207
The Canadian Mineralogist
Vol. 44, pp. 207-212 (2006)LENAITE FROM THE GIES GOLD–SILVER TELLURIDE DEPOSIT,
JUDITH MOUNTAINS, MONTANA, USA: OCCURRENCE, COMPOSITION,
AND CRYSTAL STRUCTURE
Luca BINDI§
Museo di Storia Naturale, sez. di Mineralogia e Litologia,
Università degli Studi di Firenze, via La Pira 4, I-50121 Firenze, Italy

Paul G. SPRY
Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames, Iowa 50011-3210, USA

Giovanni PRATESI
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I-50121 Firenze, Italy
and Museo di Storia Naturale, sez. di Mineralogia e Litologia,
Università degli Studi di Firenze, via La Pira 4, I-50121 Firenze, Italy

Abstract
Lenaite, AgFeS2, a very rare silver–iron sulfide, was found in a sample in the mineralogical collection of the Natural History
Museum of Florence. The sample is from the Gies gold–silver telluride deposit, Judith Mountains, Montana. Lenaite in thissample occurs as anhedral grains up to 0.2 mm across. It is steel-grey in color and has a black streak. No cleavage is observed, andthe Vickers hardness (VHN10) is 285 kg/mm2. Lenaite is greyish white, non-pleochroic, moderately anisotropic in yellowish tints,
and it shows no bireflectance. Reflectance values in air for Rmin and Rmax are 28.2, 30.6 (471.1 nm), 31.8, 35.8 (548.3 nm), 32.8,
36.5 (586.6 nm), and 30.9, 35.0% (652.3 nm), respectively. Lenaite is tetragonal and belongs to the space group I 4̄2d, with thefollowing unit-cell parameters: a 5.4371(2), c 10.8479(9) Å, V 320.69(3) Å3, and Z = 4. Electron-microprobe analyses gave thechemical formula Ag0.96Fe1.01S2.02. The crystal structure has been solved and refined to R = 3.63%. It consists of corner-sharing[4]M and [4]X tetrahedra forming cubic or pseudocubic frameworks, as in sphalerite, but with cation order. The crystal-chemical
relationships with other members of the chalcopyrite group are outlined.
Keywords: lenaite, chalcopyrite, imiterite, chemical composition, physical properties, crystal-structure determination, Gies,
Montana.

Sommaire
Nous signalons la présence de lénaïte, AgFeS2, sulfure rarissime d’argent et de fer, dans un échantillon de la collection
minéralogique du musée d’Histoire Naturelle de Florence. L’échantillon provient du gisement de tellurures d’or et d’argent deGies, dans les montagnes Judith, au Montana. La lénaïte se présente en grains xénomorphes atteignant 0.2 mm. Ils sont grisacier et possèdent une rayure noire. Nous n’avons vu aucun clivage; la dureté de Vickers (VHN10) est 285 kg/mm2. La lénaïte
est blanc grisâtre, non pléochroïque, modérément anisotrope en teintes de jaune, et elle ne montre aucune biréflectance. Les
valeurs de réflectance dans l’air (Rmin et Rmax) sont 28.2, 30.6 (471.1 nm), 31.8, 35.8 (548.3 nm), 32.8, 36.5 (586.6 nm), et 30.9,
35.0% (652.3 nm), respectivement. La lénaïte est tétragonale, groupe d’espace I 4̄2d, avec les paramètres réticulaires suivants:
a 5.4371(2), c 10.8479(9) Å, V 320.69(3) Å3, et Z = 4. Les analyses, effectuées avec une microsonde électronique, ont donné laformule Ag0.96Fe1.01S2.02. La structure cristalline a été résolue jusqu’à un résidu R de 3.63%. Elle contient des tétraèdres [4]M et[4]X à coins partagés, formant des trames cubiques ou pseudocubiques, tout comme dans la sphalérite, mais avec une distribution
ordonnée des cations. Nous décrivons les relations cristallochimiques avec les autres membres du groupe de la chalcopyrite.
(Traduit par la Rédaction)
Mots-clés: lénaïte, chalcopyrite, imitérite, composition chimique, propriétés physiques, structure cristalline, Gies, Montana.

§E-mail address: lbindi@steno.geo.unifi.it

208

the canadian mineralogist

Introduction
An unnamed Ag–Fe sulfide with the formula
AgFeS2, found in association with various Ni–Co–Fe
arsenides, was described from the Tynebottom mine,
Garrigill, Cumbria, England by Ixer & Stanley (1987).
It occurs as lamellae up to 15 mm across in chalcopyrite.
They reported it as being pale grey-buff in air, non-pleochroic and slightly anisotropic. Nekrasov et al. (1987)
also reported a mineral with the same composition
occurring as a rim 0.1 mm wide on grains of Ag-bearing
chalcopyrite in the Aid and Alfa silver deposit, Siberia,
Russia. Since then, Zhang & Spry (1994) described the
Ag–Fe sulfide (“unnamed mineral 1”) as inclusions up
to 20 mm across in a veinlet 50 mm wide coexisting withhessite and pyrite in the epithermal Gies gold–silver
telluride deposit, Judith Mountains, Montana. The namelenaite was formally given to AgFeS2 by Amuzinskii
et al. (1995); they reported it as grains up to 0.2 mm
in length in goethite pseudomorphs after magnesiansiderite from the Khachakchanskoye silver occurrence,
Lena River basin, Yakutia, Russia. It is also spatiallyassociated with acanthite, chalcopyrite, and Ag-richmembers of the tetrahedrite group in quartz–sideriteveins. The optical properties given by Amuzinskii et al.
(1995) for lenaite match those given previously for theunnamed phases by Ixer & Stanley (1987), Nekrasov
et al. (1987), and Zhang & Spry (1994).
In the course of a revision of the mineralogicalcollection of the Natural History Museum of Florence (Bindi et al. 2005, and references therein), we
discovered a sample of lenaite from the Gies deposit(catalogue number 44285/G, labeled proustite). In thispaper, we report the physical and chemical characteristics of lenaite from the Gies deposit, together witha determination of the crystal structure. We have also
re-examined the specimen of lenaite (sample 90–P13)
of Zhang & Spry (1994) by optical methods and witha scanning electron microscope.

Occurrence of Lenaite and Geological Setting
The Gies deposit is an epithermal vein system that
occurs along the contact between Cretaceous–Tertiary
alkaline intrusive rocks (monzonite, syenite, andtinguaite) and Cambrian to Cretaceous sedimentaryrocks (limestone, sandstone, and siltstone) in the Judith
Mountains, Montana. The average grade of the deposit
was about 14 g/t gold. Thirty-six ore and gangueminerals were identified by Zhang & Spry (1994) infour hypogene stages. The first three stages correlatewith quartz-forming events, whereas the last threestages contain tellurides. Sulfides are found in all fourstages, with lenaite occurring in stage-IV veinlets (<1mm wide) that cross-cut minerals of stage III. In sample
44285/G, lenaite occurs as anhedral grains up to 0.2 mm
across spatially associated with proustite, tetrahedrite,
tennantite, chalcopyrite, and pyrite.
A re-examination of sample 90–P13 of Zhang &Spry (1994) shows the presence of the following ore
minerals: hessite, pyrite, lenaite, native gold, chalco­
pyrite, polybasite, tennantite, and the rare phase­imiterite (Ag2HgS2). Lenaite occurs in contact withhessite, pyrite, and imiterite, with the last mineralcross-cutting lenaite (Fig. 1). Imiterite has been reported
previously from only two localities (Guillou et al. 1985,
Walenta & Hess 1985).
Although fluid-inclusion studies show that stages
I, II, and III formed between 270° and 320°C, there
was no suitable mineral on which to conduct a fluid­inclusion study in the stage-IV lenaite-bearing material.
However, we speculate that lenaite formed at less than
170°C because of the presence of coexisting hessite
and Au–Ag alloy in sample 90–P13 of Zhang & Spry(1994) from stage IV and the intimate spatial association between hessite and lenaite. Coexisting Au–Ag
alloy and hessite would have formed as products ofthe decomposition of the g phase, a composition intermediate between hessite and petzite, which forms only
below 170°C (Cabri 1965).

Physical and Optical Properties

Fig. 1. Back-scattered electron image of lenaite (Len) containing inclusions of pyrite (Py) and in contact with hessite(Hes). It is also rimmed and cross-cut by imiterite (Im).
Sample 90–P13; the image was obtained using Hitachi S
2460 reduced-vacuum scanning electron microscope at anaccelerating voltage of 20 kV.

Lenaite is steel-grey in color and shows a black
streak. The mineral is opaque with a metallic luster.
The micro-indentation measurements carried out witha VHN load of 10 g on sample 44285/G gave a meanvalue of 285 kg/mm2 (range: 270–300), corresponding toa Mohs hardness of about 4 – 4½. This value is slightly
lower than that found by Amuzinskii et al. (1995) for
lenaite from the Khachakchanskoye ore deposit (310kg/mm2). In reflected light, lenaite in sample 44285/G
is greyish white, non-pleochroic, moderately anisotropiclenaite from the gies au–ag telluride deposit, montana

in yellowish tints, and shows no bireflectance. There
are no internal reflections.
Reflectance measurements were performed in air
with a MPM–200 Zeiss microphotometer equippedwith a MSP–20 system processor on a Zeiss Axioplan
ore microscope on the 44285/G sample. Filament
temperature was approximately 3350 K. An interference
filter was adjusted, in turn, to select four wavelengths
for measurement (471.1, 548.3, 586.6, and 652.3 nm).
Readings were taken for specimen and standard (SiC)
maintained under the same conditions of focus. The
diameter of the circular measuring area was 0.1 mm.
Measurements of reflectivity (in %) for Rmin and Rmax
are 28.2, 30.6 (471.1 nm), 31.8, 35.8 (548.3 nm),
32.8, 36.5 (586.6 nm), and 30.9, 35.0 (652.3 nm),respectively.

Chemical Composition
A preliminary chemical analysis using EDS,
performed on the same crystal fragment as that used
for the structural study, did not indicate elements (Z> 9) other than Ag, Fe, S and very minor Sb, As and
Cu. The chemical composition was then determinedby means of a JEOL JXA–8600 electron microprobe.
Concentrations of the major and minor elements were
determined at an accelerating voltage of 20 kV and abeam current of 40 nA, with variable count-times: 30
s were used for Ag, Fe and S, and 60 s for the minor
elements Sb, As and Cu. For the wavelength-dispersion analyses, the following lines were used: AgLa,
FeKa, SKa, SbLb, AsLa, and CuKa. We employed
the following standards: pure silver (Ag), marcasite(Fe, S), synthetic GaAs (As), pure copper (Cu), andsynthetic Sb2Te3 (Sb). The crystal fragment was foundto be homogeneous within analytical uncertainty. The
average chemical composition, together with ranges of
wt.% of elements, is reported in Table 1. On the basisof four atoms, the empirical formula is Ag0.96Fe1.01S2.02,
which is similar to that reported by Amuzinskii et al.
(1995): Ag0.98Fe0.98S2.04.

Crystal-Structure Solution and Refinement
A small fragment (15 3 25 3 25 mm) from sample
44285/G was selected for the single-crystal X-raydiffraction study. The data collection intensity (3 < u <50°) was carried out by means of an Oxford Diffraction
Xcalibur 2 single-crystal diffractometer (enhanced Xray source, X-ray radiation MoKa, l = 0.71073 Å) fittedwith a Sapphire–2 CCD detector. A total of 1985 framesof data were collected at room temperature as 12 setsof omega runs with an exposure time of 40 s per frame
and a frame width of 0.75°. This afforded an overall
data-collection of 15048 reflections (836 unique). Dataframes were processed using the CRYSALIS software
package (Oxford Diffraction 2002) running on the
Xcalibur 2 control PC. The empirical method proposed209

by Blessing (1995) was applied for the absorption
correction. The merging R for the data set decreased
from 9.31% before the absorption correction to 5.24%
after this correction. Systematic absences (hkl: h + k + l= 2n; hk0: h + k = 2n; 0kl: k + l = 2n; hhl: 2h + l = 4n;00l: l = 4n; h00: h = 2n; h h̄0: h = 2n) are consistent withthe space groups I41md and I 4̄2d. In view of the close
similarity with the minerals of the chalcopyrite group,
we decided to solve the structure in the space groupI 4̄2d. The positions of silver and iron atoms were determined from a three-dimensional Patterson synthesis(Sheldrick 1997a). A least-squares refinement using
these heavy-atom positions and isotropic temperaturefactors yielded an R factor of 8.55%. Three-dimensional
difference-Fourier synthesis yielded the position of theremaining sulfur atom. The full-matrix least-squares
program SHELXL–97 (Sheldrick 1997b) was used
for the refinement of the structure. The introduction ofanisotropic temperature-factors for all the atoms led to R
= 3.63% for 651 observed reflections [Fo > 4s(Fo)] andR = 3.88% for all 836 independent reflections. Neutral
scattering curves for Ag, Fe and S were taken from theInternational Tables for X-ray Crystallography (Ibers& Hamilton 1974). Inspection of the difference-Fourier
map revealed that maximum positive and negative
peaks were 3.85 and 3.77 e–/Å3, respectively.
In contrast with what is observed for the chalcopyrite
structure (Hall & Stewart 1973), the absolute structure
cannot be determined accurately. In fact, the Flackparameter is 0.53(8) for the data presented here, and0.51(8) for the inverted structure. Experimental details
and R indices are given in Table 2. Fractional atomic
coordinates and anisotropic-displacement parameters
are shown in Table 3.

Description of the Structure and Discussion
In their mineralogical classification Strunz & Nickel(2001) considered lenaite as a member of the chalcopyrite group. There are six members: chalcopyrite,
CuFeS2 (Hall & Stewart 1973), gallite, CuGaS2 (Strunz
et al. 1958), roquesite, CuInS2 (Picot & Pierrot 1963),
eskebornite, CuFeSe2 (Delgado et al. 1992), lenaite,

210

the canadian mineralogist

AgFeS2 (Amuzinskii et al. 1995), and laforêtite, AgInS2(Meisser et al. 1999). Apart from eskebornite (spacegroup P4̄2c, Delgado et al. 1992) and lenaite (spacegroup proposed P42mc, Amuzinskii et al. 1995), all theothers are characterized by space group I 4̄2d, unit-celldimensions a in the range 5.29 – 5.88 Å, 10.42 < c <11.21 Å, and Z = 4. From the crystallographic point ofview, however, only the crystal structure of chalcopyrite
has been investigated (Hall & Stewart 1973). It consistsof corner-sharing [4]M and [4]X tetrahedra forming cubicor pseudocubic frameworks, as in sphalerite, but withorder of the cations. On the whole, the crystal structureof lenaite refined here (Fig. 2) is topologically identical to that of chalcopyrite, apart from the deviations
expected from the metal size (i.e., Ag with respect toCu). Both Ag and Fe atoms are tetrahedrally surrounded
by S atoms, with distances Ag–S of 2.4698(4) Å, and
Fe–S of 2.2458(3) Å.
One of the most interesting structural peculiaritiesof the crystal structure of lenaite is the angle that thesulfur atoms subtend with the different metals. The
two independent S–Fe–S angles of the iron tetrahedronare 111.38(1) and 105.72(1)°, whereas the S–Ag–S
angles in the silver tetrahedron are 113.40(1) and
107.54(1)° (Fig. 3). On the other hand, Hall & Stewart(1973) showed that the coordination of iron in chalco­
pyrite is perfectly tetrahedral [S–Fe–S: 109.47(4) and
109.47(3)°, respectively], whereas the coordinationof copper resembles a flattened tetrahedron [S–Cu–S:
111.06(3) and 108.68(4)°, respectively] in the z direction of the cell. The combination of distorted tetrahedrally coordinated silver and iron atoms in the crystal
structure of lenaite apparently induces a strong increasein the distortion parameter s2 (Robinson et al. 1971)of the S(Ag2Fe2) tetrahedron. In fact, the value of
this parameter, 24.05, exceeds by far that observed inchalcopyrite [s2 = 1.40 for S(Cu2Fe2): Hall & Stewart(1973)] and those observed in the stannite–kësterite
join [s2 = 2.06–3.51 for the S(M3Sn): Bonazzi et al.
(2003)].
By comparing the crystal structures of chalcopyrite(Hall & Stewart 1973), talnakhite (Hall & Gabe 1972),
mooihoekite (Hall & Rowland 1973), and haycockite(Rowland & Hall 1975), Hall (1975) pointed out thatthe relative magnitude of the thermal motion for theCu, Fe and S atoms in these minerals is remarkably
consistent. Hall (1975) noted that the average isotropic

temperature-factor of the iron atoms is consistently
lower than that for the copper atoms. The likely cause
for this difference is the stronger covalent interaction
between iron and sulfur over copper and sulfur, as indicated by the markedly shorter Fe–S bond distances withrespect to Cu-S. For the lenaite sample studied here,
we observed a very similar average isotropic temperature-factor for the Ag and Fe atoms (Table 3), thus
indicating a similar covalent interaction between metals
and sulfur atoms. It should be stressed, however, that
Hall (1975) analyzed temperature factors and distortionsof coordination polyhedra in order to find out possible
guidelines to determine the degree of metal order inthe chalcopyrite-group minerals. In the case of lenaite,
since there is a strong difference in size between silver
and iron, together with the marked difference in themean number of electrons, we were able to determine
the tetrahedral environment of both metals.
Table 4 shows the X-ray powder pattern calculatedusing the structural data obtained in this study, together
with that reported by Amuzinskii et al. (1995) for lenaite
from the type locality. It is evident that the patterns

Fig. 2. The crystal structure of lenaite. Grey, red and yellow
circles refer to Ag, Fe and S atoms, respectively. The unit
cell is outlined.

lenaite from the gies au–ag telluride deposit, montana

211

Fig. 3. Interatomic angles in lenaite. Symbols as in Figure 2.
The unit cell is outlined.

differ from one another significantly. Amuzinskii etal. (1995) reported only 14 lines in their diffraction
pattern, and the attribution of the space group P42mc
to lenaite is considered questionable. One can speculate that the original lenaite and the mineral examined
here could represent two distinct species; however, thechemical composition of the two minerals is identical.
On the other hand, possible reasons for the observed
discrepancies could be the small amount of material(very small grain-size) available to Amuzinskii et al.
(1995), as well as the possibility of intergrowths withother Ag-sulfides.

Acknowledgements
The paper benefitted by the official reviews madeby Gunnar Raade and Nigel Cook. The authors arealso grateful to Robert F. Martin for his suggestions on
improving the manuscript. The authors thank Filippo

212

the canadian mineralogist

Olmi (CNR – Istituto di Geoscienze e Georisorse– sezione di Firenze) for his help with the electronmicroprobe analyses, and Warren Straszheim (Iowa
State University) for his assistance with scanningelectron imaging. Financial support was provided by
the University of Florence (60% grant) and by P.R.I.N.,
cofinanziamento 2003, project “Crystal Chemistry of
Metalliferous Minerals”.

References
Amuzinskii, V.A., Zhdanov, Yu.Ya., Zayakina, N.V. & Leskova, N.V. (1995): Lenaite, AgFeS2, a new mineral species.
Zap. Vser. Mineral. Obshchest. 124(5), 85-91 (in Russ.).
Bindi, L., Rossell, M.D., Van Tendeloo, G., Spry, P.G. &Cipriani, C. (2005): Inferred phase relations in part of thesystem Au–Ag–Te: an integrated analytical study of goldore from the Golden Mile, Kalgoorlie, Australia. Mineral.
Petrol. 83, 283-293.
Blessing, R.H. (1995): An empirical correction for absorption
anisotropy. Acta Crystallogr. A51, 33-38.
Bonazzi, P., Bindi, L., Bernardini, G.P. & Menchetti, S.
(2003): A model for the mechanism of incorporation ofCu, Fe and Zn in the stannite–kësterite series, Cu2FeSnS4–
Cu2ZnSnS4. Can. Mineral. 41, 639-647.
Cabri, L.J. (1965): Phase relations in the system Au–Ag–Te
and their mineralogical significance. Econ. Geol. 60,
1569-1606.
Delgado, J.M., Diaz de Delgado, G., Quintero, M. &Woolley, J.C. (1992): The crystal structure of copper ironselenide, CuFeSe2. Mater. Res. Bull. 27(3), 367-373.
Downs, R.T., Bartelmehs, K.L., Gibbs, G.V. & Boisen, M.B.,
Jr. (1993): Interactive software for calculating and displaying X-ray or neutron powder diffractometer patterns ofcrystalline materials. Am. Mineral. 78, 1104-1107.
Guillou, J.-J., Monthel, J., Picot, P., Pillard, F., Protas, J.
& Samama, J.-C. (1985): L’imitérite, Ag2HgS2, nouvelle
espèce minérale; propriétés et structure cristalline. Bull.
Minéral. 108, 457-464.
Hall, S.R. (1975): Crystal structures of the chalcopyrite
series. Can. Mineral. 13, 168-172.
Hall, S.R. & Gabe, E.J. (1972): The crystal structure of talnakhite, Cu18Fe16S32. Am. Mineral. 57, 368-380.
Hall, S.R. & Rowland, J.F. (1973): The crystal structure ofsynthetic mooihoekite, Cu9Fe9S16. Acta Crystallogr. B29,2365-2372.
Hall, S.R. & Stewart, J.M. (1973): The crystal structurerefinement of chalcopyrite, CuFeS 2. Acta Crystallogr.
B29, 579-585.
Ibers, J.A. & Hamilton, W.C., eds. (1974): International
Tables for X-ray Crystallography IV. Kynoch, Dordrecht,
The Netherlands.

Ixer, R.A. & Stanley, C.J. (1987): A silver–nickel–cobalt
mineral association at Tynebottom mine, Garrigill, near
Alston, Cumbria. Proc. Yorkshire Geol. Soc. 46, 133-139.
Meisser, N., Thelin, P., Chiappero, P.J. & Maurel C. (1999):
Laforêtite, AgInS2, a new mineral of the chalcopyrite
group from the Montgros mine, Haute-Loire, France. Eur.
J. Mineral. 5, 891- 897.
N ekrasov , I.Y a ., G amyanin , G.N., G oryachev , N.A.,
Zhadov, Yu.Ya., Goryacheva, E.M. & Leskova, N.V.
(1987): Mineralogical and geochemical peculiarities ofsilver mineralization in the Verkhoyan–Kolymian folded
region. Mineral associations of ores of the silver–antimony
and gold–silver formations. Mineral. Zh. 9(6), 5-17 (inRuss.).
Oxford Diffraction (2002): CrysAlis CCD and RED. Version 1.70. Oxford Diffraction Ltd., Oxford, U.K.
Picot, P. & Pierrot, R. (1963): La roquesite, premier minérald’indium: CuInS2. Bull. Soc. Fr. Minéral. Cristallogr. 86,
7-14.
Robinson, K., Gibbs, G.V. & Ribbe, P.H. (1971): Quadratic
elongation; a quantitative measure of distortion in coordination polyhedra. Science 172, 567-570.
Rowland, J.F. & Hall, S.R. (1975): Haycockite Cu4Fe5S8: asuperstructure in the chalcopyrite series. Acta Crystallogr.
B31, 2105-2112.
S heldrick , G.M. (1997a): SHELXS–97: a Program for
Automatic Solution of Crystal Structures. University of
Göttingen, Göttingen, Germany.
S heldrick , G.M. (1997b): SHELXL–97: a Program for
Crystal Structure Refinement. University of Göttingen,Göttingen, Germany.
S trunz , H., G eier , B.H. & S eeliger , E. (1958): Gallit,
CuGaS2, das erste selbständige Galliummineral, und seine
Verbreitung in den Erzen der Tsumeb- und Kipushi-Mine.
Neues Jahbr. Mineral., Monatsh., 241-264.
Strunz, H. & Nickel, E.H. (2001): Strunz Mineralogical
Tables. Chemical-Structural Mineral Classification System(9th ed.). Schweizerbart, Stuttgart, Germany.
Walenta, K. & Hess, H. (1985): Imiterite; a new silver- andmercury-rich sulfide mineral from Ramsbeck, Sauerland.
Der Aufschluss 36, 209-215.
Zhang, Xiamao & Spry, P.G. (1994): Petrological, mineralogical, fluid inclusion, and stable isotope studies of the Gies
gold–silver telluride deposit, Judith Mountains, Montana.
Econ. Geol. 89, 602-627.

Received March 29, 2005, revised manusript accepted July 19,
2005.