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The Canadian Mineralogist
Vol. 40, pp. 1191-1198 (2002)

SEWARDITE, CaFe3+2(AsO4)2(OH)2, THE Ca-ANALOGUE OF CARMINITE,FROM TSUMEB, NAMIBIA: DESCRIPTION AND CRYSTAL STRUCTURE
ANDREW C. ROBERTS§
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

MARK A. COOPER AND FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

ALAN J. CRIDDLE†
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

JOHN A.R. STIRLING
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

ABSTRACT
Sewardite, ideally CaFe3+2(AsO4)2(OH)2, is orthorhombic, Cccm, with unit-cell parameters derived from a crystal-structurerefinement: a 16.461(2), b 7.434(1), c 12.131(2) Å, V 1484.5(6) Å3, a:b:c 2.2143:1:1.6318, Z = 8. The strongest nine lines of the
X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 4.874(90)(202), 3.473(50)(113), 3.389(60)(220), 3.167(100)(022),
3.015(50)(510), 2.988(50)(313), 2.919(70)(511), 2.503(90)(422,314) and 1.775(50)(533,026). The mineral occurs within a vug 3
cm across on a single specimen of gangue collected from the 31st level of the Tsumeb mine, Tsumeb, Namibia, as platy tocompact anhedral to subhedral masses. These masses, up to 0.3 mm in maximum size, are intimately intergrown with very darkgreen to black botryoidal masses of an intergrowth of two tsumcorite-group minerals. Individual fragments are between 50 and
100 m in size, with no obvious forms, and {100} and {011} as imperfect cleavages. Sewardite has physical properties that arevery similar to those of carminite; it is dark red in masses to a much lighter red to orange on thin edges of small fragments. The
streak is reddish brown, and the luster, vitreous. It is translucent (masses) to transparent (on thin edges), and nonfluorescent. The
hardness (Mohs) is 3½. It is brittle, and has a splintery (in the [011] direction) to uneven fracture. The calculated density is 4.156
g/cm3 (for the empirical formula). In polished section, it is light bluish grey in plane-polarized reflected light in air, with ubiquitousvery bright pink to red internal reflections; the mineral is very weakly bireflectant, non-anisotropic, without pleochroism. Averaged
results of eight electron-microprobe analyses yield CaO 11.77, ZnO 1.68, CuO 0.28, Fe2O3 31.65, As2O5 48.81, H2O [4.04], total[98.23] wt.%. The empirical formula is Ca0.99(Fe3+1.87Zn0.10Cu0.02)1.99As5+2.01O8.00[(OH)1.88(H2O)0.12]2.00, on the basis of 10
anions. The infrared-absorption spectra show bands for structural (OH), (H2O), and arsenate. The mineral’s name honors Terry
M. Seward, Professor of Geochemistry, ETH, Zürich, Switzerland, who collected the mineral specimen in 1982. The crystal
structure has been refined to an R index of 6.1% for 451 observed reflections measured with MoK X-radiation. Chains of edgeand corner-sharing Fe3+6 octahedra (: O, OH) extend along the b axis and are linked by (AsO4) tetrahedra to form
[Fe3+2(AsO4)2(OH)2] sheets parallel to {001}; these sheets link via [8]-coordinated Ca atoms. Sewardite is the Ca-dominantanalogue of carminite, ideally Pb2+Fe3+(AsO4)2(OH)2.
Keywords: sewardite, new mineral species, calcium iron hydroxide-arsenate, carminite, X-ray data, electron-microprobe data,
reflectance data, infrared spectroscopy, crystal structure, Tsumeb mine, Tsumeb, Namibia.

SOMMAIRE
La sewardite, dont la composition idéale serait CaFe3+2(AsO4)2(OH)2, est orthorhombique, Cccm, avec les paramètres
réticulaires suivants, dérivés d’une ébauche de la structure cristalline: a 16.461(2), b 7.434(1), c 12.131(2) Å, V 1484.5(6) Å3,
a:b:c 2.2143:1:1.6318, Z = 8. Les huit raies les plus intenses du spectre de diffraction X, méthode des poudres [d en Å (I)(hkl)],sont: 4.874(90)(202), 3.473(50)(113), 3.389(60)(220), 3.167(100)(022), 3.015(50)(510), 2.988(50)(313), 2.919(70)(511),§†

E-mail address: aroberts@NRCan.gc.ca
Deceased, May 2002.

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2.503(90)(422,314) et 1.775(50)(533,026). Ce minéral se présente dans une seule cavité de 3 cm dans un échantillon de gangue
prélevé au niveau 31 de la mine Tsumeb, à Tsumeb, en Namibie, en plaquettes ou masses compactes, xénomorphes à subidiomorphes. Ces masses, atteignant 0.3 mm, sont disposées en intercroissance intime avec des masses botryoïdales vert foncé ànoires qui contiennent deux minéraux du groupe de la tsumcorite. Les fragments individuels varient de 50 à 100 m en taille, sansformes apparentes, avec {100} et {011} comme clivages imparfaits. La sewardite possède des propriétés physiques très semblables
à celles de la carminite; elle va du rouge foncé où elle est massive à rouge beaucoup plus pâle ou orange en bordure mince desgrains. Sa rayure est brun rougeâtre, et son éclat, vitreux. Elle est translucide (massive) à transparente (bordure mince), et non
fluorescente. La dureté de Mohs est 3½. Elle est cassante, et possède une fracture en échape le long de [011] ou inégale. La densité
calculée est égale à 4.156 g/cm3 (d’après la formule empirique). En sections polies, la sewardite est gris bleuâtre pâle en lumière
réfléchie dans l’air (polarisation en plan), avec des reflets internes rose très brillant à rouge. Elle est très faiblement biréflectante,
non anisotrope, et sans pléochroïsme. En moyenne, les résultats de huit analyses à la microsonde électronique ont donné: CaO
11.77, ZnO 1.68, CuO 0.28, Fe2O3 31.65, As2O5 48.81, H2O [4.04], total [98.23] % en poids. La formule empirique qui en
découle, sur une base de dix anions, est: Ca0.99(Fe3+1.87Zn0.10Cu0.02)1.99As5+2.01O8.00[(OH)1.88(H2O)0.12]2.00. Les spectresd’absorption dans l’infra-rouge montrent des bandes attribuées à (OH), (H2O), et des groupes arsenate dans la structure. Le nom
du minéral honore Terry M. Seward, professeur de géochimie à l’institut ETH, Zürich, en Suisse, qui l’a trouvé en 1982. La
structure cristalline a été affinée jusqu’à un résidu R de 6.1% pour 451 réflexions observées, mesurées avec rayonnement MoK.
Des chaînes d’octaèdres Fe3+6 à arêtes et à coins partagés (: O, OH) sont orientées selon l’axe b et sont rattachées par destétraèdres (AsO4) pour former des feuillets [Fe3+2(AsO4)2(OH)2] parallèles à {001}; ces feuillets sont liés grâce aux atomes Ca,à coordinence [8]. La sewardite est l’analogue à dominance de Ca de la carminite, dont la formule idéale est Pb2+Fe3+(AsO4)2(OH)2.
(Traduit par la Rédaction)
Mots-clés: sewardite, nouvelle espèce minérale, arsenate hydroxylé de calcium et de fer, carminite, données de diffraction X,
données de microsonde électronique, données de réflectance, spectroscopie infra-rouge, structure cristalline, mine Tsumeb,
Tsumeb, Namibie.

INTRODUCTION
The Tsumeb mine, Tsumeb, Namibia (lat. 19°S,
long. 18°E) has long been a prolific source of new mineral species; over fifty-five species have been formallycharacterized from this locality since the first mineral(otavite) was described in 1906 (Gebhard 1999). Here,
we describe yet another new phase, sewardite, from thisclassic locality. The specimen in which the new phase
is found was originally collected in 1982 by Dr. Terry
Seward, but it has taken seventeen years, and a chancemeeting between Professor Seward and the senior author at the IMA meeting in Toronto in 1998, before formal characterization of the mineral commenced.
Preliminary studies by Dr. Seward in the 1980s (megascopic appearance, qualitative energy-dispersion electron-microprobe analysis and a Gandolfi camera X-ray
powder-diffraction pattern) suggested a potential newmineral; further studies in Ottawa, Winnipeg and
London have confirmed that this phase is, indeed, new
to science.
The mineral is named sewardite in honor of Terry
Maxwell Seward (b. 1940) who collected the mineral
specimen and recognized its potential as a new species.
Dr. Seward is Professor of Geochemistry at theEidgenössische Technische Hochschule (ETH), Zürich,
Switzerland, and holds dual citizenship in Canada and
New Zealand. The mineral and mineral name have been
approved by the Commission on New Minerals andMineral Names, IMA (2001–54). Holotype material,consisting of a few small aggregates in two gelatin cap-

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sules, is housed in the Systematic Reference Series of
the National Mineral Collection at the Geological Survey of Canada, Ottawa, Ontario, under catalogue number 68095. The polished section used for the quantitativeelectron-microprobe and reflectance studies is preservedin the mineral collections at The Natural History Museum, London, Great Britain, as BM 2001.36.

OCCURRENCE AND ASSOCIATED MINERALS
Sewardite was identified in a single specimen ofgangue that has since been split for scientific study; thenew mineral is rare, and we estimate that only 1–2 mgof pure sewardite is currently available for further study.
The mineral occurs within a small 3 cm-sized vug which
was collected on the 31st level of the Tsumeb mine. It is
intimately intergrown with a very dark green to black
botryoidal mass, identified as a member of thetsumcorite group (Krause et al. 1998) by routine X-ray
powder-diffraction film methods. Compositionally, thisbotryoidal mass is composed of an intimate intergrowthof two distinct phases: one conforms to the compositionof ferrilotharmeyerite; the other may be a cuprian,zincian variety of ferrilotharmeyerite or it may be theZn-dominant analogue of lukrahnite (Krause et al.
2001). We hope to pursue further studies of this matrix
material at a later date.
Recently, it has come to our attention that sewardite
has also been identified, by both quantitative electronmicroprobe analyses and Gandolfi X-ray powder camera, from Mina Ojuela, Mapimi, Durango, Mexico (U.

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SEWARDITE, A NEW SPECIES FROM TSUMEB

Kolitsch, pers. commun., 2002). There, it occurs as dark
reddish spherules and rosettes of very small thin platy
crystals.

PHYSICAL PROPERTIES
Sewardite occurs as platy to compact anhedral tosubhedral masses that do not exceed 0.3 mm in maximum size and are randomly scattered throughout thedark green to black matrix. A photomicrograph of bothan isolated fragment and one in matrix is shown in
Figure 1. Individual fragments are 50–100 m in sizeand have {100} imperfect and {011} imperfect cleavages (orientations confirmed by a precession singlecrystal study). There are no obvious crystal forms, andtwinning was not observed megascopically or in X-ray
single-crystal studies. Most physical properties are very
similar to those of carminite; megascopic discrimination between the two minerals is virtually impossible.
The color of sewardite varies from dark red in masses
to a much lighter red to orange on thin edges of crystal
fragments. The streak is reddish brown, and the luster isvitreous. Masses are translucent, whereas thin edges offragments are transparent. The mineral is brittle, with asplintery (in the [011] direction) to uneven fracture, andis nonfluorescent under both long- and short-wave ultraviolet radiation. The Mohs hardness is estimated to be
3½. The density could not be measured because of thesmall size of available fragments and the dearth of material. The density, calculated on the basis of the empirical formula and unit-cell parameters derived froman investigation of the crystal structure, is 4.156 g/cm3.

OPTICAL PROPERTIESIn plane-polarized reflected light, sewardite is lightbluish grey with weak, but measurable, bireflectance,
no anisotropy and no pleochroism. However, undercrossed polars, the mineral shows ubiquitous very brightpink to red internal reflections. Reflectance measurements in air were made relative to a Zeiss silicon carbide standard, SiC no. 472, and results are presented inTable 1. Reflectance measurements were made in oil[index of refraction (ND) 1.515]; the values are so lowthat they are considered unreliable and are not given
here. This indicates, as do the low values in air, that themineral is sensibly transparent. The indices of refraction, as calculated from the reflectances in air at 589
nm, are n1 = 1.87 and n2 = 1.92 (n̄ = 1.895). The Vickers
micro-hardness was not measured because there wasonly a single grain-mount of material available for

FIG. 1. Photomicrograph of an isolated fragment of sewardite as well as a fragment ingreen to black botryoidal matrix. Scale bar: 0.1 mm.

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study, and the VHN values obtained would not havebeen reliable.

Debye–Scherrer powder films. Differences are in fact
significant enough that it was not initially obvious thatthe two minerals are structurally related.

X-RAY CRYSTALLOGRAPHY
A small fragment of sewardite, 0.009  0.021 
0.023 mm in size, was investigated with a Bruker P4
four-circle X-ray diffractometer equipped with a CCD
detector. Systematic absences are consistent with thespace group Cccm. The measured unit-cell parameters
are: a 16.461(2), b 7.434(1), c 12.131(2) Å, V 1484.5(6)Å3, a:b:c 2.2143:1:1.6318.
The unit-cell parameters, a 16.436(8), b 7.425(3), c
12.116(5) Å, V 1479(1) Å3, a:b:c 2.2136:1:1.6318, Z =8, were refined from 28 powder reflections representing d values between 3.473 and 1.347 Å for which unambiguous indexing was possible on the basis of thecalculated intensities derived from the crystal structure.
A fully indexed powder pattern is presented in Table 2.
Powder data for sewardite and carminite are similar,
though readily distinguishable, even on 57.3 mm

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CHEMICAL COMPOSITION
Polished fragments of sewardite were analyzed witha Cameca SX–50 electron microprobe at an operating
voltage of 15 kV, a beam current of 20 nA, a count timeof 20 s on peak and 10 s on background, and a beam 10
m in diameter. An energy-dispersion scan indicatedthe absence of any other elements with atomic number
greater than 9 except those reported here. There is apossibility of very minor Na; however, a significant interference with a Zn L-line does not allow quantitative
measurement of the Na peak. Therefore, Na2O is estimated to be below 0.2 wt.%. Al, V, Mn, Co, Ni and Pb
were sought, but are below detection limits. The following standards were used: diopside (Ca), gahnite (Zn),azurite (Cu), fayalite (Fe) and cobaltite (As). Data werecorrected using the PAP procedure of Pouchou &8/23/02, 9:35

SEWARDITE, A NEW SPECIES FROM TSUMEB

FIG. 2.

Infrared-absorption spectrum for sewardite.

Pichoir (1984, 1985). The valence states for both Fe andAs, as well as the number of O atoms, were determinedby crystal-structure analysis prior to final interpretationof the electron-microprobe results. The paucity of purematerial prevented quantitative determination of H2O.
However, the presence of H as (OH) and as very minor(H2O) was confirmed both by crystal-structure analysis
and by powder infrared-absorption study; the amountof H2O was therefore calculated by stoichiometry. The
average of eight determinations (and standard deviations) gave CaO 11.77 (0.05), ZnO 1.68 (0.20), CuO
0.28 (0.14), Fe2O3 31.65 (0.37), As2O5 48.81 (0.37),
H2O [4.04], total [98.23] wt.%. With a total of 10 anions, the empirical formula for the Ca-analogue ofcarminite is Ca0.99(Fe3+1.87Zn0.10Cu0.02)1.99As 5+2.01
O8.00[(OH)1.88 (H2O)0.12]2.00. The end-member formula,
CaFe3+2 (AsO4)2(OH)2, requires CaO 12.10, Fe2O3
34.44, As2O5 49.57, H2O 3.89, total 100.00 wt.%. Because a precise index of refraction could not be determined, a meaningful index of compatibility could not
be calculated.

INFRARED SPECTROSCOPY
The procedures for acquiring the infrared-absorption
spectrum of sewardite are identical to those reported byRoberts et al. (1994). The sample was analyzed using a
Bomem Michelson MB–100 FTIR spectrometerequipped with a wide-band mercury–cadmium telluridedetector. The transmittance spectrum (Fig. 2) shows astrong absorption band, centered at 3150 cm–1, attribut-

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FIG. 3. Sewardite structure projected onto (010). The (Fe3+6) octahedra (: O, OH) are blue, (AsO4) tetrahedra are yellow, andCa atoms (in framework interstices) are green.

able to O–H stretching in the hydroxyl groups and a very
weak absorption band, centered at 1560 cm–1, which ismost likely due to H–O–H bending in the absorbed(H2O). Characteristic bands for arsenate are very strong
and are centered at 895, 817 and 768 cm–1.

CRYSTAL STRUCTURE
The (very small) crystal selected for collection of Xray intensity data was mounted on a Siemens P4 automated four-circle diffractometer equipped with a 1K
CCD detector and MoK X-radiation. Subsequent to thecollection of the intensity data, all frames were examined carefully for visually observed reflections. Becauseof the small size of the crystal (Table 3), no data wereobserved above 2 = 50°, and hence the refinement isbased on data with 2 ≤ 50°. A total of 8155 intensities
was collected to 50° 2 using a frame width of 0.2°, aframe exposure time of three minutes, and a crystal-to-

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detector distance of 4 cm. The cell dimensions (Table 3)
were determined from 1673 reflections with I > 10 I.
All calculations were done with the SHELXTL
PC(PLUS) system of programs; R indices are of theform given in Table 3. Using the atom parameters ofcarminite [Kharisun et al. (1996); see also Olmi &Sabelli (1995)] the structure of sewardite was refined toconvergence at an R-index of 6.1%. Because of the paucity of data due to the small size of the crystal, only
isotropic-displacement parameters were used in the refinement. Final positions and displacement parameters
are listed in Table 4, selected interatomic distances are
given in Table 5, and a bond-valence table is given as
Table 6. Structure factors may be obtained from The
Depository of Unpublished Data, CISTI, National Research Council, Ottawa, Ontario K1A 0S2, Canada.
Subsequent to the collection of the X-ray intensity data,
the crystal was embedded in epoxy and quantitatively
analyzed with a Cameca SX–50 electron microprobe as
described above.

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Sewardite is the Ca analogue of carminite, andTables 5 and 6 also list the data for carminite for comparison. A prominent feature of the structure is a chainof (Fe3+6) octahedra (: O, OH) in which the octahedra link by alternately sharing edges and corners; theedge-sharing dimers in this arrangement have one pairof corners bridged by an (AsO4) tetrahedron, and thesecond tetrahedron is linked to an octahedron cornertrans to the other tetrahedron–octahedron linkage. The“free” tetrahedron vertices in this chain then link to octahedron corners in adjacent chains to form a framework. Ca occupies [8]-coordinated sites withininterstices in this framework. The environments of Ca
in sewardite and Pb2+ in carminite (Tables 5, 6) are very
similar, and thus Pb2+ in carminite does not show
stereoactive-lone-pair behavior. Such similarities incoordination of Ca and Pb2+ occur in several mineralpairs (e.g., conichalcite–duftite, calciovolborthite–mottramite), and raise the possibility of solid solution
in such minerals. This Ca–Pb substitution is not generally observed in minerals, but this may be a geochemical constraint rather than a crystal-chemical constraint.
It would be interesting to examine this issue via attempted synthesis of solid solutions of the relevant compositions.

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ACKNOWLEDGEMENTS
The authors thank E. Moffatt (Canadian Conservation Institute) for the infrared spectrum of sewardite,P. Hunt (GSC) for the photomicrograph reproduced in
Figure 1, M. Clarke (GSC) for redrafting Figure 2, and
K. Mooney (GSC) for typing the manuscript. FCH was
supported by a Canada Research Chair in Crystallography and Mineralogy, and by Research and Major Equipment grants for the Natural Sciences and Engineering
Research Council of Canada. Finally, we thank Uwe
Kolitsch, Ernie Nickel, Allan Pring, Joe Mandarino and
Bob Martin for their comments, which helped to improve this paper.

REFERENCES
GEBHARD, G. (1999): Tsumeb: A Unique Mineral Locality. GG
Publishing, Grossenseifen, Germany.
KHARISUN, TAYLOR, M.R., BEVAN, D.J.M. & PRING, A. (1996):
The crystal structure of carminite: refinement and bondvalence calculations. Mineral. Mag. 60, 805-811.
KRAUSE, W., BELENDORFF, K., BERNHARDT, H.-J., MCCAMMON,
C., EFFENBERGER, H. & MIKENDA, W. (1998): Crystal
chemistry of the tsumcorite-group minerals. New data on
ferrilotharmeyerite, tsumcorite, thometzekite, mounanaite,
helmutwinklerite, and a redefinition of gartrellite. Eur. J.
Mineral. 10, 179-206.

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THE CANADIAN MINERALOGIST________, BLASS, G., BERNHARDT, H.-J. & EFFENBERGER, H.
(2001): Lukrahnite, CaCuFe3+(AsO4)2[(H2O)(OH)], thecalcium analogue of gartrellite. Neues Jahrb. Mineral.,Monatsh., 481-492.
OLMI, F. & SABELLI, C. (1995): Carminite from three localities
of Sardinia (Italy): crystal structure refinements. Neues
Jahrb. Mineral., Monatsh., 553-562.

lysis (J.T. Armstrong, ed.) San Francisco Press, San
Francisco, California (104-106).
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WILLIAMS, R.S., CURETON, F.F., II & JENSEN, M.C. (1994):
Mcalpineite, Cu 3TeO 6•H 2O, a new mineral from the
McAlpine mine, Tuolumne County, California and fromthe Centennial Eureka mine, Juab County, Utah. Mineral.
Mag. 58, 417-424.

POUCHOU, J.-L. & PICHOIR, R. (1984): A new model for quantitative analysis. 1. Applications to the analysis of homogeneous samples. La Recherche Aerospatiale 3, 13-38.
________ & ________ (1985): “PAP” ( – –z) procedure for
improved quantitative microanalysis. In Microbeam Ana-

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Received May 5, 2002, revised manuscript accepted June 28,
2002.

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