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Stibioclaudetite ASSB[O.sub.3] a new mineral from Tsumeb, Namibia: Stibioclaudetite has been found at the Tsumeb mine, Namibia, in bladed crystals to 6 mm association with leiteite, ludlockite, smithsonite and quartz. Previously identified specimens of claudetite from Tsumeb may well be stibioclaudetite instead.

ABSTRACT

Stibioclaudetite is a new mineral species with ideal chemistry [AsSbO.sub.3]. The mineral has monoclinic symmetry, [P2.sub.1]/n, with a = 4.5757(4) [Angstrom], b = 13.1288(13) [Angstrom], c = 5.4216(5) [Angstrom], [beta] = 95.039(4)[degrees], V = 324.44(5) [[Angstrom].sup.3], Z = 4, and [d.sub.calc] = 5.009 g/[cm.sup.3]. The strongest X-ray lines (calculated) are 3.512 (100), 3.282 (82), 3.238 (71), 2.279 (34), and 4.995 (32). The average of ten microprobe analyses is 45.15% [As.sub.2][O.sub.3] and 55.77% [Sb.sub.2][O.sub.3], total 100.92, corresponding to [As.sub.1.088][Sb.sub.0.912][O.sub.3]. Stibioclaudetite forms adamantine, colorless transparent bladed crystals to 6 mm, bound by {010}, {110}, {111}, and {[bar.1]01}. The mineral is flexible with perfect cleavage on {010}. The hardness is < 2; indices of refraction are > 2.00. Stibioclaudetite occurs with leiteite, ludlockite, smithsonite and quartz in a vug within massive tennantite from the Tsumeb mine, Tsumeb, Namibia. Stibioclaudetite is isostructural with claudetite, specifically an Sb-substituted ordered analog, and the name denotes the relationship. The crystal structure consists of corrugated sheets of corner-sharing As[O.sub.3] and Sb[O.sub.3] trigonal pyramids arranged in an ordered, alternating pattern. Raman spectra of stibioclaudetite, claudetite, and leiteite are presented and compared.

INTRODUCTION

Mineral dealer David W. Bunk obtained an unusual Tsumeb specimen containing a well-formed leiteite (Zn[As.sub.2][O.sub.4]) blade, red fibrous ludlockite, quartz, and an undetermined mineral occurring as colorless crystals to 6 mm in length. In situ, non-destructive examination of the unknown mineral with Raman spectroscopy failed to match its pattern from a large Raman spectral database that the Department of Geosciences at the University of Arizona is currently constructing. Raman spectroscopy confirmed that three separate crystals are of the same unknown. Similarities to the Raman spectrum of leiteite indicated an A[s.sup.3+]-bearing structure, and preliminary electron-dispersive spectroscopy (EDS) on an SEM indicated the presence of As, Sb and O (and no other elements with Z > 8). Since no known mineral contained only As, Sb and O, the authors initiated a full characterization of the material.

Crystal structure determination (Origlieri et al., 2009) and quantitative electron-probe microanalysis identified this phase as naturally occurring AsSb[O.sub.3]. Bodenstein et al. (1983) studied synthetic AsSb[O.sub.3], which they demonstrated to be isostructural with claudetite (A[s.sub.2][O.sub.3]) (Pertlik, 1978). The crystal structure of this new mineral consists of corrugated sheets of corner-sharing As[O.sub.3] and Sb[O.sub.3] trigonal pyramids, with sheets stacked along [010]. The Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association approved the mineral (proposal IMA2007-028) and mineral name before publication. We have deposited type material at the United States National Museum of Natural History (Smithsonian Institution) in Washington, D.C. under catalog number 174550. The mineral name, stibioclaudetite, denotes the structural relationship with claudetite, as an ordered Sb-substituted analog.

Strunz et al. (1958) first reported claudetite from Tsumeb as gypsum-like platelets. Strunz (1959) further elaborated, describing 1-3 mm colorless to white crystals with unit cell dimensions a = 5.3 [Angstrom], b = 13.0 [Angstrom], c = 4.56 [Angstrom], and [beta] ~ 94[degrees]. He sublimated the mineral in a closed glass tube and condensed minute octahedral crystals. This microchemical behavior is consistent with the known behavior of claudetite, which condenses into octahedral crystals (i.e. arsenolite). However, these tests are not sufficient to distinguish claudetite from stibioclaudetite. Synthetic [AsSbO.sub.3], also has a cubic modification with the same crystal structure as cubic A[s.sub.2][O.sub.3] (arsenolite) (Hayek et al., 1963). Consequently, sublimation of either claudetite or stibioclaudetite would produce octahedral crystals. The unit cell reported by Strunz (1959) lacks the precision required to reliably distinguish stibioclaudetite from claudetite. Keller et al. (1979) reported another occurrence of claudetite from Tsumeb in association with warikhanite, unfortunately without specifying the identification method. The original identification of claudetite from Tsumeb could be in error; therefore Tsumeb specimens labeled "claudetite" warrant re-examination.

Hayek et al. (1963) showed that cubic A[s.sub.2][O.sub.3], (arsenolite) and cubic S[b.sub.2][O.sub.3] (senarmontite) are miscible, forming a complete solid solution series. Consequently, ordinary solid solution between arsenolite and senarmontite might yield cubic [AsSbO.sub.3] without structural ordering of As and Sb atoms. In that case, a cubic dimorph of stibioclaudetite would simply be an intermediate of the arsenolite-senarmontite series, and would not qualify as a new mineral species. The ordering of Sb into a single As position of the claudetite structure is apparently unique to the claudetite and stibioclaudetite structure (Origlieri et al., 2009). A literature search failed to locate any report of monoclinic S[b.sub.2][O.sub.3]; however, an orthorhombic phase which bears the mineral name valentinite is well known.

Mineralogist Sidney A. Williams identified hexagonal [AsSbO.sub.3], among mine fire products from Nevada (Gibbs, 1985).

OCCURRENCE AND PARAGENESIS

The new mineral occurs within a cavity in a massive tennantite sample (4 X 5 X 7 cm) from the Tsumeb mine at Tsumeb, Namibia. The cavity measures 3 cm across, and hosts quartz crystals to 3 mm, a single terminated leiteite blade 7 by 20 mm, red fibers of ludlockite, smithsonite and crystals of stibioclaudetite to 6 mm. Figure I shows a photograph of the largest group of stibioclaudetite crystals. Although we do not know the precise original location of the specimen within the Tsumeb mine, the association with leiteite leads to certain conclusions. Leiteite occurs in the second and third oxidation zones at the Tsumeb mine (Gebhard 1991, 1999). Type leiteite occurs with tennantite, chalcocite, smithsonite and schnei-derhohnite (Cesbron et al., 1977). Our present leiteite sample occurs on tennantite matrix with quartz, ludlockite and smithsonite. This assemblage suggests that its specific origin within the Tsumeb mine may be distinct from other known leiteite occurrences.

Although antimony-dominant minerals are not typical of the arsenic-rich assemblages at Tsumeb, primary tennantite contains substantial antimony (Moritz, 1933). Previous investigators have reported five mineral species from the Tsumeb mine with essential antimony: famatinite, stibnite, stibiconite and nadorite (Schneider, 1992), and biehlite (Schluter et al., 2000). Schneider (1992) quantifies the 1988 production of NaSb[(OH).sub.6] at the Tsumeb smelter at 156 metric tons. Oxidation of host tennantite could readily supply both the arsenic and antimony sufficient to form stibioclaudetite. Moritz (1933) further notes a substantial zinc content in Tsumeb tennantite, which could supply both the zinc and arsenic required to form leiteite (Zn[As.sub.2][O.sub.4]).

Monoclinic [As.sub.2][O.sub.3] (claudetite) forms above 250[degrees] C, while cubic [As.sub.2][O.sub.3] (arsenolite) has a melting point near 275[degrees] C (Schulman and Schumb, 1943). Hayek et al. (1963) report a melting point of 315[degrees]C for claudetite. In other words, claudetite remains stable at higher temperatures than arsenolite. Bodenstein et al. (1983) synthesized their monoclinic AsSb[O.sub.3] at temperatures near 347[degrees] C. These data conservatively bracket the formation of stibioclaudetite between 300[degrees]C and 400[degrees]C.

APPEARANCE AND PHYSICAL PROPERTIES

Stibioclaudetite forms bladed crystals to 6 mm bound by major {010}, major {110}, minor {111}, and very minor {[bar.1]01}. Stibioclaudetite is colorless and transparent with an adamantine luster and a white streak. The mineral does not show fluorescence under ultraviolet radiation. Figure 1 is a close-up of the largest stibioclaudetite crystals on the holotype, and Figure 2 shows the terminal morphology in a scanning electron micrograph. Figure 3 is a line drawing of the ideal morphology. Stibioclaudetite crystals mimic the morphology of claudetite from Imperial Valley, California as illustrated by Palache (1934), shown in Figure 3. Hardness is ~2. The mineral has perfect cleavage on {010}, readily obtained. Cleavage plates are flexible, and deform similarly to gypsum. The mineral shows strong relief under n = 2.00 index fluids, indicating an index of refraction above 2.00.

[FIGURE OMITTED]

[FIGURE 3 OMITTED]

CHEMISTRY

We conducted electron probe microanalysis on a cleavage plate of the stibioclaudetite attached to a glass disc. Qualitative WDS scans showed only As, Sb and O, and no other elements with Z > 8. Standardized quantitative WDS analysis employed a Cameca SX-50 electron microprobe at the Lunar and Planetary Sciences Department, University of Arizona. Operating conditions were 15 kV and 30 nA with a beam diameter of 1.5 [micro]m. Enargite (As) and stibiotantalite (Sb) served as standards. Data reduction and correction followed the PAP method (Pouchou and Pichoir, 1984).

Table 1 lists the results often separate electron probe spot analyses. The average of these weight percent analyses with standard deviations is: 55.77(1.07)% [Sb.sub.2][O.sub.3], 45.15(0.95)% [As.sub.2][O.sub.3]; total 100.92%. Normalized to three oxygen atoms, the average composition is [As.sub.1.088][Sb.sub.0.912][O.sub.3]. The composition remained homogeneous over the sampled regions. In the solution of the crystal structure, use of the idealized formula AsSb[O.sub.3] produced a smaller residual error than the empirical electron probe formula (Origlieri et al., 2009). The crystal structure analysis indicates that AsSb[O.sub.3] more accurately represents the chemistry of stibioclaudetite than the empirical electron microprobe chemistry given in Table 1. (Origlieri et al., 2009).
Table 1. Electron probe microanalysis data for stibioclaudetite with
corresponding atomic compositions normalized to three oxygen atoms. The
average of these ten analyses, with standard deviations is 45.15(0.95)%
[As.sub.2][O.sub.3], 55.77(1.07)% [Sb.sub.2][O.sub.3], total 100.92%.
Normalized to three oxygen atoms, the average composition is
[As.sub.1.088][Sb.sub.0.912][O.sub.3]. Ideal AsSb[O.sub.3] contains
40.43% [As.sub.2][O.sub.3] and 59.57% [Sb.sub.2][O.sub.3].

% [As.sub.2][O.sub.3]  % [Sb.sub.2][O.sub.3]  Total

45.66                          55.03          100.69
44.57                          55.46          100.02
44.82                          56.39          101.22
43.95                          55.64           99.59
44.30                          56.13          100.43
46.62                          56.33          102.95
44.54                          56.77          101.31
46.75                          56.76          103.51
45.50                          53.16           98.67
44.74                          56.05          100.80

% [As.sub.2][O.sub.3]               Composition

45.66                  [As.sub.1.100][Sb.sub.0.900][O.sub.3]
44.57                  [As.sub.1.084][Sb.sub.0.916][O.sub.3]
44.82                  [As.sub.1.079][Sb.sub.0.921][O.sub.3]
43.95                  [As.sub.1.076][Sb.sub.0.924][O.sub.3]
44.30                  [As.sub.1.075][Sb.sub.0.925][O.sub.3]
46.62                  [As.sub.1.099][Sb.sub.0.901][O.sub.3]
44.54                  [As.sub.1.072][Sb.sub.0.928][O.sub.3]
46.75                  [As.sub.1.097][Sb.sub.0.903][O.sub.3]
45.50                  [As.sub.1.116][Sb.sub.0.884][O.sub.3]
44.74                  [As.sub.1.081][Sb.sub.0.919][O.sub.3]


X-RAY CRYSTALLOGRAPHY

We obtained single-crystal X-ray diffraction data using a Bruker X8 Apex diffractometer equipped with a 4K Apex II CCD detector. We used monochromatic MoK[alpha] radiation generated at 50 kV and 35 mA. A cleavage fragment of 30 X 70 X 220 [micro]m produced diffraction spots with streaking along constant 2[theta]. Despite the poor appearance of the data, the reflections yielded a merged [R.sub.int] value of 3.08%. A data collection strategy resulted in the acquisition of 1863 frames in 6 scans, from which the Bruker software generated the calculated powder pattern given in Table 2. We used Bruker Saint 7.16b to fit the unit cell parameters from the positions of 6609 reflections collected to 82[degrees] 2[theta], and Bruker Shelxtl 6.14 to determine the space group. Table 3 compares the unit cell parameters for stibioclaudetite and claudetite (Origlieri et al., 2009) in Table 3.
Table 2. Calculated X-ray powder diffraction data for stibioclaudetite.

d       I/[I.sub.0]  h   k  l

4.995       32        0  1  1
3.645       11       -1  0  1
3.512      100       -1  1  1
3.400       18        0  3  1
3.342       14        1  0  1
3.282       82        0  4  0
3.238       71        1  1  1
3.157       24        1  3  0
2.8048      39        0  4  1
2.8006      31       -1  3  1
2.7003      23        0  0  2
2.6559      28        1  3  1
2.6450      24        0  1  2
2.2790      34        2  0  0
2.2692       8       -1  2  2
2.2454       5        2  1  0
2.1401       5       -2  1  1
2.1304       9       -1  5  1
2.1188       8        1  2  2
2.0853      17        0  4  2
2.0646      13        1  5  1
1.8825      10        0  5  2
1.8720      21        2  4  0
1.8223       8       -2  0  2
1.8096       6       -2  4  1
1.8050       5       -2  1  2
1.7344      16        1  7  0
1.7305       7        2  4  1
1.7270       5       -1  0  3
1.6649       7        0  3  3
1.6574       6        2  1  2
1.6263       7        1  0  3
1.6064       6       -1  3  3
1.5932       6       -2  4  2
1.5702      17        0  8  1
1.4876       7       -3  1  1
1.4572       7        1  4  3
1.3087       7       -2  8  1

Table 3. Comparison of the unit cells of claudetite and
stibioclaudetite.

            Stibioclaudetite          Claudetite *

idealized   AsSb[O.sub.3]         [As.sub.2][O.sub.3]
formula

space       [P2.sub.1]/n          [P2.sub.1]/n
group

a           4.5757(4) [Angstrom]  4.5460(4) [Angstrom]

b           13.1288(13)           13.0012(14)
            [Angstrom]            [Angstrom]

c           5.4216(5) [Angstrom]  5.3420(5) [Angstrom]

[beta]      95.039(4)[degrees]    94.329(2) [degrees]

V           324.44(5)             314.83(5)
            [[Angstrom].sup.3]    [[Angstrom].sup.3]

Z           4                     4

calculated  5.009 g/[cm.sup.3]    4.174 g/[cm.sup.3]
density

* Origlieri et al. (2009)


RAMAN SPECTROSCOPY

Raman spectroscopy provides a nondestructive and rapid means to distinguish claudetite from stibioclaudetite. Samples compared include the stibioclaudetite fragment from our X-ray study; claudetite from Jachymov, Czech Republic (University of Arizona Mineral Museum 16128; RRUFF R050313); and leiteite from Tsumeb, Namibia (RRUFF R040011). We collected Raman spectra with a benchtop 100 mW Ar-ion laser centered at 514.532 nm and a Jobin Yvon Spex HR 460 spectometer equipped with a liquid nitrogen cooled Princeton Instruments 1152 X 256 pixel CCD detector.

Using a 1200 grooves [mm.sup.-1] grating centered at 530.4 nm and Roper Instruments Winspec/32 software, we collected the shifted region from 113 to 1016 [cm.sup.-1].

Figure 4 compares the Raman spectra of stibioclaudetite, claude-tite and leiteite, all in undetermined orientations. The stibioclaudetite spectrum shows 22 vibrational modes. Raman selection rules for the claudetite and stibioclaudetite structures allow for 15 [A.sub.g] modes and 15 [B.sub.g] modes, not all of which may be visible. Table 4 lists the principal Raman peak positions for stibioclaudetite, claudetite and leiteite. Additionally, Raman spectroscopy in the region between 3000-4000 rel [cm.sup.-1] showed no active Raman modes of greater significance than background, demonstrating that the mineral is nominally anhydrous.

[FIGURE 4 OMITTED]
Table 4. Principal Raman peak positions (shifted [cm.sup.-1]) of
stibioclaudetite, claudetite, and leiteite.

Stibioclaudetite  Claudetite  Leiteite

115
125                             125
                                138
155                             150
                                168
171                   175       179
183
                      193       201
202                             205
210                   218       220
                      248       256
232                   259       269
273                   284
298                             307
                      354       368
323                   356       379
342
414
430
468                   459       459
477
517
                      541       550
                                603
620                   626       649
631                   632
726
766                             764
817                   814       806


ACKNOWLEDGEMENTS

We graciously acknowledge Michael Scott for supporting the creation of a Raman database of all known mineral species. The authors appreciate the careful review of Andrew Roberts.

REFERENCES

BODENSTEIN, D., BREHM, A., JONES, P. G., SCHWARZMANN, E. and SHELDRICK, G. M. (1983) Darstellung und Kristallstruktur von monoklinem Arsen(III)antimon(III)oxid, AsSb[O.sub.3]. Zeitschrift fur Naturforschung, 38B, 901-904.

CESBRON, F. P., ERD, R. C, CZAMANSK1, G. K., and VACHEY, H. (1977) Leiteite, a new mineral from Tsumeb. Mineralogical Record, 8(3), Tsumeb! issue, 95-97.

GEBHARD, G. (1991) Tsumeb: eine deutsch-afrikanische Geschichte. Verlag Christel Gebhard, Giesen, Germany. 239 pp.

GEBHARD, G. (1999) Tsumeb II. CG Publishing, Waldbrol, Germany. 328 pp.

GIBBS, R. B. (1985) The White Caps mine, Manhattan, Nevada. Mineralogical Record, 16, 81-88.

HAYEK, E., INAMA, P., and SCHATZ, B. (1963) Mischkristall-bildung von [As.sub.2][O.sub.3] und [Sb.sub.2][O.sub.3]. Monatshefte fur Chemie, 94, 366-372.

KELLER, P., HESS, H., and DUNN, P. J. (1979) Warikhanit, [Zn.sub.3][[([H.sub.2]O).sub.2]|(As[O.sub.4]).sub.2], ein neues Mineral aus Tsumeb, Sudwesta-frika. Neues Jahrbuch fur Mineralogie Monatshefte, 389-395.

MORITZ, H. (1933) Die sulfidischen Erze der Tsumeb-Mine von Ausgehenden bis zur XVI. Sohle (-460 m). Neues Jahrbuch fur Mineralogie, Geologie, und Palaontologie, 67A, 118-154, Tafeln XIII-XIV.

ORIGLIERI, M., DOWNS, R. T., and CARDUCCI, M. D. (2009) Crystal structures of stibioclaudetite and claudetite. Canadian Mineralogist (in press).

PALACHE, C. (1934) Contributions to crystallography: claudetite, minasragrite, samsonite, native selenium, indium. American Mineralogist, 19, 194-205.

PERTLIK, F. (1978) Verfeinerung der Kristallstruktur des Minerals Claudetit, [As.sub.2][O.sub.3] ("Claudetit I"). Monatshefte fur Chemie, 109, 277-282.

POUCHOU, J. L. and PICHOIR, F. (1984) Un nouveau modele de calcul pour la microanalyse quantitative par spectrometrie de rayons X. Partie I: Application a I'analyse d'enchantillons homogenes. La Recherche Aerospatiale, 3, 167-192.

SCHNEIDER, G. I. C. (1992) Antimony. In The Mineral Resources of Namibia. First Edition. Namibia Geological Survey.

SCHULMAN, J. H. and SCHUMB, W. C. (1943) The polymorphism of arsenious oxide. Journal of the American Chemical Society, 65, 878-883.

SCHLUTER, J., KLASKA, K.-H., ADIWIDJAJA, G., FRIESE, K., and GEBHARD, G. (2000) Biehlite, [(Sb,As).sub.2]Mo[O.sub.6], a new mineral from Tsumeb, Namibia. Neues Jahrbuch fur Mineralogie, Monatshefte, 234-240.

STRUNZ, H. (1959) Tsumeb, seine Erze und Sekundarmineralien, insbesondere der neu aufgeschlossenen zweiten Oxydationszone. Forschritte der Mineralogie, 37, 87-90.

STRUNZ, H., SOHNGE, G., and GEIER, B. H. (1958) Stottit, ein neues Germanium-Mineral und seine Paragenese in Tsumeb. Neues Jahrbuch fur Mineralogie Monatshefte, 85-96.

Marcus J. Origlieri

Department of Geosciences, University of Arizona

Tucson, Arizona 85721-0077 USA

marcus@mineralzone.com

Robert T. Downs

Department of Geosciences, University of Arizona

Tucson, Arizona 85721-0077 USA

rdowns@u.arizona.edu

William W. Pinch

19 Stonebridge Lane, Pittsford, New York 14534 USA

Gary L. Zito

Department of Metallurgical and Materials Engineering

Colorado School of Mines, Golden, Colorado 80401 USA
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