1519
The Canadian Mineralogist
Vol. 45, pp. 1519-1523 (2007)
DOI : 10.3749/canmin.45.6.1519

CALVERTITE, Cu5Ge0.5S4, A NEW MINERAL SPECIES FROM TSUMEB, NAMIBIAJOHN L. JAMBOR§
Leslie Research and Consulting, 316 Rosehill Wynd, Tsawwassen, British Columbia V4M 3L9, Canada

ANDREW C. ROBERTS
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

LEE A. GROAT
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

CHRIS J. STANLEY AND ALAN J. CRIDDLE†
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

MARK N. FEINGLOS
Duke University Medical Center, Box 3921, Durham, North Carolina 27710, U.S.A.

ABSTRACTThe sulÞde-rich matrix of a partly oxidized specimen from the Tsumeb mine, Tsumeb, Namibia, consists mainly of renierite,tennantite, gallite, chalcocite, and the new mineral calvertite, simpliÞed as Cu5Ge0.5S4, which is named after Lauriston D. Calvert(1924–1993) of the National Research Council, Ottawa, Canada. Calvertite occurs as anhedral grains, up to 100 m across,that are outlined by a network of chalcocite veinlets of micrometric width. In reßected light, calvertite is opaque, pale bluishgrey, and isotropic. Reßectance percentages in air are 26.3 (470 nm), 23.1 (546), 22.2 (589), and 21.5% (650). The mineral isbrittle, macroscopically black with a black streak, and has a hardness of 4 to 5, a conchoidal to uneven fracture, and no cleavage.
Single-crystal X-ray study indicated F-centered cubic symmetry, a 5.337(3) Å; the structure has not been solved, and thepowder-diffraction pattern has only four lines [d in Å(I)(hkl)]: 3.053(100)(111), 2.639(10)(200), 1.869(90)(220), and 1.595(30)(311). An electron-microprobe analysis gave Cu 63.10, Fe 1.66, Zn 0.55, Ge 5.76, As 1.50, Ga 0.36, V 0.05, S 26.63, total 99.60
wt.%, corresponding to (Cu4.782Fe0.143Zn0.041Ga0.025V0.005)4.996(Ge0.382As0.096)0.478S4 on the basis of S = 4 atoms and Z = 1. It
is likely that the structure is disordered, and the measured unit-cell is the subcell of a compound whose ordered equivalent has
the simpliÞed formula Cu10GeS8.
Keywords: calvertite, new mineral species, Cu–Ge sulÞde, electron-microprobe analysis, reßectance data, Tsumeb mine,Namibia.

SOMMAIRE
La matrice riche en sulfures d’un échantillon partiellement oxydé provenant de la mine Tsumeb, à Tsumeb, en Namibie,
contient surtout renierite, tennantite, gallite, chalcocite, et la nouvelle espèce minérale calvertite, ayant la formule simpliÞéeCu5Ge0.5S4. Le nom honore Lauriston D. Calvert (1924–1993), du Conseil National de la Recherche à Ottawa, Canada. Lacalvertite se présente en grains xénomorphes atteignant un diamètre de 100 m; leur pourtour est décoré par un réseau de veinules
micrométriques de chalcocite. En lumière réßéchie, la calvertite est opaque, gris bleuâtre pâle, et isotrope. Nous en avons mesuré
la réßectance dans l’air: 26.3 (470 nm), 23.1 (546), 22.2 (589), et 21.5% (650). Le minéral est cassant, et macroscopiquement
noir avec une rayure noire. Sa dureté est entre 4 et 5, elle possède une fracture conchoïdale à inégale, et semble dépourvue declivage. Les données de diffraction obtenues sur monocristal indiquent une symétrie cubique, un réseau avec F centré, et unparamètre réticulaire égal à a 5.337(3) Å; nous n’avons pas résolu la structure, et le spectre de diffraction (méthode des poudres)ne montre que quatre raies [d en Å(I)(hkl)]: 3.053(100)(111), 2.639(10)(200), 1.869(90)(220), et 1.595(30)(311). Une analyse†§

Deceased 02 May 2002.
E-mail address: jljambor@aol.com

1520

THE CANADIAN MINERALOGISTavec une microsonde électronique a donné Cu 63.10, Fe 1.66, Zn 0.55, Ge 5.76, As 1.50, Ga 0.36, V 0.05, S 26.63, total 99.60%(poids), ce qui correspond à (Cu4.782Fe0.143Zn0.041Ga0.025V0.005)4.996 (Ge0.382As0.096)0.478S4 sur une base de S = 4 atomes et Z =1. Il semble probable que la structure est désordonnée, et que la maille élémentaire mesurée est une sous-maille d’un composé
dont l’équivalent ordonné possède la formule simpliÞée Cu10GeS8.
Mots-clés: calvertite, nouvelle espèce minérale, sulfure de Cu–Ge, données de microsonde électronique, données de réßectance,mine Tsumeb, Namibie.

INTRODUCTION

OCCURRENCE

The Tsumeb mine at Tsumeb, Namibia, is one of theworld’s most famous mineral localities. The deposit,
which formed a vertically zoned, discordant pipe-likestructure in fractured and brecciated Precambrian
dolomite and sandstone (Lombaard et al. 1986), hasbeen the source of many museum-quality oxidationinduced minerals and is the type locality for morethan 50 mineral species. Mining of the Tsumeb deposit
occurred intermittently for decades at a small scale, andthe subsequent main exploitation period was from 1948
to nominal closure in 1996. Total production was about
30 Mt grading 10% Pb, mainly as galena, 3.5% Zn,
mainly as sphalerite, and 4.3% Cu, principally as chalcocite, bornite, tennantite, and enargite (Lombaard et al.
1986, Melcher 2003). However, the Tsumeb ores also
contained notably high concentrations of various otherelements, such as Ag, Cd, As, Sb, and Ga, and the minealso represents the world’s largest occurrence of Ge insulÞde minerals (Melcher 2003). Within the orebody
were pockets, some attaining several tens of tonnes,
that contained high-grade ores of germanite [Cu26Fe4Ge 4 S 32 ] and renierite [(Cu,Zn) 11 (Ge,As) 2 Fe 4 S 16 ](Lombaard et al. 1986).
During the examination of the specimen that hosts
holotype gallobeudantite, PbGa 3 [(AsO 4 ),(SO 4 )] 2(OH,H2O)6, from the Tsumeb mine (Jambor et al.
1996), the matrix of the specimen was observed toconsist largely of partly oxidized Cu-rich massive
sulfides, among which was a sulfide mineral thatremained unidentiÞed. Further study of the unidentiÞed mineral has led to the deÞnition of a new speciesof simpliÞed composition Cu5Ge0.5S4. The mineral isnamed calvertite in honor of Lauriston (Larry) Derwent
Calvert (1924–1993) of the National Research Council,Ottawa, Canada, in recognition of his numerous studiesof metallic phases and his important contributions tothe Powder Diffraction File produced by the International Centre for Diffraction Data. The new mineraland name have been approved by the Commission on
New Minerals, Nomenclature and ClassiÞcation, IMA(2006–30). Cotype polished sections of calvertite arein the Canadian Museum of Nature, Ottawa (CMNMC
85731), and the Natural History Museum, London, U.K.
(BM 2004, 78).

The calvertite-bearing specimen was originallyabout 3  5  1.5 cm and is now smaller because ofoffcuts for the two polished sections of the sulÞde-richmatrix. The unoxidized portion of the matrix consists
largely of renierite and calvertite, with local tennantite
and inclusions of gallite [CuGaS2]. The major sulÞdes,i.e., renierite and calvertite, are concentrated in bands
rather than uniformly intergrown, and calvertite locallycuts renierite and inclusions of tennantite in renierite(Fig. 1). Brittle fracturing apparently led to the deposition of numerous late-stage chalcocite-Þlled veinlets
of micrometric width. In calvertite-rich portions,the chalcocite forms a network that has produced agranular texture in which the calvertite is anhedral andcommonly elongate to elliptical (Fig. 1a); the largest
homogeneous grains rarely attain 100 m across.
Along the contact between calvertite and the networkof veinlets of chalcocite is another Cu sulfide, or
possibly a mixture of digenite and djurleite. Amongthe associated non-sulÞde minerals in the specimen are
goethite, hematite, quartz, veinlets of Hg-rich nativesilver, stolzite [PbWO4], otjisumeite [PbGe4O9], andvugs that contain various members of the alunite supergroup. The principal sulÞde minerals are of hypogene
origin, and the oxide minerals are supergene. However,this assemblage does not necessarily indicate that thecalvertite-bearing specimen was collected near thesurface of the Tsumeb deposit; the pipe was mined toa depth of about 1700 m below surface, and within theorebody were large but discontinuous zones of oxidation that extended to a depth of >1300 m (Lombaard etal. 1986, Melcher 2003).

PROPERTIES AND COMPOSITIONCalvertite is macroscopically black and has ametallic luster, a black streak, brittle tenacity, an irregular to conchoidal fracture, and no cleavage, parting, orßuorescence. The Mohs hardness is 4 to 5, and VHN25
is 283 (268–307) kg/mm2. The mineral is opaque, andin reßected light is pale bluish grey and without internal
reßections, pleochroism or bireßectance. The veining
by chalcocite suggests an anhedral morphology, andthe chalcocite-enclosed grains of calvertite are opti-

CALVERTITE, A NEW Cu-Ge SULFIDE MINERAL

1521

FIG. 1. SEM images of (a) the typical appearance of massive calvertite, which makes up the bulk of the image. The granular
texture is made apparent by the presence of a thin rind of chalcocite (white) on the calvertite grains. The interstitial material consists of pits and heterogeneous oxidation-derived products. (b) Renierite (ren) with inclusions of gallite (gall) cut bycalvertite (calv); all are cut by thread-like veinlets of chalcocite (white).

FIG. 2. Plots showing the distinct differences in the reßectances of calvertite (calv) andgermanite (ger) in air and in oil. Data for germanite are by R. Caye and J. Pasdeloup(Criddle & Stanley 1993).

cally uniform and isotropic. Reßectance percentages
measured in air and in oil are given in Table 1. The data
were obtained using a SiC standard, an oil with an index
of refraction of 1.515, and the methodology of Stanleyet al. (2002). The optical properties of calvertite do notmatch those of germanite or any mineral in the COM
data Þle (Fig. 2).
Electron-microprobe analysis of eight grains ofcalvertite gave the composition listed in Table 2. The
microprobe, a JEOL 8900 instrument at CANMET,Ottawa, was operated in the wavelength-dispersion

mode at 20 kV and 20 nA, with a nominal beamdiameter of 1 m. We used CuS as a standard for Cu(K), chalcopyrite for Fe (K) and S (K), sphaleritefor Zn (K), InAs for As (L), GaS for Ga (K), andGe and V metals for Ge (K) and V (K), respectively. Counting times were 40 s except for Cu (10 s)
and S (30 s). Minimum detection-limits ranged from180 ppm for Zn to 560 ppm for As. The results of theanalyses, on the basis of four atoms of S per formulaunit, correspond to the general formula (Cu,Fe,Zn,Ga,V)5.00(Ge,As)0.48S4, simpliÞed as Cu5Ge0.5S4. The

1522

THE CANADIAN MINERALOGIST

simpliÞed formula requires Cu 65.88, Ge 7.53, S 26.59
wt.%. The data in Table 2, and results of other analyses,indicate that there is little compositional variation fromgrain to grain. The electron-microprobe compositionof the associated renierite corresponds to (Cu10.55
Zn0.49)11.04(Fe3.80Ga0.10)3.90(Ge1.65As0.39)2.04S16, andthat of tennantite approximates Cu6Cu4(Zn1.7Cu0.1Fe0.1
Ga0.1)2.0(As3.9Ge0.1)4.0S13.
The empirical formula for calvertite and the singlecrystal unit cell give a Dcalc of 5.239 g/cm3 for Z = 1.
The composition of calvertite differs substantially fromthat of germanite (Table 3), and calvertite does notseem to be related to any other Cu or Cu–Ge sulÞde,either natural or synthetic. In terms of compositional
similarity, a phase synthesized by Wang (1976) wasreported to be Cu4.33Ge0.67S5, which for S = 4 atoms per
formula unit corresponds to Cu3.47Ge0.54S4. The phaseis hexagonal, and the X-ray powder-diffraction patternis unlike that for calvertite. An orthorhombic compound
synthesized by Onoda et al. (1999), given as Cu5GeS4.5
in PDF 89–6686, likewise has little similarity to the
X-ray character of calvertite.

X-RAY RESULTS
One of the concerns in deÞning calvertite as a newspecies was that its X-ray powder pattern has only fourmeasurable diffraction lines (Table 4), and the lines arenot deÞnitive in identifying the mineral. For example,the compound Cu3GeS4 synthesized by Naiding Wang
(PDF 41–1032), although not compositionally closeto calvertite, also has only four powder-diffractionlines, all of which agree moderately well with those ofcalvertite. Moreover, the powder pattern of calvertite
also agrees well with the strongest diffraction linesof renierite (tetragonal), germanocolusite (cubic), andgermanite (cubic).
Single-crystal X-ray precession data for an analyzed
grain of calvertite removed from a polished section
were obtained with Zr-Þltered Mo X-radiation. Theresults conÞrmed the optical indication that the mineralis cubic. The precession and powder data indicate an aof 5.29(1) Å. The observed systematic extinctions forhkl are h + k = 2n, k + l = 2n, and l = 2n; for hhl, h + l= 2n; for 0kl, k = 2n, l = 2n; and for 00l, l = 2n. Thus,
the allowed space-groups are Fm3m, F432, and F43m,
and the diffraction aspect is F*3*. The same crystal
on the same mount from which the precession results
were obtained in 1998 no longer yielded diffraction
spots when examined with a CCD unit in 1999 at theUniversity of Manitoba, nor when re-examined in 2005
with an up-to-date CCD unit at the University of British
Columbia. Despite this indication that calvertite might
be unstable, a re-run of the original powder-diffraction
mount showed that the X-ray pattern of the mineral had
not changed after 11 years.
Single-crystal data from a second analyzed grain
were collected by J.F. Britten in 2000 at McMaster
University. The collection, which was done with aSiemens/Bruker instrument with a rotating-anodegenerator and a CCD detector, yielded 317 reßections

CALVERTITE, A NEW Cu-Ge SULFIDE MINERAL1523

REFERENCES
CRIDDLE, A.J. & STANLEY, C.J., eds. (1993): Quantitative
Data File for Ore Minerals (3rd ed.). Chapman & Hall,London, U.K.
JAMBOR, J.L., OWENS, D.R., GRICE, J.D. & FEINGLOS, M.N.
(1996): Gallobeudantite, PbGa3[(AsO4),(SO4)]2(OH)6, a
new mineral species from Tsumeb, Namibia, and associated new gallium analogues of the alunite–jarosite family.
Can. Mineral. 34, 1305-1315.
LOMBAARD, A.F., GÜNZEL, A., INNES, J. & KRÜGER, T.L.
(1986): The Tsumeb lead – copper – zinc – silver deposit,South West Africa/Namibia. In Mineral Deposits of Southern Aftrica (C.R. Anhaeusser & S. Maske, eds.). Geol. Soc.
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(29 unique in F43m) and a cubic cell with a 5.3365(6)Å, thus conÞrming the original precession-based results.
However, a structure model satisfactorily commensuratewith the microprobe composition of calvertite could not
be established.
In accordance with the structure description ofputzite [(Cu,Ag)8GeS6] (Paar et al. 2004), wherein Ge
is represented as a stoichiometric entity in the formula,a separate position has likewise been allocated to theGe in calvertite. The small unit-cell of calvertite, andthe possibly unstable character, suggest that calvertite isa metastable, highly disordered mineral whose ordered
equivalent would be generalized as Cu10GeS8 with a =2a’calvertite = 2  5.337 Å and Z = 4. Thus, part of thedifÞculty in solving the structure of calvertite is that
only a subcell rather than the true cell is detectable.
Charge balance for these types of sulÞdes (e.g.,germanite: Spiridonov 1987) is commonly obtained byusing heterovalent states for the metals. For calvertite,
only the Cu needs be partitioned (Table 5). However,
if Ga is assigned to the Ge position, as has been doneby Spiridonov et al. (1992) for germanocolusite, andif the valences of the minor elements are changed(Table 5), the Cu…:Ge…:S proportions of calvertite
are still reasonable in terms of charge balance and theproposed general formula.

ACKNOWLEDGEMENTS
We thank J.H.G. Laflamme and D.R. Owens,
formerly at CANMET, Ottawa (now retired), for theelectron-microprobe data, and visiting CANMET
scientist Ania Peregoedova for concerted attempts tosynthesize calvertite. We are also grateful to W.H. Paarand A. Peregoedova for critical reviews, and to R.F.
Martin for editorial comments.

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& K RAPIVA , L.Y A . (1992): Germanocolusite Cu 26V 2(Ge,As)6S32: a new mineral. Vestnik Mosk. Univ., Ser. 4,Geol., 1992(6), 50-54 [abstr. in Am. Mineral. 79, 387,1994].
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Received December 13, 2006, revised manuscript acceptedJune 14, 2007.