American Mineralogist, Volume 78, pages 85-95, 1993
The distribution of Ag and Sb in galena: Inclusions versus solid solution
Tnorras G. Sn.lnpx
Departmentof Geology,Arizona StateUniversity, Tempe,Arizona 85287-1404,U.S.A.
PBrBn R. Busncr
Departments of Geology and Chemistry, Arizona State University, Tempe, Arizona 85287-1404,U.S.A.
Ansrnacr
The distributions of Ag and Sb in galena samplesfrom La Paz and Zacatecas,Mexico,
were investigatedusing electron microprobe analysis,backscattered-electronimaging, and
high-resolution transmission electron microscopy. Both samples contain numerous rodshapedinclusions of diaphorite (PbrAg3Sb3S8),
and the LaPaz samplesalso contain franckeite [(Pb,Sn).SnrSbrFeS,o].
Although diaphorite is a common Ag-bearing inclusion in galena, franckeite has not been previously reported. Both sampleshave Ag concentrations
nearly equal to those of Sb, indicating coupled substitution of Ag* and Sb3*for 2 Pb,*.
The average Ago,Sbo,S contents of the bulk La Paz and Zacatecassamples (including
inclusions)are approximately1.37 and0.44 molo/0,respectively.Of thesetotal AgorSborS
amounts, 0.48 and 0.32 molo/ooccur in solid solution in galenain the La Paz andZacatecas
samples,respectively.
Inclusions ofdiaphorite are from 20 nm to 50 pm long, but no evidencefor Ag-Sb atom
clusterswas found. Rod-shapeddiaphorite inclusions are oriented with their c axes (rod
axes)parallel to the ( 100) directions ofgalena and their a and b axesparallel to the ( I l0)
galenadirections. The diaphorite-galenainterfacesare semicoherent,with misfit dislocations every 6 to 10 nm. Disk-shaped inclusions of franckeite in the LaPaz samplesare
oriented with their a*, b, and c axes parallel to the (100) directions of galena.The top
and bottom interfacesofthe disks are parallel to the (100) layers offranckeite and appear
to be coherentwith the galena.The franckeite inclusions have a modulated-layer structure
with a wavelength of 3.55 nm, which is significantly smaller than that of previously described franckeite. Both diaphorite and franckeite inclusions appearto have resulted from
coherentexsolution at low temperatures.Diaphorite appearsto occur metastably relative
to freieslebenite(AgPbSbSr),suggestingthat there is a coherent solvus betweendiaphorite
and galena.
INrnooucrroN
The distribution of precious metals in sulfide minerals
is of economic, crystallographic, and geologic importance. Accurate knowledge of precious metal distributions is of economic importance for maximizing metal
recovery and improving metallurgical techniques. The
crystallographic significanceof precious metal distributions is in understanding how "foreign" atoms are accommodated in sulfide structures and in the crystallographiccontrolson exsolution.Understandingthe primary
vs. secondarynature of precious metals, as well as the
processesand conditions of ore deposition, is of significant geologicimportance.
The occurrenceof precious metals in sulfide minerals
and their economic importance has been addressedby,
among others, Cabri (1987, 1992) and Chryssoulis and
Cabri (1990). Distinguishing submicroscopic inclusions
- p..*"t
Bayerisches
Geoinstitut,UniversitiitBay"Odress:
reuth,Postfach
10 12 51,8580Bayreuth,
Germany.
0003-o04x/93/0102-o085$02.00
85
from material that is structurally bound in solid solution
is crucial for understandingthe distributions ofprecious
metals and the processesof their incorporation and recovery. This is particularly difficult for invisible Au, which
occurs as either submicroscopicparticles or in solid solution with sulfides, sulfarsenides,and arsenides.Ag occurs in more minerals than Au and is common in many
base-metalminerals, either in the form of Ag-bearing inclusions or in solid solution. By overlooking Ag in such
common minerals, significant amounts of Ag are lost in
processing(Cabri, 1987, 1992).
Microbeam techniquesare very useful for determining
the concentrations and distributions of precious metals
in minerals (Cabri, 1992), but such techniques cannot
always distinguish precious metals in solid solution from
microscopic inclusions of precious-metal-bearingminerals. If such inclusions are not accountedfor in analyses,
the result could be unrealistically high solid-solution estimates or incorrectly interpreted substitution mechanisms. SEM imaging of samplescan be usedto determine
86
SHARP AND BUSECK: Ag AND Sb IN GALENA
the presenceof microinclusions that are larger than about
100 nm, but the high spatial resolution of transmission
electron microscopy (TEM) is neededto observe smaller
particles.In addition, the structural relations betweeninclusions and the host mineral can be obtained with TEM
to interpret better the origins of inclusions. High-resolution transmission electron microscopy (HRTEM) has recently been applied to the problem of invisible Au in
arsenopyrite(Cabri et al., 1989) and disseminatedAu in
Carlin-type deposits(Bakken et al., I 989). In the first case
(Cabri et al., 1989),Au particles were not observed,suggestingthat the Au is structurally bound, whereasBakken
et al. (1989) found Au particles less than 20 nm in diameter that had not been previously observed.
High-sensitivity analytical techniques,such as protoninduced X-ray emission (Cabri, 1987; Cabri et al., 1984,
1985)and secondary-ionmassspectrometry(Mclntyre et
al., 1984;Chryssouliset al., 1986; Chryssoulisand Cabri,
1990; Cabri et al., 1989, l99l), combined with careful
inspection for inclusions, have been used to show that
trace concentrations of precious metals occur in many
sulfide minerals. Samplesof galena containing relatively
high concentrationsofAg, Bi, and Sb have been studied
using electron-microprobe analysis (EMPA) (Laflamme
and Cabri, I 986a,I 986b;Foord et al., I 988),but HRTEM
(Sharp and Buseck, 1989) and scanning tunneling microscopy(STM) (Sharp et al., 1990)have only recently
been applied to Ag-bearing galenas.
ConcentrationsofAg in galenaare variable and depend
on the presenceof other elementssuch as Sb and Bi. The
simple substitution of Ag for Pb (2Ag- : Pb'*) is limited
to a maximum of 0.4 molo/oAg.S at 700 oC and less at
lower temperaturesbecausehalf of the Ag must go into
interstitial sites(Van Hook, 1960;Karup-Moller, 1977).
Where Ag* substitution for Pb2* is coupled with substitution of Sb3*or Bi3* [Ag* + (Sb,BD3*: 2Pb"f, the charge
and cation-anion ratio are balanced, allowing higher Ag
concentrations. Galena with combined Ag, Sb, and Bi
concentrationsgreaterthan 0.5 wto/ois called galenasolid
solutionby Foord et al. (1988),who report samplescontaining as much as 9.4 and 18.5 wto/oAg and Bi, respectively. Whereas excessAg relative to Sb or Bi is limited
by interstitial substitution of Ag, excessSb or Bi is more
easilyaccommodatedby the substitutionmechanism2(Sb,
Bi;'* * tr : 3Pb'?*),resulting in vacancieson Pb sites.
The lattice constantsfor the AgSbSr-galenasolid solution
show a negative deviation from ideality, which is interpreted as evidence for clustering of the solute atoms in
the solid solution (Wernick, 1960; Godovikov and Nenasheva,1968).
Galena solid solution and the sulfosalt inclusions may
be useful as indicators of depositional conditions. The
coexistenceofgalena, galenasolid solution, and a variety
of sulfosaltswas interpreted as evidence for multiple stages
of mineralization in the Round Mountain and Manhattan
Au districts, Nevada (Foord et al., 1988).Experimentally
determined phaserelations in the AgrS-SbrS'-PbSsystem
indicate a solvus between freieslebenite(AgPbSbSr)and
galena solid solution below 420 to 390 "C (Hoda and
Chang, 1975; Amcofl 1976) thar may be useful as an
indicator of deposition temperatures.However, our results, as well as previous studies (Czamanskeand Hall,
1976;Laflamme and Cabri, 1986a, 1986b),suggestthat
diaphorite (PbrAg.SbrSr),rather than freieslebenite,is the
major inclusion type in Ag- and Sb-bearinggalena.
The purpose ofthe present study is to investigate the
distribution of Ag and Sb between solid solution and inclusions in galena samples from two Ag deposits. The
compositions of the galenaand inclusions are determined
with EMPA. The type and size distribution of the inclusions are determined with TEM, as are the structural relations to the host galena.The origins of the inclusions
are discussedin terms of their morphologies and crystallographic relations to galena.
TECHNTQUES
S,l'rvrpr-rsAND CHARACTERIZATToN
The Ag- and Sb-bearing galena samples used in this
study were from the La Paz and Zacalecas Ag districts,
Mexico. Most publishedstudiesof Ag-bearinggalenahave
focused on coupled substitution of Ag with Bi; for this
study sampleswere chosenthat contain up to I wto/oAg
and Sb, with no detectableBi and only trace amounts of
As, Sn, and Te. Both samplescontain Ag-Sb inclusions,
predominantly diaphorite, and minor amounts of other
sulfosalts.Thesegalenasampleswere also chosenfor their
large crystal size, which was required for previously reported STM investigations(Sharpet al., 1990).
In addition to the Ag- and Sb-bearingsamples,Ag-free
galena (from an unknown location) and synthetic Agbearing and Sb-Bi-free galena(provided by Louis Cabri)
were examined. These sampleswere used as controls to
determine whether observed defectscorrelate with Ag and
Sb concentrations.
Chemical analyseswere obtained with a Jeol JXA 8600
Superprobeelectronmicroprobe using an acceleratingpotential of 15 kV. Cleavage fragments of galena were
mounted in epoxy resin, polished,and C coated,resulting
in polished surfacesnearly parallel to (100) planes.Backscattered-electronimaging (BEI) was used to locate sulfosalt inclusions and to observetheir textural relations to
the galena host. Inclusions and the galenahost were analyzed with wavelength-dispersive X-ray spectroscopyfor
Pb, S, Ag, Sb, Bi, As, and Sn, using common sulfidesand
Bi metal as standards.The galena host (between inclusions), was analyzedusing a 50-nA beam current to obtain high count rates for Ag and Sb. Basedon 1o counting
statistics, the precision of these analysesis +0.02 wto/0.
Analysesof the host galenaplus inclusions were obtained
by rasteringthe beam over areas 120 x 120 pm. Because
the rastered-beamanalysesand those of the micrometersized inclusions have contributions from inclusions and
matrix, the results are only semiquantitative.
For HRTEM studies,cleavedslabsof galenawere glued
to 3-mm Cu grids, mechanically thinned to approximately 50 p.m, and ion milled with 5-kV Ar ions until
87
SHARP AND BUSECK: AS AND Sb IN GALENA
termined by HRTEM imaging of the inclusions and their
boundaries.
Ag-Sb DISTRIBUTIoNBETwEEN
GALENA AND INCLUSIONS
f
,
1O irm,
Fig. l A backscattered-electron
imageof La Pazgalenawith
diaphorite(Dph)and Sn-bearing
(Sn)inclusions.A large,irregularlyshapeddiaphoritegrain,aswell asthe typicalrod-shaped
inclusions,areindicated.The orientationofthe diaphoriterods
alongthe galena( 100)directionsproducesthe dark orthogonal
Iinesand spotsdepictedhere.An exampleof the Sn-bearing
materialoccursat the end ofthe largediaphoritegrain
the slabswere perforated.Low-voltagemilling (1.6 kV)
was done to remove excessbombardment-damaged
material. Electrical contact betweenthe samplesand the Cu
grids was achievedby either applying a light C coat or by
making a bridge with conductive C paint. Crushed samples,suspendedon holey-carbongrids,were examinedto
monitor ion-bombardmentdamagein the milled samples.
HRTEM experiments were conducted with a Jeol
2000FX using an acceleratingpotential of 200 kV. Imaging and selected-areaelectron diffraction experiments
(SAED) weredone alongthe ( I 00) axesof galena.SAED
was used to identify inclusions and to determine their
orientation and crystallographicrelations to host galena.
Structural and interfacial relations with salena were de-
Although the Ag- and Sb-bearing inclusions in these
samplesare not visible by reflected-lightmicroscopy,they
stand out in high-magnification and high-contrast BEI
micrographs.The predominant inclusionsin both samples are diaphorite, which appear as dark linear features
that
and nearlyround spots(Fig. 1).This texturesuggests
the inclusions are orthogonally oriented rods within the
galena.Most rods appearto be 0.5 to 2.0 1rmin diameter
and as long as 50 prm,although larger and less regularly
shapeddiaphoriteinclusionsoccur in the LaPaz sample.
Diaphoriteinclusionsof similar sizeand shapealsooccur
in galena from the Brunswick 12 mine (Laflamme and
Cabri, 1986a,1986b).
In addition to diaphorite,the La Paz samplecontains
significantamounts of Sn-bearinginclusions.These inclusionsappear in BEI micrographs(Fig. l) as irregularly
shapedgrainsthat are commonly associatedwith diaphorite. Many display variable contrastin such images,indicating chemical heterogeneity and possibly a multiphasecharacter.The Sn-bearinginclusionsseenwith BEI
werenot observedwith HRTEM, but the Sn-Ag-Sbmineral, franckeite,was commonly encounteredin the La Paz
samplewith HRTEM. Additional inclusionssuchasbouand someAg-rich tellurideswereoblangerite(PbrSboS,,)
servedin the La Paz andZacatecassamples,but they are
rare.
Electron microprobe analysis(EMPA) of the inclusions
is limited by their small sizes;many analysesof the diaphorite inclusions have excessPb, indicating that even
for the larger inclusions there can be contributions from
the surroundinggalena.The compositionsof diaphorite,
Sn-bearinginclusions, and boulangerite are presentedin
Table 1, togetherwith the stoichiometriccompositions
TreLe 1. Electronmicroprobedata for inclusionsin La Paz galenaand Ag and Sb contentsof La Paz and Zacatecasgalena(+
inclusions)
La Paz
J
Ag
Sb
Pb
5n
Fe
Total
J
Ag
Sb
Pb
Sn
Fe
Total
Diaphorite
t6l
ldeal
diaphorite
1I 9s(0 30)
22 92(0.53)
26.55(0.41)
30.86(073)
18.86
23 80
26 86
30 48
99.28(0.60)
8.00
288
295
2.02
1585
Sn
p h a s e[ 3 ]
16.44(2.0)
8.44(10)
1s.51(12)
59.90(21)
1.13(0.84)
ldeal Pb- Boulang
ldeal
franckeite t11
b o u l a n g . G a l e n a[ 2 1 ]
2188
1 18 7
40 39
23.14
272
1813
0.02
2524
55 57
0.32
+ inclusions n 2l
Zacatecas
G a l e n a[ 2 4 ]
+ Inctus i o n s[ 1 2 ]
18.80
2596
5523
022(012]'
0 28(014)
0.62(006)
0.14(0.08)
0.81(0.06) 0 1e(0.09)
10000
10142e.0\ 10000
9928
9999
fncfusionstoichiometry
Ag and Sb
basedon givenno.ot S atomsand ato/o
8.00
14.00
1 4 . 0 0 1 10 0
1 10 0
3.00
1.98
0.69(0.07)
0.24(013)
3.00
0.79(0.06)
3.28
200
403
400
0.27(013)
2.00
8.21
4 00
5.22
5 00
0.27
4.00
0 05
1.00
16.00
27 74
2 5 . 0 0 1 93 0
1 90 0
0.19(0.07)
0.27(006)
0 16(0.09) 0 22(008)
0 18(0.0s) 0 27(006)
Note. The numbersin bracketscorrespondto the numberof analysespresented,whereasthe numbersin parenthesescorrespondto the standard
deviationsof the data.
SHARP AND BUSECK: AE AND Sb IN GALENA
88
a
b
0.020
Ac = Acanthite
qgzs
Bl = Boulangerite
sl1
Pbssb4
Dph = Diaphorite
Pb2Ag3Sb3SB
Fri = Freieslebenite PbAgSbS,
Gn = Galena
PbS
Mr = Miargyrite
AgSbS,
Stb = Stibnite
sb2s3
La Paz
AgSbPb-z
0.015
0.010
AgC Pb
-0.5 -0.s
0.005
0,000
0.000 0.005 0.010 0.015 0.020
AgSbPb-,
1.0
sbtr Pb- 1 . 5
0.5
c
0.020
calecas
AgE .rPb*.u
AgSbPb.,
0.015
0.010
AgQo.uP9o.
0.005
iF
PbS
0.33
0.67 Sb.67S
sbtro.sPhl.s
0.000
0.000 0.005 0.010 0.015 0.020
toEo.rto-r.u
Fig. 2. Chemographicand substitution relations (a) in the AgrS-PbS-Sbo
urSsystemshowing substitution vectors relating galena
The PbS corner of the triangle is enlargedto show the
freieslebenite,
and
diaphorite.
to acanthite, stibnite, miargyrite, boulangerite,
substitutions of Ag and Sb in the (b) La Paz and (c) Zacatecassamples.Many data plot along or slightly below the AgSbPb '
coupled-substitutionvector.
of these minerals and Pb-rich franckeite for comparison.
The analysesofdiaphorite and boulangeriteare near their
stoichiometric compositions, but that of the Sn-bearing
materials does not closely resemble franckeite. The Snbearing inclusions contain high concentrationsofAg and
Sb and little Sn relative to franckeite and may represent
a mixture of minerals.
The Ag and Sb concentrationswere quite variable, reflecting the distribution of dissolved Ag and Sb, as well
as inclusions too small to be resolved with BEI, and averagevalues are presentedin Table l. Concentrationsof
Ag rangedfrom 0.11 to 0.69 wto/o(mean : 0.22 ! 0.12
wto/o)for the La Paz sample and from 0.04 to 0.36 wto/o
(mean : 0. 14 + 0.08 wt0/0)for the Zacalecassample.The
Sb concentrationsrange from 0. l8 to 0.84 wto/o(mean :
0.28 + 0.14 wto/o)for the LaPaz sampleand from 0.08
to 0.45 wto/o(mean : 0. 19 + 0.09 wto/o)for the Zacatecas
sample.
The mechanismsof Ag and Sb substitution aro examined by plotting the concentrationsin terms of substitu-
tion vectorsAgtr orPb-., and Sbtro'Pb-'' (Fig. 2). The
chemographic and substitution relations among galena,
stibnite, acanthite, miargyrite, diaphorite, and freieslebenite are illustrated (Fig.2a), with the right triangle defined by the components AgrS, PbS, and SbourS,(normalized to one S atom). In this diagram, application of
the Agtr orPb-., vector two times transforms PbS into
AgrS (acanthite)and application of the Sblo.'Pb , , vector 0.67 times transformsPbS into Sbou,S(stibnite).The
galena corner of this triangle is enlarged in Figures 2b
arrd 2c to illustrate the correlation between Ag and Sb
substitutions in the galenasolid solution. The Ag and Sb
substitution (y and x, respectively) are calculated from
the atom percentsusing the generalformula Pb'-'r"-orrsystem.Becausethe catAg"Sb"Sfor the Ag'S-PbS-SbourS
ion-anion ratios changein this system, except along the
coupled-substitution vector, the substitution is not linearly related to the atom percent.However, at the low Ag
and Sb contents of these samples,the substitution is approximately two times the Ag and Sb atom percent. As
89
SHARP AND BUSECK: Ag AND Sb IN GALENA
TABLE
2. Dimensionalrelationsamong galena,diaphorite,freieslebenite,
and franckeite
Unit cell (nm)
% misfit relativeto galena
Subcell-
Space Gp.
G a l e n a( 1 )
Diaph.(2)
Freies.(3)
Frnk.T (4)
Frnk.H (4)
F4lm32lm
Q'la
P'ln
0.594
1.584
0 753
1.72
't.72
0.594
3.208
1.279
0.579
0.365
0.594
0.590
0.588
0.581
0.63
9e
90.1
0'
9214',
a\f212
al4'
al2.
a\f212
blg'
bl3
1 04
-4.5-.
1.4*
-2.51
-38.61
-0.51
-1.01
-2.01
6.7t
/VotejCrystallographic
data from (1) Wycoff,1963, (2) Helner,1957a, (3) Helner,1957b,and (4) Williamsand Hyde,1988.
. Subcellshown in Figure3.
-. Misfit relativeto galena
410.
t Misfit relativeto galena dloo.
can be seenin Figure 2, Ag vs. Sb substitutions for both
samplesplot along but slightly below the AgSbPb_, vector, indicating coupled substitution with a slight excess
of Sb relative to Ag, presumably accompaniedby vacancies on Pb sites.
The results ofthe rastered-beamanalyses(galenaplus
inclusions, Table l) indicate that the total Ag and Sb
concentrations in the l-a Paz sample are approximately
0.62 (+9.66; and 0.81 (+0.06) wt0/0,respectively,whereas the total Ag and Sb concentrations in the Zacatecas
sampleare approximately0.19 (+0.07) and0.27 (+0.06)
wto/0,respectively.The distribution of Ag and Sb between
galenaand inclusions can be estimatedby subtractingthe
AgrSborS contents of the galenafrom those determined
by rastered-beamanalyses.In the l-aPaz sample, the total AgorSborScontent is 1.37 mol0/0,with 0.48 molo/oin
the solid solution and 0.89 molo/oas inclusions, principally diaphorite. The total Ago,SborScontent of theZacatecassample is 0.44 mol0/0,with 0.32 molo/oin solid
solution, leaving only 0.12 molo/oAg"rSborSto account
for diaphorite inclusions.
Franckeite [(PbSn)uSnrSbrFeS,o]
is a layered mineral
consisting of alternating (along a) pseudotetragonal(T)
and pseudohexagonal(H) sheetsthat have an incommensuraterelation (Makovicky, 1974, 1976; Moh, I 984, I 987;
Williams and Hyde, 1988; Wang and Kuo, 1991).The
T-sheet structure is either PbS-like or slightly sheared,
making it SnSJike, with Sb3* and Ag* substituting for
Pb2*or Sn2t;its thicknessis approximately 1.2 nm along
a. The cell parametersthat are parallel to the sheet,D' :
0.579 and cr : 0.581 nm (Williams and Hyde, 1988),are
Galena
[100loph[100lFrl
-r-*
[1lolcn
Srnucrun-q.L RELATIoNSBETwEENGALENAAND
INCLUSIONS
The structuresof galena,diaphorite, and freieslebenite
are closely related, allowing coherentintergrowth and interface relations such as those observed in HRTEM images.Unit-cell and subcellrelations among theseminerals
are summarized in Table 2 and illustrated in Figure 3.
Galena has the NaCl structure, with octahedrally coordinated Pb and S defining a face-centeredcubic lattice.
The structuresofdiaphorite and freieslebeniteare derivatives of galena,with the same octahedral arrangement
of cations and S anions,but with Ag* and Sb3*substituted
for Pb2*.Both diaphorite and freieslebenitehave galenalike subcells(Hellner, 1957a, 1957b),as illustrated in Figure 3. The presenceof Sb3*(radius 0.89 nm; Shannon,
1976), which is considerably smaller than Pb2* (radius
1.33 nm), results in the diaphorite subcell parameters(a"
: a/4, b": b/8, and c. : c) somewhatsmaller than a"t/2/
2, and a" of galena.The freieslebenitesubcell (a": a/2,
b": b/3, and c" : c) is also smaller than the primitive
galena subcell, but with most of the misfit along the
freieslebenitea axis.
Freieslebenite
a = 0.75nm
b = 1.28nm
Diaphorite
a = 1.58nm
b = 3.21nm
lilolcnI totoloon
Y lololFri
Fig. 3. Schematicillustration of the unit-cell relations among
galena,diaphorite, and freieslebeniteas viewed along ther c axes.
The box illustrating the diaphorite unit cell is truncated to save
space.The cation (small circles)and anion (largecircles)arrangement and the subcell (shadedbox) common to all three structures are illustrated. The galena face diagonal, along [110], is
significantly longer than the freieslebenitea parameter,illustrating the large misfit betweentheseminerals along a.
90
SHARP AND BUSECK: Ag AND Sb IN GALENA
HRTEM
OBSERVATIONS
Diaphorite inclusions
Diaphorite inclusions are abundant in both theL,aPaz
arrd Zacatecassamples. Most are rod shaped, with the
rod axis parallel to the diaphorite c axis; many are nearly
round, but some have partial {120} faces (Fig. 4a) or
irregular shapes.Diaphorite rods in the La Paz sample
are commonly 300-600 nm in diameter, and those in the
Zacatecassample 100-200 nm, but inclusions as large as
several micrometers in diameter and as small as 20 x
200 nm have been observed.The smallestof theseinclusions are not visible by BEI and would therefore be incorporated into analysesof the solid solution. The rare
occurrenceofthese inclusions on the scaleof20-200 nm
suggeststhat galenaanalysesreflect primarily Ag and Sb
in true solid solution.
The orientation relation suggestedby the textures in
BEI micrographs is confirmed by TEM observations.
Diaphorite rods (c axis : rod axis) are parallel to the three
( I 00) directions in galena.This relation is a result of the
almost identical values of the c cell parameter of diaphorite (0.590nm) and a cell parameterof galena(0.594
nm). SAED patterns of the diaphorite [001] and the galena [001] zone axes (Fig. ab) show that the a* and b*
directionsofdiaphorite are parallel to the (ll0)* directions ofgalena. The substructureofdiaphorite is indicated in the [001] SAED pattern (Fig. ab) by high-intensity
reflectionsthat occur very near the galenareflections.The
NaCl arrangement of cations and anions in diaphorite
results in a substructurediffraction pattern that is essentially the same as that of galena.The diaphorite 400 and
080 difraction spots are examples of subcell reflections
that are located adjacent to the correspondinggalenareFig. 4. (a) TEM imageof a diaphoriterod viewedalongthe flections, indicative of a diaphorite subcell that is someSAEDpattern.The traceof
rod axisand (b) the corresponding
what smaller than that of galena.
interfaceand dislocations
extendingfrom
the diaphorite-galena
theinterfaceappearasdarklinesin theimage.Theinterfacehas
flattenedsidesthat are subparallelto the {120} planesof dia- Diaphorite-galenainterfaces
phoriteand { 100}planesofgalena.The SAEDpatternalongthe
The interfacesbetweendiaphorite and galenaare semidiaphoriteandgalena[001]zoneaxesillustratesthat the a* and coherent, with periodic steps (Fig. 5a) and misfit dislob* directionsofdiaphoriteareparallelto the (110)directionsof cations (Fig. 5b). Becauseof the smaller diaphorite subgalena.The intensesubstructure
reflectionsof diaphorite400 cell, a semicoherentrelation requires extra atomic planes
and 080 arenearlycoincidentwith the D0 and220 reflections
on the diaphorite side of the boundary. The terminations
of galena.
ofthese layersat the boundary resembledislocationsand
can be describedin terms of Burgersvectors and circuits.
Burgers circuits drawn around misfit dislocations (Fig.
parameter
galena
4sand 5b) indicate extra (240) and (240) atomic planesin diaphcell
only slightly smaller than the
galena
is a orite analogousto the {200} planes of galena. The corThe
H
sheet
fit
well
into
the
structure.
can
derivative ofthe berndite (SnSr)structure, with Fe3"sub- respondingBurgersvectors are 0.28 nm along diaphorite
stituted for Sno*;it is approximately 0.5 nm thick, with
I l0] or [1T0], where only one extra atomic plane occurs,
:
:
parallel
and
0.40 nm along [00] or [010], wheretwo orthogonal
cell parametersb, 0.365 nm and cH 0.63 nm
planes occur together. In Figure 5b, the Burgers
extra
misfit
(Williams
1988).
The
large
Hyde,
and
the
sheet
to
between the H and T sheetsresults in the incommensu- circuit on the left side of the figure revealsonly one extra
atomic plane, whereas the circuits in the center and on
rate structure. An alternating sequenceof the T and H
the right side show two extra planes.A singleextra atomsheetsproduces a layered structure 1.72 nm thick, with
plane (0.28-nm Burgersvector) would result in anions
ic
with
variable
and
a
modulation
along
c*
sinusoidal
a
wavelengthof 4.2 nm (Williams and Hyde, 1988)to 4.7 on cation sites(Fig. 6) and a chargedstackingfault, which
would be highly unlikely in a galenalike structure. The
nm (Wang and Kuo, 1991).
SHARP AND BUSECK: AS AND Sb IN GALENA
91
Fig. 5. HRTEM images of a diaphorite (Dph) inclusion in galena (Gn), viewed along [001], illustrating the structure and
morphology ofthe diaphorite-galenainterface. The orientation relationship (a) is illustrated by the a and b lattice vectors ofboth
minerals. The interface is curved with facets(at arrows) parallel to the {100} planes of galena.A higher magnification image (b) of
this interface shows misfit dislocations illustrated by the Burgers circuits (boxes).The gaps at the bottoms ofthe Burgers circuits
representprojected Burgersvectors of |za[100] (0.28 nm) and,(y2/2)all101 (0.40 nm) relative to the galenastructure.
92
SHARP AND BUSECK: Ae AND Sb IN GALENA
Subcell
Fig.6. Schematic
illustrationofthe apparentBurgersvectors
infor the misfit dislocations
observedat the galena-diaphorite
terfaceofFig. 5.Thesmalldarkcirclescorrespond
to thecations,
to the anions,andthe box
the lightly shadedcirclescorrespond
indicatesthe subcell.The v, Burgersvectors,alongthe galena
( 100)directions,areunlikelybecause
theyconnectanionto cation sites.The v, vector,alongthe galena[ 10]direction,is 0.40
nm longand connectsequivalention sites.
apparent0.28-nm dislocation probably has an equivalent
0.28-nm screwcomponent along c that is invisible in this
projection. If this model is correct, all the misfit dislocations are equivalent to the 0.40-nm type, but with some
accommodating misfit along c as well as along a or b.
The spacingsof the misfit dislocations are consistent
with the values expectedfrom the calculated misfit between diaphorite and galena. Dislocations with Burgers
vectorsof0.40 nm along a on a diaphorite (010) boundary would accommodate the 5.7o/omisfit if spacedevery
7.0 nm along a. The distance between the two dislocations with 0.40-nm Burgers vectors in Figure 5b is 7.8
nm. The separation expectedbetween a 0.28-nm dislocation and a 0.40-nm dislocationis 5.3 nm, as compared
with the 5.6-nm value observedin Figure 5b. On a larger
scale(Fig. 5a), the diaphorite (010) boundary consistsof
small {ll0} facetsthat are associatedwith misfit dislocations.The spacingof thesefacetsis 6.3-8.3 nm, which
is comparable to the distance between dislocations with
0.40-nm Burgersvectors.
indicates nearly identical atomic spacings in the two
structures. Streaking along a* in the T-sheet rows indicatesa stackingdisorder ofthe sheets.
The closely spaced reflections along the modulation
vector q (crossingthe rows of T-sheet reflections) correspond to a 3.55-nm modulation of the T sheets.This
3.55-nmwavelengthis considerablysmallerthan the 4.24.7 values reported for franckeite (Williams and Hyde,
1988;Wang and Kuo, l99l) and may be a result of high
Ag and Sb contents.Although Ag*, Sn2*and Pb'* all have
comparableionic radii in sixfold coordination (=0.130
nm), the radiusof Sb3*is only 0.089nm (Shannon,1976).
Excesssubstitution of Sb3*(and thus Ag*) for Pb2* and
Sn'?*in the pseudotetragonalsheet of franckeite would
increasethe misfit between T and H sheets,requiring a
greatermodulation of the structure.
The layers offranckeite are further resolved into their
T and H sheetsin Figure 8. The images show ofsets of
the lattice fringes acrossboth sheets,indicating sheared
structures. The problem of the PbS-like vs. SnS-like
structure of the T sheetwas investigatedby Williams and
Hyde (1988),who presentedimage calculationsthat can
be used to distinguish between the PbS- and SnSlike
structures.Their image simulations indicate fringes normal to the T layers for the PbS-typestructure and fringes
oblique to the T layers for the SnS-type structure. The
image presentedhere (Fig. 8a) resolvesthe lattice fringes
in the T sheet,showing that they are oblique to the sheets,
similar to the image calculationsof the SnSJike structure
presentedby Williams and Hyde (1988).
Franckeite-galenainterfaces
The franckeite inclusions are coherently intergrown with
galena.The most extensive of the boundaries is parallel
to the franckeite (100) layersand the galena(100) planes.
Here the (020) lattice fringesof galenaare continuous into
the franckeite, with minor offsets but with no apparent
misfit dislocations (Fig. 8). This observation implies that
a small amount of misfit at the interface is accommodated by homogeneousstrain. An interesting form of interfacial strain is evident in franckeite inclusions that are
imaged along [010] (Fig. 8a), where the amplitude of the
modulations decreasestoward the (100) boundary. Such
a flattening of the modulation apparently resultsin a betFranckeite inclusions
ter fit betweenthe H sheetoffranckeite and the galenaat
In addition to diaphorite, franckeite has been observed the interface. A step is present along the boundary in
in the La Paz sample(Fig. 7). Theseinclusions have disk- Figure 8a, where a 1.7-nm layer of franckeite terminates
like morphologiesparallel to their (100) layers.The disks into the galena.The step is evident as the termination of
are 60-150 nm thick and occur parallel to the galena an H sheet,but with a smooth transition from the galena
{100} planes.In the HRTEM imagesof franckeite inclu- to the T sheet below the terminated H sheet.The pressionsviewedalong[0 I 0] (Figs.7a, 8a),the I .72-nm layers enceof such stepsat these interfacessuggestsa layer-byand their sinusoidal modulations are clearly visible. The layer growth mechanism for the franckeite inclusions.
SAED pattern for this orientation (Fig. 7b) suggestsa
topotaxial relation betweengalenaand franckeite, where Defects in galena
Defectsthat resembleGuinier-Preston(G-P) zones(Fig.
b, a*, and c* offranckeite are nearly parallel to the ( 100)
directions of galena and the T-sheet structure nearly 5a) are abundant in ion-milled galena,but are absent in
matchesthat of galena.The fact that the T-sheet diffrac- all of the crushed samples,suggestingthat they are artition rows along a* coincide with the galena reflections facts ofion thinning.
SHARP AND BUSECK: Ae AND Sb IN GALENA
Fig. 7. (a) TEM image of a franckeite(Fk) inclusion in galena
(Gn) and (b) the correspondingSAED pattern. The image shows
the end of a plate-shapedinclusion of franckeite viewed along
the [010] zone axis. Fringes that are visible in this inclusion
correspondto the 1.7-nmlayersalonga and their 3.55-nmmodulations. The SAED pattern is a superpositionofthe franckeite
[010] and galena [001] pattems. The a,* and c'* directions and
the modulation vector qr of franckeite are nearly parallel to the
(100) directions ofgalena. The streaksalong franckeite ar* indicate layer-stackingdisorder.
DrscussroN
Ag and Sb are distributed between galena solid-solution and microscopic inclusions, most of which are diaphorite, in proportions dictated by bulk concentrations
and the extent of exsolution. Microscopic inclusions of
diaphorite may be common in Ag-Sb-bearinggalenabut
overlooked becauseof their small size and optical similarities to galena.Most diaphorite inclusions that we have
observedare rods less than I pm in diameter, but larger
diaphorite inclusions have been reported by Laflamme
and Cabri (1986a, 1986b) and Czamanskeand Hall
(1976). Although the optical properties ofdiaphorite are
similar to those of galena(Ramdohr, 1980),they are easily observedusing BEI with high magnification and con-
93
Fig. 8. HRTEM imagesof interfacesbetweengalena(Gn)
andfranckeite(Fk)viewedalongthe franckeite(a) [010] and(b)
(T) and
[001] zoneaxes.In both imagesthe pseudotetragonal
pseudohexagonal
(H) sheetsof franckeiteare resolved;the H
sheetsappearsmearedand perhapsbeamdamagedand the T
Thegalena-franckbut slightlysheared.
sheets
appeargalena-like
eiteinterfaces
arecoherentwith no apparentmisfit dislocations,
althoughthereis somedistortionofthe galena(020)fringesat
(a) in the modulationamplitudeof
the interfaces
anda decrease
the franckeitelayers.A stepalongthe interface(a) is evidentas
a termination(t) of an H sheet.
trast. We have seen no evidence of inclusions or atom
clusters smaller than 20 nm in the HRTEM results, but
STM experiments on cleavagesurfaceshave shown distorted structure that may be a result of AgSb clusters
(Sharpet al., 1990).
The rodlike morphology, orthogonal orientation, and
homogeneousdistribution of the diaphorite inclusionsare
strong evidencefor exsolution. Although franckeite exsolution in galena has not been reported previously, diskshapedinclusions with topotaxial relations to the galena
suggestthat they are also products of exsolution. Both
franckeite and diaphorite inclusions have topotaxial relations to galena,with coherentor semicoherentinterfac-
94
SHARP AND BUSECK: Ae AND Sb IN GALENA
es. Thesecrystallographicrelations result from the structural similarities between galena and the galena-derivative
structuresofdiaphorite and the T layer in franckeite.The
interfaces and orientations of exsolution features are a
function of the misfit between the two phasesand their
elastic properties (Yund and Tullis, 1983, and references
therein). Without knowledge of the elastic constants for
diaphorite and franckeite, we can only consider the relations in terms of misfit.
Becausethe diaphorite substructure is only slightly
smaller than the galenastructure,the misfit at diaphoritegalenainterfacesis relatively small and is accommodated
by misfit dislocations.The interfacial strain is minimized
by the rod morphology of the inclusions along c, the axis
with the leastmisfit. The relativelyAgo,Sbo,S-richgalena
from La Paz (l.37 molo/o)probably experienceddiaphorite exsolution at a higher temperaturethan the Z,acalecas
sample,which containsonly 0.44 molo/oAgorSborS. As a
result of higher Ag and Sb concentrations and temperatures, the La Paz sample contains coarser and more irregularly shaped diaphorite inclusions. Comparing rod
lengths and widths in backscattered-electronimages, an
average aspect ratio for diaphorite rods in the La Paz
galenais 16:l, whereasthat determined for the Zacalecas
galena is 47:1. The higher aspectratios of Zacatecasdiaphorite reflect the increased importance of interfacial
strain energy during exsolution in the samples less rich
in Ag and Sb.
The exsolution of diaphorite in both samplessuggests
that a solvus exists betweengalenaand diaphorite in the
PbS-AgSbS, system. However, experimental investigations of phaseequilibria in the PbS-AgSbSr-SbrS.
system
above 300'C (Hoda and Chang, 1975;Amcofi, 1976)
indicate a solvus between galena and freieslebenite
(AgPbSbS,).One explanation is that equilibrium was not
attained in the experimental studiesand that freieslebenite exsolvedmetastably.This seemsunlikely becauseboth
studies found immiscibility between galena and freieslebenite in PbS-richsamples,and Hoda and Chang(1975)
found a second miscibility gap between diaphorite and
freieslebenite.A secondexplanation for the occurrenceof
diaphorite in galenais that it exsolvesmetastablybecause
of a coherentsolvus at lower temperaturesthan the freieslebenite-galenasolvus. This explanation is possibleifdiaphorite meshes with the galena structure more easily
than freieslebenite.A comparison of the misfit values for
diaphorite and freieslebenite(Table 2) indicates that the
strain is more evenly distributed along a and b for diaphorite-galenaintergrowths. Although the misfit differences
appear small, they are apparently large enough to reduce
the nucleation energy for diaphorite relative to freieslebenite in some Ag-Sb-bearinggalena.
AcxNowr-rocMENTS
we thank Donald Burt and Julio Pinto for thel,aPaz sample,Michael
Sheridanfor the Zacatecassample,and Louis Cabri for the synthetic Agbearing samples.We also thank Nancy Brown, Gerry Czamanske,and
Tamsin McCormick for helpful reviews of the manuscript, as well as Su
Wang for discussionsconcerningfranckeitestructureand JamesClark for
assistancewith the electron microprobe This work was funded by NSF
grant EAR-8708 529. The electron microprobe facility is funded by NSF
grant EAR-8408529and the HRTEM at Arizona StateUniversity is funded by the NSF and ASU
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