1697 (6) 0.0307 (17)
#Occupancy = 0.881 (8).
+Occupancy = 0.119 (8).

Table 2
Pb-O and Mo-O bond lengths (Å)

  This work Lugli et al. (1999)[Lugli, C., Medici, L. & Saccardo, D. (1999). Neues Jahrb. Mineral. Monatsh. 6, 281-288.]
Pb1-O1 2.619 (7) 2.611 (3)
Pb1-O2 2.643 (8) 2.636 (3)
Mo2-O1 1.772 (7) 1.769 (3)
Pb3-O2 2.620 (7) 2.611 (3)
Pb3-O1i 2.635 (8) 2.636 (3)
Mo1-O2 1.763 (8) 1.769 (3)
Symmetry code: (i) x, y + 1, z.

Racemic twinning was indicated during the isotropic refinement and the volume ratio of the twin pairs was refined to 0.50 (6). All atoms were treated anisotropically. An extinction parameter was also refined, and an occupancy constraint was applied for the 2b site.

Data collection: CrystalClear (Rigaku, 2008[Rigaku (2008). CrystalStructure. Rigaku Corporation, The Woodlands, Texas, USA.]); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]); software used to prepare material for publication: SHELXL97.


Supplementary data for this paper are available from the IUCr electronic archives (Reference: KU3043 ). Services for accessing these data are described at the back of the journal.


Acknowledgements

The authors express their sincere thanks to Alajos Kálmán and László Párkányi for their helpful comments and suggestions, and to Tamás Holczbauer for his help. We also thank the HAS CRC for carrying out the X-ray data collection. Financial support, administered through OTKA grant Nos. 68562 and K-75869 and the ARRS L1-2232 project, is gratefully acknowledged.

References

Arakcheeva, A. & Chapuis, G. (2008). Acta Cryst. B64, 12-25.  [ISI] [CrossRef] [details]
Flack, H. D. (1983). Acta Cryst. A39, 876-881.  [CrossRef] [details]
Gürmen, E., Daniels, E. & King, J. S. (1971). J. Chem. Phys. 55, 1093-1097.
Hibbs, D. E., Jury, C. M., Leverett, P., Plimer, I. R. & Williams, P. A. (2000). Mineral. Mag. 64, 1057-1062.  [ChemPort]
Higashi, T. (2002). NUMABS. Rigaku Corporation, Tokyo, Japan.
Hurlbut, C. S. (1955). Am. Mineral. 40, 857-860.  [ChemPort]
Kay, M. I., Frazer, B. C. & Almodovar, I. (1964). J. Chem. Phys. 40, 504.
Leciejewicz, J. (1965). Z. Kristallogr. 121, 158-164.  [ChemPort]
Lugli, C., Medici, L. & Saccardo, D. (1999). Neues Jahrb. Mineral. Monatsh. 6, 281-288.
Recnik, A. (2010). In Minerals of the Lead and Zinc Ore Deposit Mezica. Ljubljana: Bode Vlg.
Rigaku (2008). CrystalStructure. Rigaku Corporation, The Woodlands, Texas, USA.
Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.  [CrossRef] [details]
Spek, A. L. (2009). Acta Cryst. D65, 148-155.  [ISI] [CrossRef] [details]
Zalkin, A. & Templeton, D. H. (1964). J. Chem. Phys. 40, 501-504.  [ChemPort]
Zavasnik, J., Recnik, A., Samardzija, Z., Meden, A. & Dódony, I. (2010). Acta Mineral. Petrogr. Abstr. Ser. 6, 727.


Acta Cryst (2011). C67, i33-i35   [ doi:10.1107/S0108270111015769 ]