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The Canadian Mineralogist
Vol. 42, pp. 1771-1776 (2004)

EYSELITE, Fe3+Ge4+3O7(OH), A NEW MINERAL SPECIES FROM TSUMEB, NAMIBIA
ANDREW C. ROBERTS§
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

TERRY M. SEWARD AND ERIC REUSSER
Institut für Mineralogie und Petrographie, ETH Zentrum, Sonneggstrasse 5, CH–8092 Zürich, Switzerland

GRAHAM J.C. CARPENTER
Materials Technology Laboratories, CANMET, 568 Booth Street, Ottawa, Ontario K1A 0G1, Canada

JOEL D. GRICE
Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

SIMON M. CLARK AND MATTHEW A. MARCUS
Advanced Light Source, Lawrence Berkeley National Lab, 1 Cyclotron Road, MS 6R2100,
Berkeley, California 94720-8226, U.S.A.

ABSTRACT
Eyselite, ideally Fe3+Ge4+3O7(OH), is orthorhombic with a P lattice, and unit-cell parameters refined from powder data: a8.302(4), b 9.718(4), c 4.527(2) Å, V 365.2(3) Å3, a:b:c 0.8543:1:0.4658, Z = 2. The strongest eight lines of the X-ray powderdiffraction pattern [d in Å (I)(hkl)] are: 4.105(40)(011), 3.681(100)(111), 3.121(60b)(220,121), 2.921(100)(211), 2.512(40)(131),
2.403(90)(320), 1.646(80)(322) and 1.624(50)(142). The mineral occurs on a single specimen originating from the Tsumeb mine,
Tsumeb, Namibia, as very fine-grained aggregates in vugs of massive renierite – germanite – tennantite ore. There are no otherassociated secondary phases. Individual subhedral to rarely euhedral crystals are platy to very thin prismatic, elongate [001], witha length-to-width ratio of approximately 3:1 and not exceeding 20 m in maximum length. Typical crystals are 20  14  1 min size. Forms are {100} major, very thin {010}, rounded {011}, and very thin rounded {001} minor. Crystals show growth steps
on {100} and are partly hollow. The mineral is dirty brown-yellow (aggregates) to yellow-tan (crystals), opaque (aggregates) totransparent (crystals), with a brownish yellow streak, and vitreous luster. Eyselite is brittle, with an uneven fracture, and is
nonfluorescent. The aggregates are soft; D (calc.) is 3.639 g/cm3 for the empirical formula. It is biaxial positive, and two indicesof refraction exceed 1.80; 2V (meas.) is large; pale yellow in plane-polarized light, with no pleochroism, low birefringence,
moderate anisotropy, and length-fast. Averaged results of electron-microprobe analyses yield CaO 0.06, Fe2O3 18.54, Ga2O3
1.01, GeO2 77.75, H2O [2.64] (by difference), total [100.00] wt.%. The empirical formula, on the basis of O = 8, is(Fe3+0.93Ga3+0.04)0.97Ge4+2.98O6.90(OH)1.17. The infrared-absorption spectrum shows bands for structural (OH). A micro-XAS
study shows that all the Fe is trivalent, and the Ge atoms are probably in octahedral coordination. The mineral’s name honors thelate Walter H. Eysel, Professor of Crystallography, Ruprecht-Karls-Universität, Heidelberg, Germany, for his contributions to thestudy of germanates and for his numerous contributions to the Powder Diffraction File.
Keywords: eyselite, new mineral species, iron germanium oxide hydroxide, X-ray powder data, electron-microprobe data, infrared spectroscopy, micro-XAS study, selected-area electron diffraction, Tsumeb mine, Tsumeb, Namibia.

SOMMAIRE
La eyselite, dont la formule idéale est Fe3+Ge4+3O7(OH), est orthorhombique avec une maille P, et possède les paramètres
réticulaires suivants, affinés à partir de données en diffraction X obtenues sur poudre: a 8.302(4), b 9.718(4), c 4.527(2) Å, V
365.2(3) Å3, a:b:c 0.8543:1:0.4658, Z = 2. Les huit raies les plus intenses du spectre de diffraction, méthode des poudres [d en Å(I)(hkl)] sont: 4.105(40)(011), 3.681(100)(111), 3.121(60b)(220,121), 2.921(100)(211), 2.512(40)(131), 2.403(90)(320),§

E-mail address: aroberts@nrcan.gc.ca

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1.646(80)(322) et 1.624(50)(142). Le minéral, découvert sur un seul échantillon provenant de la mine Tsumeb, en Namibie, se
présente en aggrégats de grains infimes dans des cavités d’un minerai à renierite – germanite – tennantite massif. Aucune autrephase secondaire ne lui est associée. Les cristaux individuels sub-idiomorphes à rarement idiomorphes sont des plaquettes ou desprismes très minces, allongés selon [001], avec un rapport longueur à largeur d’environ 3:1, et n’atteignant que 20 m au maximum. Les cristaux typiques sont 20  14  1 m. Les formes sont {100} majeure, {010} très mince, {011} arrondie, et {001}mineure, très mince et arrondie. Les cristaux montrent des paliers de croissance sur {100} et sont en partie caverneux. Ils sontbrun jaunâtre sale (aggrégats) à jaune brunâtre (cristaux), opaques (aggrégats) à transparents (cristaux), avec une rayure jaune
brunâtre et un éclat vitreux. La eyselite est cassante, avec une fracture inégale, et non fluorescente. Les aggrégats sont mous; D(calc.) est 3.639 g/cm3 pour la formule empirique. Elle est biaxe positive, et deux des indices de réfraction dépassent 1.80; 2V(mesuré) est grand; jaune pâle en lumière polarisée, sans pléochroïsme, faible biréfringence, anisotropie modérée, et allongement
positif. Les résultats moyens des analyses faites avec une microsonde électronique donnent CaO 0.06, Fe2O3 18.54, Ga2O3 1.01,
GeO2 77.75, H2O [2.64] (par différence), pour un total de [100.00]% (poids). La formule empirique, sur une base de O = 8, est(Fe3+0.93Ga3+0.04)0.97Ge4+2.98O6.90(OH)1.17. Le spectre d’absorption infrarouge montre des bandes attribuées à l’hydroxyle structural. Une étude en micro-absorption des rayons X montre que tout le fer est trivalent, et les atomes de Ge seraient en coordinence
octaédrique. Le nom choisi honore feu Walter H. Eysel, professeur de Cristallographie, Ruprecht-Karls-Universität, Heidelberg,
Germany, pour ses nombreuses contributions à l’étude des germanates et à la banque de données en diffraction X sur poudres.
(Traduit par la Rédaction)
Mots-clés: eyselite, nouvelle espèce minérale, oxyde hydroxylé de fer et germanium, diffraction X sur poudre, données demicrosonde électronique, spectroscopie infrarouge, spectroscopie micro-XAS, diffraction des électrons sur aire choisie, mine
Tsumeb, Tsumeb, Namibie.

INTRODUCTION

OCCURRENCE AND ASSOCIATED MINERALSThe Tsumeb mine, Tsumeb, Namibia (lat. 19°13’S,long. 17°43’E) has been, over the years, a prolific sourceof new mineral species. Herein, we describe anothernew phase, eyselite, from this classic locality. The specimen on which the new mineral occurs was acquired in1977 by one of us (T.M.S.), from a collection of altered
Ge-bearing ore samples in the possession of the late Sid
Pieters of Windhoek, Namibia. The secondary phase on
the ore sample was X-rayed by routine powder-diffraction methods in the early 1990s, but was unidentified
and remained so until a portion of the material was sent
to the senior author for further characterization in late
2002.
The mineral is named eyselite in honor of the late
Walter Hans Eysel (1935–1999), Professor of Crystallography at the Ruprecht-Karls-Universität, Heidelberg,
Germany, for his contributions to the study ofgermanates (as models for silicates) and for his numerous contributions to the Powder Diffraction File produced by the International Centre for Diffraction Data
(ICDD). A fitting tribute to Dr. Eysel was published byPentinghaus & Tovar (2000). The mineral and mineralname have been approved by the Commission on New
Minerals and Mineral Names, IMA (2003–52). A small
portion of the holotype specimen used for study (3 
1.5  1.5 cm) is housed in the Systematic Reference
Series of the National Mineral Collection at the Geological Survey of Canada, Ottawa, Ontario, under catalogue number 68093. The larger matrix sample and theelectron-microprobe mount have been retained by
T.M.S.

Eyselite was identified in a vug (4  5 mm) on asingle specimen (1.6  2.3  3.5 cm) of renierite –germanite ore with minor associated tennantite. The newmineral is very rare, and we estimate that only 10 mg ofpure eyselite remains for further study. The mineral has
formed from the alteration of the Ge-bearing sulfide
minerals by aqueous solutions (i.e., groundwaters) migrating through the Tsumeb orebody. No other secondary mineral occurs in the vug in direct association witheyselite.

PHYSICAL AND OPTICAL PROPERTIES
Eyselite occurs on the holotype specimen within the
4  5 mm vug as loosely attached, very fine-grained
aggregates of tiny crystals (Fig. 1) coating the sulfide
phases that host the cavity. A SEM photomicrograph(Fig. 2) shows that the megascopically anhedral aggregates are composed of randomly oriented subhedral torarely euhedral crystals that generally do not exceed 20
m in size and average approximately 20  14  1
m. Individual crystals are platy to very thin prismatic,
elongate along [001], with a length-to-width ratio that
does not exceed 3:1. The dominant forms are {100}major, with minor {010}, {011} and {001}; the latter
two are usually somewhat rounded in appearance.
Growth steps are present on the {100} face, and crystals appear to be partly hollow if viewed down the [001].
The color of the mineral varies from dirty brown-yellow (aggregates) to yellow-tan (crystals), and the streakis brownish yellow. The luster is vitreous, and crystals

EYSELITE, A NEW MINERAL SPECIES FROM TSUMEB, NAMIBIA

are transparent, whereas aggregates are opaque. Eyseliteis brittle, with an uneven fracture, and is nonfluorescent
under both long- and short-wave ultraviolet light. Twinning and cleavage were not observed. Individual crystals are too small to accurately measure hardness, thoughaggregates are soft. The density could not be measured
because of the small size of available crystals and thedearth of material. The density calculated, on the basisof the empirical formula and unit-cell parameters refinedfrom powder data, is 3.639 g/cm3.
Optically, eyselite is very pale yellow in plane-polarized light and has a low birefringence, moderateanisotropy, parallel extinction, and it is length-fast(negative). It is nonpleochroic, biaxial positive, with alarge 2V and with at least two indices of refraction
greater than 1.80. The crystal size (10–20 m grains)
did not allow us to acquire more precise optical data.

CRYSTALLOGRAPHY
Eyselite, at least those samples that have been
examined to date, is unsuitable for study by traditional
X-ray single-crystal techniques. A single-crystal precession study was attempted, but failed; the crystals are too
small to adequately diffract. Even multiple crystallites

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did not diffract with sufficient intensity on orientation
films. Therefore, hand-picked material was subjected toconvergent beam electron-diffraction (CBED) study
with a TEM, and the resulting microdiffraction patterngave an orthogonal two-dimensional lattice with averaged spacings of 9.76 and 4.46 Å and no systematic
extinction conditions. Using the results of the CBED
study, the X-ray powder-diffraction pattern (Table 1)
was successfully indexed by the Ito method on a primitive orthorhombic unit-cell. Regarding potential spacegroup choices, inspection of the indexed powder data
given in Table 1 suggests that if the systematic absence
condition h0l with h + l = 2n is present, then the permissible space-group choices are Pmnm, Pmn21 or P21nm(diffraction aspect P*n*). However, if there is no systematic h0l absence condition, then the highest symmetry-related primitive orthorhombic space-group is
P212121, which would indicate only systematic absence
extinctions along the principal axial directions. It should
be noted that the size of the individual crystals, coupledwith the presence of growth steps on the predominantface, and the partly hollow nature of the crystals, precluded an attempt at a CCD-based crystal-structure determination (F.C. Hawthorne, pers. commun., 2003).
There is most certainly too little material available foran ab initio Rietveld refinement.
Eyselite is thus orthorhombic with a P lattice, andthe refined unit-cell parameters, a 8.302(4), b 9.718(4),c 4.527(2) Å, V 365.2(3) Å3, a:b:c 0.8543:1:0.4658, Z =2, are based on 22 powder reflections, representing dvalues between 4.105 and 1.350 Å for which unambiguous indexing was possible. The powder data are uniqueand bear no resemblance to any germanate, silicate or
any other inorganic phase listed in the Powder Diffraction File. The only other Fe–Ge-oxy compounds foundin nature are brunogeierite (Ge2+,Fe2+)Fe 3+2O 4 andstottite Fe2+Ge4+(OH)6.

CHEMICAL COMPOSITION
Electron-microprobe study

FIG. 1. Photomicrograph of eyselite aggregates. Scale bar: 0.5mm.

Aggregates of eyselite were mounted in polishedsection and chemically analyzed with a Cameca SX–50
electron microprobe, using an operating voltage of 15
kV, a beam current of 10 nA, and a beam diameter of
0.2 to 0.5 m. The following standards were used: wollastonite (Ca), hematite (Fe), synthetic GaAs (Ga) andsynthetic GeO2 (Ge). An energy-dispersion scan indicated the absence of any other elements with atomic
number greater than 9, except those reported here. The
paucity of material prevented quantitative determinationof the amount of H2O. However, the presence of H as
(OH) was confirmed by powder infrared-absorption
study; the amount of H2O was therefore calculated bydifference. Germanium was assumed to be Ge4+, as thisis the most common valence state for the element at
Tsumeb. Iron was assumed to be Fe3+ on the basis of

1774

THE CANADIAN MINERALOGISTthe color of the mineral and of its occurrence as a secondary mineral within a supergene ore zone. This assumption was subsequently confirmed by a qualitative
micro-XAS study (see below), which unambiguously
indicated that all iron is Fe3+. The average of nine determinations (and ranges) gave CaO 0.06 (0.03–0.09),
Fe2O3 18.54 (16.97–20.55), Ga2O3 1.01 (0.73–1.31),
GeO2 77.75 (75.65–79.61), H2O [2.64], total [100.00]
wt.%. With O = 8, the empirical formula is (Fe3+0.93
Ga3+0.04)0.97Ge4+2.98O6.90(OH)1.17. The ideal formula,
Fe3+Ge4+3O7(OH), requires Fe2O3 19.83, GeO2 77.93,
H2O 2.24, total 100.00 wt.%. Because precise values of
the indices of refraction could not be determined, ameaningful index of compatibility could not be calculated.
Micro-X-ray absorption spectroscopy (XAS) study
The polished section used for quantitative electronmicroprobe analyses was sent to the Advanced Light
Source (ALS) facility in Berkeley, California for further study to determine the valence state of the Fe atomin eyselite. Analytical characterization by micro-XRD,
micro-XRF (X-ray fluorescence), and micro-XAS weredone at Beamline 10.3.2 [see Manceau et al. (2002) for
a comprehensive review of the equipment and procedures available at ALS]. Micro-XRD patterns weretaken with a 16  7 m spot size, at an operating voltage of 14 keV, and showed that the eyselite grains aremosaic, with a spread of several degrees, without detectable (h0l) reflections. Micro-XRF mapping wasdone with a 5  5 m beam spot size at operating volt-

ages of 12 and 14 keV. The study revealed that the
Fe:Ge ratio is constant between grains and within eachgrain. The results are consistent with a daltonide (stoichiometric) compound. Minor amounts of Ca and Ga
were detected (as previously determined by electronprobe); in addition, trace amounts of Rb and Sr also weredetected. Micro-XAS was carried out at both the FeK
and GeK edges. For the near-edge (XANES) region ofFe, goethite, fayalite and metallic Fe (alloy addition inaluminum tape) were used as standards. The XANES(X-ray Absorption Near-Edge Spectroscopy) results areshown in Figure 3, and demonstrate that the Fe in
eyselite is trivalent. Magnetite, ZnO and -GeO2 wereused as EXAFS (extended X-ray absorption edge finestructure) standards. EXAFS measurements at the Fe Kedge yielded an average Fe–O first-shell average distance of 2.00(2) Å, which is consistent with that in otherFe3+ oxides. EXAFS measurements at the Ge edge
yielded a Ge–O first-shell average distance of 1.88 Å,which is considerably larger that that found in –GeO2(1.74 Å), but is close to that found in –GeO2 (1.86 Å).
These two polymorphs of GeO2 differ in that the -form
has Ge in tetrahedral coordination, and the -form has
Ge in octahedral coordination. Thus we can postulatethat the Ge atoms in eyselite are most likely in octahedral coordination. For both the Fe and Ge edges, several higher shells are visible; this feature suggests thatthe mineral is structurally well ordered.

INFRARED SPECTROSCOPY
The procedures for acquiring the infrared-absorption
spectrum of eyselite are identical to those reported byRoberts et al. (1994). The sample was analyzed using a
Bomen Michelson MB–100 FTIR spectrometerequipped with a wide-band mercury–cadmium telluridedetector. The transmittance spectrum (Fig. 4) shows amedium-intensity, somewhat shallow absorption
band with a peak at 2957 and a distinct shoulder at3389 cm–1, attributable to O–H stretching in the hydroxyl group. There is no evidence of a band in the
1600–1700 cm–1 region, which would be characteristicof H–O–H bending in structural H2O. Bands at 1007,
1085 and 1230 cm–1 are unassigned, but are inferred topertain to Ge–O interactions.

ACKNOWLEDGEMENTS
The authors thank E. Moffatt (Canadian Conservation Institute) for the infrared spectrum of eyselite, P.
Hunt (GSC) for the photomicrograph reproduced in Figure 2, M. Clarke (GSC) for redrafting Figure 3, and K.
Mooney (GSC) for typing the manuscript. The fayalite
micro-XAS data were taken by M. Heuer and T.
Bounassissi (ALS), and the goethite micro-XAS data
were taken by S. Fakra (ALS). The EXAFS data for
Fe 3O 4 come from the archive maintained by M.
Newville (ALS). The Advanced Light Source is sup-

EYSELITE, A NEW MINERAL SPECIES FROM TSUMEB, NAMIBIA

1775

FIG. 2. SEM photomicrograph showing individual crystals of eyselite. Scale bar: 20 m.

FIG. 3. XANES plots for three individual grains of eyselite (solid lines), goethite (dashedlines), fayalite (fine dashed lines) and metallic Fe (symbols). The differences between
eyselite grains may be due to polarization effects.

ported by the Director, Office of Science, Office ofBasic Energy Sciences, Materials Sciences Division,
United States Department of Energy, under Contract
Number DE–AC03–76SF00098 at the Lawrence Ber-keley National Laboratory. The authors thank Drs. T.
Scott Ercit, Andrew M. McDonald, Joseph A.
Mandarino and Robert F. Martin for constructive comments regarding the manuscript.

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FIG. 4. Infrared-absorption spectrum for eyselite.

REFERENCES
MANCEAU, A., MARCUS, M.A. & TAMURA, N. (2002): Qualitative speciation of heavy metals in soils and sediments bysynchrotron X-ray techniques. In Applications of Synchrotron Radiation in Low-Temperature Geochemistry andEnvironmental Science (P.A. Fenter, M.L. Rivers, N.C.
Sturchio & S.R. Sutton, eds.). Rev. Mineral. Geochem. 49,
341-428.
PENTINGHAUS, H.J. & TOVAR, M. (2000): Obituary: Walter
Hans Eysel, 1935–1999. Powder Diff. 15, 1.

ROBERTS , A.C., ERCIT, T.S., CRIDDLE, A.J., J ONES, G.C.,
WILLIAMS, R.S., CURETON, F.F., II & JENSEN, M.C. (1994):
Mcalpineite, Cu 3TeO6•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.

Received April 23, 2004, revised manuscript accepted July 27,
2004.