RENDlOONTl Socleta ltalitlna Ilf Mfne ralogfa
<'l
Petrologfa, 35 (1), 1979: PJI. 71-9O
GrAN PrERO BERNARDINI ·, GIUSEPPE MAZZETIl·
PHASE RELATIONS IN THE Cu-Fe-Se SYSTEM
AT 900" AND 7W' C"
RIASSUNTO. - Per la definirione delle rehuioni fm le fasi esistenti nd sistema Cu·Fe-Se
a diverse temperature sono state studiate per il momento le isoterme a 900° e 700" C mediante
esperienze di quenching e analisi diffrattometriche, ottiche e tennodifferenziali.
A 900" C un largo campo a un liquido ternario {L) domina la regione centrale del sistema.
At di sopra del suo limite ricco in Se esiste un canlJ?O a due liquidi non miscibili fra L e Se
liquido. A questa temperatura esistono solo due fasi solide, olm: a Cu e Fe metallid:
C\b..~Se, oon 0,00 .;;; X <: 0,10, oorrispondente al minerale berzelianite {bz •.•. ) e Fe._xSe, oon
0,00 <: X <: 0,25, oorrispondente 0.1 minerale achavalite (ach •.•. ). Fra questi due composti e il
liquido temario L si stabilisoono delle tie·lines che individuano due campi a due fasi:
bz•.•. + L e ach •.•. + L. Nella regione del tematio al di sono del 33 % e del 50 % at. di
Se, rispettivamente sui binati Cu-Se e Fe-Se, sono stati individuati i seguenti campi: due
campi estremi a due fasi: Cu •.•. + bz•.•. e Fe•.•. + ach e tre campi a tre fasi : Cu •. •. + bz•.•. +
+ Fe•. •. , bz•.•. + L + Fe •.•. e Fe •.•. + L + ach.
A 700· C illiquido ternario si ritira verso il bordo Cu-Se e verso il venice Se del sistema.
Ndla regione ricca in Se si ha, petlanto, la oomparsa di un largo campo a tre fasi: SeL + L +
+ ach •.•. e di uno picoolo a due: SeL + ach•.•. vicino 0.1 bordo Fe-Se. A questa temperatura
la punta piu povera in Se del campo a un liquido ternario raggiunge quasi il 50 % at. di Se
e, pertanto, nd!a zona centrale del sistema, oompaiono un campo univariante: bz•.•. + L + ach •.•.
e uno bivariante: bz•.•. + aeh..... lndagini a raggi X di alto. temperatura hanno pennesso di
stabilire che, anche a 700" C, non esistono fasi temarie nella regione a composizione corri_
spondente a quella ddl'eskebomite (CuFeSe.) oontrariamente a quanto avviene nel sistema
Cu-Fe-S. La oomparsa del campo bivariante: bz •.•. + ach •.•. , che proibisce la coesistenza del liquido L con Fe•.•. , provoca la $COmparsa, ndla regione ricca in metalli, del campo a tre fasi:
Fe •.•. + L + ach, sostituito, vicino al bordo Fe-Se, da quello a due fasi: Fe•.•. + ach •.•..
ABSTRACT. - As a part of a complete investigation of phase relations and properties of
oompounds in the Cu-Fe-Se system, the phase relations at 9(X)o and 700" C have been investigated
by silica tube quenching experiments using microscopic, differential thermal and X-ray analyses.
At 900" C a homogeneous liquid field (L) dominates the central region of the system.
Above it$ Se·rich limit, which varies from 60 % at. on the Cu-Se border to almost 75 % at. on
the Fe-Se, an extensive liquid immiscibility field spans the Se-rich portion of the system. At this
temperature, apatt from the solid phases on the metal system, nOI discussed here, only twO
solid phases exist: CUr_.Se, with 0.00 ...: X <: 0.10, berzelianite solid solution (bz •.• .)
and Fe,_.Se, with 0.00 ...: X ...: 025, achavalite solid solution (ach•.•. ). Between these
{WO solid phases and the ternary liquid two two.phase fields are established: bz •. •. + Land
ach •.•. + L. In the region of the system below 33 % and 50 % at. Se, respectively on the Cu·Se
and Fe-Se systems, the following fields were detected: Cu, with a maximum Fe<ontent of 2 % at.
(Cu •. •. ) plus bz •.•. and Fe, with a maximum Cu-content of nearly 4 % at. (Fe •.•. ) plus ach, and
three three-phase fields: Cu •.•. + bz •.•. + Fe•.•. , bz •.•. + L + Fe•.•. and Fe•.•. + L + aeh.
* Istituto di Mineralogia, Petrografia e Geochimica ddl'Universita, Via Lamarmora 4,
50121 Firenze.
** Lavoro eseguito con il oontributo finanziario del CN.R. - Centro di studio
per la Mineralogia e Geochimica dei Sedimenti.
72
C. P. BE RNARDINI. G . M ... ZZETT I
At 700· C the large L-liquid field ~lfeats towards the eu-Se border together with the
liquid immiscibility field, with the consequent appearan~ of 9. large th ree.phase field:
Set. + Beh •.•. + L and a very small two-phase field, near the Fe-Se border: Sec + aeh •.•..
At this temperature the Se-poor point of the L-liquid field reaches a Se-content of almost .50 % at.
Therefore, in the central region of the system, an univariant field: hz •.•. + L + 3eh •.•. and a
divariam one: bz•.•. + aeh •.•. appear. This last field prohibits the L-Iiquid coexisting with
Fe •.•.. High.temperature X-ray anaiys("S of charges with composition corresponding 10 that of
eskebornitc (CuFeSe.), have permitted to establish that even at 700~ C no ternaty phases are
present in the eu-fe-Se system in C;)ntraSI with the situation in the Cu-Fe-S system. The appearance of the divariant field: bz •.•. + ach •.•. causes the disappearance, in the metal rich region,
of the three-phase field: Fe•.•. + L + aeh which is substimted, near the Fe·Se border, by the
two-solid.phase field: Fe •.•. + ach •.• .
In troduction
During the last two decades systematic investigations of phase relations in
sulfide systems have yelded significant information on the thermodynamic stability
of sulfide minerals and on the conditions of their formation. Most of the experimental
work is concerned with sulfides, arsenides and sulfo-salts while little attention h:ls
been paid to selenides and sulfo-selenides despite their importance especially for
industrial application. Studies on phase relations in various selenide systems have
been mainly conducted by solid-state physicists because of the semiconduCling
properties of Se<ompounds.
In the overall interest for trace elements in ore deposits as possible indicators
of geochemical conditions of mi neral formatio n, special consideration has been given
in recent literature to seleni um ,especially for part icula r geological settings such as
roll-type sandstone uranium deposits, as poi nted out by GF.NT (1976) in his annotated
bibliography of the geology of selenium and by H OWARD (1977) in a recent work
on the geochemistry of selenium ro which the reader is referred for reference o{
detai led geochemical studies.
T he Cu-Fe-Se system was selected fo r study because knowledge of its phase
relations is a neeessary prerequisite to the study nf the phase relations ;n the
quaternary Cu-F e-S-Se system which could shed some light on the role played by Se
in stabi lizi ng copper-iron suI fides and on the significance of the Se/S ratio as a
useful indicator of the physico-chemical environments of deposition from sulfide
ore solutions. While the phase relations in the Cu-Fe-$ system have been studied
for a long time (M£RWIN & Lo:-'IBARD, 1937) and are to date well known, especially
at high and moderate temperatures (efr. CRAIG, 1974; SUGAKl et al., 1975 and the
bibliography therein), those in the Cu-Fc-$e system have not been investigated
sistematieally and only FRANZ (1970, 1971 a and 1975) has conducted investigations
on some joins of the Cu-Fe-Se, Cu-Fe-S-Se and Cu-F e-Sn-S-Se systems .
The final object of our study is to determine the stable assemblages in the
entire Cu-F c-Se system over a wide range of temperature. The necessity of collecting
as many data as possible in order to portray the major features of the phase diagram
at whatever temperature, up to now un known, suggested studying the 900" and
PHASE RU.... T IONS IN TilE C.- F. -S. SYSTE M AT
9000
AND
700 0 C
73
700" C isotherms first as equilibrium CQuld be :mained, at these temperatures, over
a relatively short period of time.
Phases a nd p h a!le relat ion s
The natural and synthetic phases present in the two binary systems Cu-Se and
Fe-Se , and in the ternary Cu-Fe-Se are reported in fig. 1, while their crystallographic
characteristics and thermal stabilities are summarized in table L No attempt has
been made to include all published studies relating to these phases but rather to
provide a brief summary of the most significant to the present work.
The phase relations in the Cu-Se system have been studied by HEWING (1966),
DIES (1%7), BERNARD!Nl & WTAN I (1968), BERNARDIN I et al. (1972), MURRAY
& HEWING (1975) and recently reviewed by BERNAROINI et al. (1976, 1977) in their
..
..So,
• ,.,.11.
".<''' ••• ,••
c.s.
•• "._ .... ". C.'S"
..... _11. C•• h,
C.',S' ,
• •
•.•• b.'."
C.,s. < • • "." •• ".
""'do",
,.
c.
Fig. I. -
Min~rals
and synthetic phases in the Cu·Fe·Se system.
studies on the phase relations in the Cu-S-Se system. Reviews of the phase relations
in the metallic system Cu-Fe may be found in HANSEN & ANDERKO'S, c Constitution
of binary alloys J. (1958) and in &L1arr's (1965) and SHUNX'S (1969) supplements.
No reference to more recent literature will be given here because the phase relations
in this system are beyond the scope of this work. The phase relations in the Fe-Se
system are not clearly defined, especially at low temperatures, altough. since the
first work of H AGG & K INDSTROl'-I (1933), several authors (TEGN"F..R, 1938; HARALOSEN
& GWNVOLD, 1944; HARALDsEN, 1952; H ERONE & CHIIIA, 1956; TROFTEN & KULLERUD.
1961; K ULLERUD, 1968; D UTRI7.AC et aI., 1968 a and b) have studied this system.
Of the above mentio ned works the first complete study on the Fe-Se system is that
of K ULLERUD (1968) who portrayed the general features of the phase diagram above
400" C using quenching and D.T.A. experiments. In the same year DUTRIZAC et al.
(1968 b) worked out indipendently the phase relations in this system. The main
differences are in the high-temperature Fe-rich region and in the region around
the Fet .xSe composilion. No mention, in fact, is made by D UTR lZAc et al. (1968 b)
74
C. P . BERNARDlNI, G. MAZZETTI
T ABLE 1
Natural and synthetic phases in the Cu-Se, Fe-Se and Ctt-Fe-Se systems
Kt ...... l
or
COIIIpoo ltlon
CrY OU.lloqraphy, thenoal nal>111ty o nd
C"25<o
orth .
Cu.Se
UH.
C"2_x Se
~,.
Cu Se
2
l
utr. -
Cuss."
orth .
Cus.
1""-hex ._ hlqh_hu ._ Cu2_~S .
refeunc..
oynthe Uc phue
8eruUIUllu
"'hab ..
".! ••
Uocuonnit ..
~cul>. ~l1q . ( 1, 2,),12J
'"
11.2. ' ,5)
,,,·C
C"2·,,5. • CuS ..
60 ' C
syn thut"
11 . 2 . 20.22)
IH)
l8. · c
orth. <H2 ' C
cUI),
(1,2 . 1~)
+ Se llq .
(1,2 ,11,1 6,26)
(15)
'Ye -se" .yn...
l'eS"
•
tHr. (PbO-type)
~
tetc .
~
hex. (HI ... ·typel • Fe (7 , 9.12 . 1J . 27)
he x. (Nl,o..- oype) !l5-812"C: ...Fe,_ " Se
~l1q .
19 . 13 , 17,271
HO_298'C
320-l8S ' C
ut"l. ,C _ lC
hex . (N1II. -type) le _ 2e _ le,
s yntheU c
_
hex. (N1"._type)
12J,H . 25)
ns'c
~ U - 'Yptl
.yntheU"
..,nOC1. ~
tun.tHon) h"x.IN 1As-type)
727"C
hu. _ F" ,_ xS. •
l1q.
(17.21.25.27)
hno . . Ut.
·Cu-Fe - S." . yn ....
,eo'!;
cub . _
Elkebornl te
-
(Fe . CuISe
(cu . r . )S"2_x
Cu _ Se '
.eU .
2 x
~
CUlS"2-s1. llar Hq.
CuF" S"x<2- .i.. llar Hq .
~
(1 ,1 0 ,29 )
17)
• According with references (12) a nd (2 7) [hi. phase has surplu. iron atom . in interstitial position s.
( I) BERN .... OINI and CAT .. NI (1968): (2) BUN"RDlNI, CoRSINl and T~OSTl (1972); (3) BO ETTCtlER,
HHSE and TaElJPEt. (1955): (4) BoaCH1'.l\T (1945); (5) BnRCIlERT and PATMK (1955); (6) DE MOl<·
TIIEUIL (1970); (7) F UNz (1970); (8) FUNz (1971 b); (9) FRANZ (1972); (10) Fu"" (1975); ( 11) GATTOW ( 1965): (12) GRJJNVOLO (1968); (13) HAc..:; anu K 'N OSTROM (1933); (H) H .... RIS. CASRL and
KA1).... N ( 1969); ( 15) JOtlAN, PICOT, P[UROT a nd K v.. cEK (1972); (16) K] EKSHlJS, R.. xXE and AI<DIIESEN (1974); (17) K ULLulJD ( 1968); (18 ) K ULL Ek UD and DosN"" ( 1958); (19 ) LIPPh"'I<N (1962);
( 20) MOIlIMOTO and KOTO ( 1966); (21) MOIllMOTO a nd K UI.LERlJn (1959); (22) MOlllMO"ro and UCtllM1ZU (1967): (23) Ox."ZAKl ( 1959); (2 ~) O K..UK[ (196 1): (25) OK .. U XI and H ,Il AK.. WA (1956):
(26) S.. o"'.... o .. ( 1966); (27) SVE,,-oHS ( 1972): (28)
TroSH
( 1938); (29)
T1SClIENDOlIF
( 1960).
PHASE REI..ATlONS IN Til E C' ·f.·S. SYSTEM AT
900"
AND
7000 C
75
of the stability field of F eaSe4 which, according to K llLLE.RUD (1968), melts
incongruently at Wo C into Fel.:rSe plus liquid. On the other hand the fo rmer
authors have investigated in detail the high.temperature region only inferred by
the latter. SVEN"DSEt-I (1972), in his investigation on the decomposition pressures
and standard enthalphy of formation for iron selenides, reported the region of the
Fe-Se system ranging from 30 % to 100 % at. Se, above 400° C. According to
this author the FeaSC4 phase exists only as a monoclinic deformation, derived from
the increased vacancies in the Fe.strata, of the NiAs·like structure with approximate
composition Fe1Ses. Therefore below 725° C, which is the temperature of the
A.-type t ransition undergone by FeaSe.. to change into the hexagonal high-temperature structure, the two-phase field: Fel ....Se + FeaSe4 inferred by K ULLERUD (1968),
should not exist.
As concerns the phase relations in the Cu-Fe-Se system, FRANZ (1970) investigated
some «combinations :. at 400" C by X-ray and differential thermal analyses ;.llld found
two new phases: 1) (Cu,Fe)Se2.x, cubic pyrite-type structure, with a" ranging from
6.061 A for the (Cuo.82Feo.18)Se~." composition to 6.000 A for the (Cuo.~,~Feo.48)Se2.s.
T his new phase should melt incongruently at 530° C into CU2.xSe plus a CuFeSe~­
-similar liquid; 2) (F e,Cu)Se, tetragonal space group P4/nmm, with ao = 3.79 A
and Co = 5.50 A. With increasing Cu..content the Ct> of this phase decreases down
to 5.30 A. The same author in 1975 gave the powder diagram of the (Cu,Fe)Se2.%
phase for the (Cuo.HFeo.26)Se2.x composition and established the compositional stability field of eskebornite at 400" C, which seems to have an elliptical fo rm ranging
from CU1.o4Fetul6Se2 to Cuo. 8s F el.12Se2 and from CuFeSe1.ll to CuFeSe2.2.
Me thods of experimental inve stigation
Experimental charges were synthesized in evacuated silica·glass capsules, as
described by KUU-ER UD (1971), either using the single tube technique or the double
tube for some charge on and near the Fe-Sc oorder. Du ring quenching, in fact,
transitions in solid selenides caused frequent craking of the sample tubes. Reactants
were pure elements as specified by supplier's analyses (AIEa Inorganics Ltd., Ventron,
Beverly, Mass.). Iron powder was reduced in a stream of hydrogen at 900~ C at
least for fou r hours. Charges were heated in Nichrome wound furnaces, whose
temperature was controlled within ± 3° C or better (as measured on calibrated
chromel-alumel thermocouples), quenched in iced water, repeatedly ground under
acetone and reheated for periods of several weeks to a maximum of four months
for the 700" C isotherm. Over 120 specific compositions were prepa red in t his way
in order to study the phase relations at 900° and 700° C.
The quenched products were investigated by visual examination trough the
transparent walls of the silica tubes, in order to ascertain the presence of liquids
or liquid immiscibility, by reAected light microscopy and by X-ray diffraction
analyses with CoKa radiation on a Philips X-ray diffractometer. In some instances
chemical composition of the phases present was determined by electron probe analyses
76
C. P. BERNARD INI, C. MAZZETTI
with aPhil ips Nordco ARM/3 microprobe using pure elements and synthetic euse
and Fe-rich sphalcrite as standards, while some element distribution maps were
taken with a Jeol Sca nning Electron Microscope U /3.
Silica tube quenching experiments are not suitable for the study of unquenchable
phases as frequent in selenide as in sulfide systems. To investigate such phases
D.T.A. experiments were conducted, using thermocouple well tubes ( B ERNARD INI,
19(6) and pure quartz as reference material, on a T ern-Press DT 712 with heating
and cooling rate of 3" C/min. Few charges W Cfe also investigated by high-temperature X-ray analyses performed with a Rigaku-D enky D chye camera equipped with
a Pt-wound furnace controlled at ± 5° C through a Pt-Pt/Rh thermocouple.
Experimenlal re8uhe
a) The 900" C isotherm
T he phase relations in the Cu~Fe-Se system at 900" C 3re shown in fig. 2. The
Se-rich portion of the system is occupied by a homogeneous liquid field (L) whose
T
= 900 DC
"' . ... . l . F ••.• .
Co
F.
• t. ""
Fig. 2. - Ph~sc re1~tions in the Cu.Fe·Se system at 9000 C. _ L = ternary liquid; bz •.•. = berzelianite solid solution; ach •.•. = achava litc solid solution; Cu •.•.
copper solid solution; Fe •.•.
iron
solid solution. All phases and phase assemblages c""xi,t with vapour.
=
=
upper limit varies from nearly 60 '70 at. Se on the Cu-Se border (HEYDIN"G, 1966)
at. on the Fe-Se (SVEN"DSEN, 1972) and by an extensive liquid immiscibility field above these limits.
The presence of various solid phases in the X-ray diffraction patterns of the
quenched charges from these two liquid fields, the results of some of which are
reported in table 2, may be explained by the complicated non-<=q uilibrium structures
observed in the relative polished sections. These structures, as shown in fig. 3,
to almost 75
ro
PHASE REL ATI ONS IN THE C.,~.'S. SYSTEM AT
TABLE
Selecud
'"'
'"
,,.
."
'"
'"
'"
,,.
'"
'"
'"
on
,
.....
'"
'"
'"
."
...
'"
'"
,,.
'"
'"
'"
'"no
'"
'"
m
,'"
..
'"
'"
'"
'"
'"
'"
,
",
,
,."
"
"
" "
" " "
", .," "
"
,
"
" "
" " "
,." , "
., ,.
"
,
" "
" " "
"
,."
" "
" " "
", ,." "
"
", "
"
" "
" " "
" .," "
"
"
" "
" " "
,." ",
"
,
" "
" , ,."
"
" ",. "
"
"
.," " "
"
..
..
..
....
..
..
..
..
..
• Scc tcxt for ~xp)analion.
From qu~nched ad, •. •. Term s.
77
at 90(1' and 7()(J' C
Ph ..... e . detected et room t","pere t ure
by x-uy and micro.cope .tudi u
,.
..."
7000 C
AND
2
~xp~rim~ntal r~Sltlts
Bulk comp.
at""'- 1
900 0
[[re ,_xse " fe Se ?] .. (cu . fe)se 2 _ x l
l 4
[(cu,p. )S " 2_X .. CuSe .. [pe,_~se .. relSe,?])
[(Cu . felse _
[re,_ xse .. fe 3 Se 4 ?]I
2 X
p"3se.7] .. {C uFese ... (Cu . relse 2 _ x (traCUI)
[Fe,_~s.
{cupese. . . (Cu .Pels"2 _x .. [Fe,_ xSe .. pe 3se 4 ?])
["" ,_xse .. p"3se(7) .. [cyp" se 2 }
reSe .. fe'_xS@7
{peSe) .. re ( t uc ... )
CU _ S" .. cupe s.,.( ao euo lution 1""", 1 1"10) .. {(Cu . pelse _x l
2
2 x
A-ph ..e . .. {PeSe }
A_phase" CuFeS"2 ( .. e x. olut i on lUlellae)
A-phase
A-phase" Fe .. [FeSe)
{FeSe) .. Fe .. A-phase (tuce.)
A-phase .. Cu .. Fe
A-ph ... e .. re
A-ph ... e .. Fe
A-phue" cuPeS"2(". e x.olutlon 1 ...... 11",,) .. {reSa)
A-pha". . . {cupeSe ("lso present as ex.olution lamellael)
2
[re ,_x se .. Fe Se . ?] .. Hcu . r "lse 2 _ x )
3
{(Cu . r,,)se 2_x .. CuSe .. [ rel _Xse .. pe 3se,?)}
[pe' _xS B • Fe]se.?) .. !lcu . pe)se._ x }
{\cu . l'e)Se _ • CUF"S" . .. cu _ se (traces))
2 x
2 x
[pe,_ xse .. F&)S".1] .. CU 2 _ x S" .. {cupes" ... (cu . pe)Se 2 _ x J
PeSe
PeSe
••
Cu2_ xSe .. [cupese.) (also present ... exso l utlon lamella,,)
A-phase .
.. feSe • 1'.. (trace s )
A-phue .. copese.( ... e x solution 1 ...... 11 ..10)
A- phase
FeSe .. re
~
A-ph ..e (nace.)
FeSe
A-ph ..e .. Cu .. Fe
[Fe _ S8 .. F,,)S" 4?] .. {{CU . Fe)S"._x )
1 x
(cup"S" ... (Cu . Fe)Se _ ) .. cU2_x Seltrace.)
2 x
A- ph.. e .. cuF"s"2 ( n exsolutlon 1""",1 he) .. FeSe
A- ph ... e .. peSe
{} Ph3 ~' obt:l.in~d from quenched liquids.
[] Iron ~elenidC!l derived
78
G. P. BERNAROtNI, G. MAZZElTt
Fig. 3. - PhOlomicrogr~ph of charge n. 903 (oil immersion, parallel nitals, 300 x~ Remnants of elongated
primary phase (white) ",rrounded by ~rytetical1y·formcd dcndritcs (greyish wbite) 3.Ild melt (dar k grey).
Fig. 4. - Photomicrograph of charge n. 927 (oil immersion, parallel nicols,
300 x). A-phase grain with oriented cxrolution lamellae of eskcbornite.
prove that the solid phases present in the quenched products are cauSl::d by
subsequent solidification procc;sses such as synthetic or perythectic reactions. At the
reaction tem~rature, in fact, apart from the solid phases present on the eu-Pe
P H-,SE REl.J.TION S IN TH E C-P_S. S YSTEM AT
900"
M'ID
70()<, C
79
me:tal syste:m (not discussc:d he:re:). only two solid phasc=s e:xist: thc bc=rzd ianite: solid
solution (CU2..,Sc=. with 0.00 ~ X :S;O.lO) a nd the: achavalite: solid solution (Fe:l ..,Sc=.
with 0.00.:s;;; X .:s;;; 025). The: Ixrzdianiite: solid solution (bz..•. ) exte:nds through a
conside:rable distance: into the: ternary system. the: maximum solid solut io n of this
"-c~.'.
Iv-'
'--
_~I\.A
)
.' .n
A.~~A
A
. ' 117
A.,
E... ~.,.u.
A./
"•
A
"•
..
'
J
,,'
"•
..
•
2•
Fig. 5. - X·ray powder diffraction pattern, of charg~. nn. 933 and 927 compared
with those of pure 0:.0..& and ~,kd:>ornin:: (Fe·filtered CoKa radiation).
phase: re:achi ng 10 % at. Fe:, while the achavalite solid solutio n (ach •.•. ) dissolves a
maximum Cu<ontent of ne:arly 5 % at.
bz".•. terms, depending on the: x value, dther the
In the: quenche:d products
a-Cu2Sc= orthorombic phase: or the: cubic CU2.XSc= we:re dete:cte:d being the P-Cu&
cubic high-te:mperat ure form unq uenchable ( BER NA ROI NI et aI., 1976). The que:nchw
or
80
G. P. RE RNARD IN I , G. MAZZETTr
ro
product of charge n. 933, with 5
at. Fe, proved to be formed by an optically
and chemically homogeneous phase with a compositiona] formula, as determined by
electron probe analysis, of Cu{;o.5F e:u$e3U which corresponds to a hypothetical
CU6.6F eo.45c4.0 compound i n which the metals to selenium ratio is of the same
order as the one established by M ORO,IOTO & GyoBU (1971) for the iron-stabilized
digenite at room temperatu re. T he X-ray pattern of this phase, reported in fig. 5,
shows, in addition to poorly crystallized reRections similar to those of the onhorambie a-Cu2Se, some very weak reAections not reported for this phase. While a
situation similar la the onc found by M ORIMOTO & GYOBU (1971) may be inferred for
the Cu-Fe-Se system at room temperature, studies on the crystallographic
characteristics of this new phase, from now on indicated as A-phase, are in progress .
The approxi mate boundary of the bz•.•. field, towards the central portion of the
system, has been determined on the basis of the appearance of solid phases coming
from the quenched L-liquid such as e.g. the (Cu,Fe)Se2.S compound in charge
n. 923. On the contrary no evidence of a liquid phase, present at the reaction
temperature, was observed in the polished section of charge n. 927 w hich has the
same Fe-content and a lower Se-content. T he exsolution lamellae in the A-phase
grains of this charge, shown in fig . 4, proved to be eskebornite as a result of
unmixing processes during the quenching mechanism. The diffraction patterns of
the quenched products of charges nn. 933 and 921 are compared in fig . 5 with those
of pure d -Cu2Se and eskebornite. The minimum eu/Fe ratio of bz" .•. terms is
smaller than the of the Se-ana logous of bornite which is never been detected either
in synthetic or natural products.
The quenched products of achs.•. terms are made of one or two iron selenides
the relations of which arc not clearly defined, as already mentioned. As in the case
of bz" .• , therefore, the approximate boundary of the achu. field has been mainly
inferred by comparing the textura l relationship of the assemblages observed in the
polished sections of the charges with a Cu-content to a maximum of 8 '70 at. with
t heir X-ray diffraction results (efr. e.g. run n. 918).
Between the bz...•. and the ternary liquid L and between the ach •.•. and the
same liquid, two t.wo-phase fields are established, as revealed by the assemblages
detected in the quenched products of charges nn. 960 and 911. F ig. 6 shows the assemblage observed in this last charge where grains of Fel .•Se, with exsolution lamellae
of probable Fe:Sei, sorrounded by a microcrystalline matrix derived from the ternary
liquid, are observable. The ooundaries of the two two-phase fi elds (bz.. •. + L
and aeh •.• + L) were determined by the interpretation of the differeOl solid phases
observed at room temperature in numerous charges. The liquid-phase limits are
dictated by the assemblages observed e.g. in the quenched products of charges no. 917
and 968 respectively fo r the ach•.•. + L and bz•.•. + L fields. The metal-rich limit
points to a liquid of nearly FeSe-composition as revealed by the analysis of the 924
quenched product. Fig. 7 shows the relative assemblage made of large grains of
A-phase with star-li ke inclusions and borders of a microcrystalline matrix derived
PHASE RELATIONS IN THE C.-F. -S . SYSTEM AT
900"
AND
7000 C
81
from a FeSe-similar liquid as determined by the SEM element distribution maps
and by the line carried out at the electron microprobe.
In the metal-rich region of the system the followi ng fields were detected; two
extreme two-solid-phase fields: Cu, with a maximum Fe<antent of nearly 2 % at.
(H A.NSEN & ANDERKO, 1958 and supplements) indicated as eu •.•. , plus bz•.•. and Fe,
with a maximum Cu-content of nearly 4 % at. (efr. above reference) indicated as
Fe...., plus achavalite, and three three-phase fields : Cu •.•. + bz. .•. + Fe.... , hz •.•. +
+ L + Fe., •. and L + ach + Fe•.•.. The boundary between the first two univariant
fields has been drawn on the basis of the presence of eu contemporary with the
absence of the FeSe-similar liquid and viceversa in numerous runs (efr. e.g. runs
Fig. 6. - Photomicrograph of charge n. 911 (oil immersion, parallel nicols, 300 x). Grains of 3ch •.•. ,
with exsolution lamellae of probable F~., sorrounded by a microcrystalline aggrega te of OJ.FeSe.
and (Cu,Fe)Se._. crystallized from L-liquid.
nn. 953 and 940). Figs. 8 and 9 show the assemblages observed in the polished
sections of these charges respectively characteristics of the first and second field.
No exsolution lamellae of eskebornite were ever observed in the A-phase grains of
all these charges, thus making it possible to differentiate them from those present
in the quenched products of the bz•.•. + L divariant field. In the A-phase grains
of run n. 940 skeletal inclusions derived from the quenched liquid of nearly FeSe-composition, as already shown in fig. 7, are dearly visible. In the 953 polished
section the absence of such a liquid is accompanied by the appearance of eu traces.
How fa r the poor Se-content prong of the FeSe-similar liquid has retreated fro m
the Fe--Se border at 900" e, was inferred by comparing the results of runs nn. 920
and 944 which have t he same amount of eu (5
at.) and different Fe/Se ratio.
ro
c. P.
82
BERN~ROI NI,
C. MAZZETTI
"C-./
"V
c,
UJ
0
0'
rv
I!f"
F.
~
I
"""I.'
Fig. 7. - Polished section
of charge n. 924. Top:
reRected light photomicrograph (oil immersion, pa·'
rallc1 nieol!, 300 x) and
!clnning electron image
(3000 x) of the particular
in the black square Grains
of A-phase with intergranular borders and star.like
inclusions crystalliud from
a F(Se_similar liquid. Center: characteristic X-ray
.canning pictures. Bottom:
traces of linear sun~ along
'he a-,,' line.
PHASE 1U;LAT IONS 1N THE C.-F.-S. S YSTEM AT
900"
AN D
700" C
83
Fig. 8. - Photomicrograph of chrge n. 953 (oil im mef$ion, parallel nicols, 300 x).
Simple inlcrgrowlh of A.phne grains (g rey) and Cu- and Fe·grains (while).
Fig. 9. - Phou>microguph of ,hargc n. 940 (011 immenion. ~rallcl nkol,. )00 x). Grain of bz •.•.•
wilh ;nlCrgu nular bordcr and liar_like indu,ion, "yslalliud from a FcSe-sim ilar liquid. and small
inlcrgnnular Fe-J!uin (while).
84
G. P . BERNARDINI, C. MA'ZZETTI
W hile, in fact, the exact composition of this liquid is still uncertain, the presence
of A~ph asc traces in charge n. 944 and their absence in n. 920 suggests that the
FeSe-similar liquid cannot have a Cu-content lower than 551'0 at.
s.
Cu
Fig. 10. in fig.
~s
al.'"
Fe
Pha<e relations in the Cu-F".& syst ... m al 700 0 C (abbreviations
2),
All
pha.."
and
phase
3ssc:mblages
coexiSl
with
vapour.
Th~
700" C isotherm
At 700° C the large L-liquid field retreats, as shown in fig. 10, towards the
(:.u-Se border together with the liquid immiscibil ity field , as evidenced by the
b)
-... -
'---
-
,•
-.•
Fig. 11. _ High-temperature powder diifuclion patterns of synthetic eskebornitc: A, B, C, D and
F = CurcSc. : E = Cu. _.Se + Fe...Se (film cassette ~ 90 mm; Fc·filterffi CoKa radiation).
comparison of the results of runs nn. 702, 703 and 704. rC(XIrted in table 2, with
those of the same com(XIsition at 900" C. Consequently a three-phase field:
SeL + L + ach•.•. and a very narrow two-phase field: SeL + ach•.•. appear near
the Fe-Se border. The problem of the solid phase present in these fields,
PHASE RELATIONS IN THE C.-F ... S . SYSTEM AT
9Q0<'
AND
7000 C
85
which could be the FeaSe.., is still open as already mentioned. In t he
quenched products of the charges of the univariant field, F e3Se4 is present as
exsolution lamellae in the grains of Fel.XSe which arc sorrou nded by a microcrystalline mat rix produced by the quenched liquids. The L-liquid field retreats
also towards the Se-apex, its metal-rich border reaching a Se-content slightly lower
than 50 % at. (efT. run n. 757). A three.phase field : bz.. o. + L + ach u . appears,
therefore, in the region with a Se<ontent variable between 43 70 and 50 % at.
which includes the theoretical composition of eskebornite (CuFeSe2). The high-temperature X-ray analysis of eskebornite synthesized at 300" C from pure elements
(fig. 11) proves that at 700° C CuFeSe2 breaks into CU2.XSe and Fel.XSe. Moreover
the D.T.A. run of eskebornite shows, in fairly good agreement with FRAm's data
(1970), an endothermic peak at 540-550" C, probably due to the incongruent melting
of CuFeSe2 into CuFeSe2.x plus seleni um liquid, followed by another large peak
at 600-630" C which might be explained by the decomposition of CuFeSet.• into
CU2 .• Se plus Fel .• Se and a ternary liquid of composition approaching that of
(Cu,Fe)Se2.x. T he possible existence of CuFeSe2 .• , which might have a cubic simmetry similar to that of i ~.•. in the Cu-Fe-S system (CABRl, 1973), and its stability
field below 700" C. might be ascerta ined by successive investigation at lower
temperatures.
Fig. 12 shows the assemblage observed in the quenched product of run n. 716,
with stoichiometric CuFeSC2 composition, where grains of Fel .• Se, with exsolution
lamellae of probable FeaSe4, and grains of CU2'XSe, with inclusions of a microcrystalline matrix derived from the quenched liquid, are evident. The relative
electron probe i n ve~tigation, carried out along the line b-b', and the element
distribution maps, shown in lh ~ same fig ure, confirm the presence of the phases
detected at room temperature.
In order to determine t he metal-rich bou ndary of the univariant field: bz..•. +
numerous runs were performed in the region with a Se<ontent
variable between 3770 and 50 70 at. I n charges nn. 761 and 724, with 50 % and
43 % at. Se respectively, only two solid phases were detected : A-phase and FeSe,
plus probable Fe traces in the second. The A-phase grai ns observed in the polished
section of charge n. 761 did not show exsolution lamellae of eskebornite, thus
suggesting the bz..... - ach~.•. join pointing to :l. bz. .•. term of pure A-phase composition. On the other hand the probable Fe traces in charge n. 724 may suggest
this join pointi ng to a compound with a metal to selenium ratio = 1/1, towards
the Fe-Se border. A very narrow divariant field: bz..•. + ach •.•. is, therefore,
established below the univariant field: b7". ~. + L + ach •.•.. Its Se-rich limit varies
from almost 43 % at. in the bz..•. region, as proved by the exsolution lamellae
of eskebornite observed in the A-phase grai ns of run n. 7W with this Se<ontent,
to nearly 50 70 at. close to the Fe-Se border; the Se-poor limit goes from 37 % at.
in the bz..•. region to the same 50 % at., as accounted for by the results of charge
n. 717 which. with this las[ Se<ontent and with 12 % at. Cu and 3870 at. Fe, is
+ L + ach•.•. ,
86
G. P. BERNARDIN I, G. MAZZE'ITI
Fig. 12. -
Polished sectIOn
of charge o. 716. Top:
reRected light photomicrograph (oil immenion, paralld n;cols, 300 x) and
>Ca lming ciectron image
(3000 x) of the particular
in the black square. Intergrowth of ach.... grains
b
b
located in the univariant field: bz..•.
b'
'----,b'
+ L + achu..
(whitish), with exolutinn
bmcllac of probable Fe.Se.,
and bz •.•. grain. (grey)
with buck-shot inclusions
(darkish grey) crystalli=3
from a CuFcSer-similar liquid. ec.ntcr: characteristic
X-ray scanning pictures.
Bottom : ITaCeS of linear
scans along the bob' line.
The establishement of the di-
variant field between hz...•. and ach8.l. causes the disappearance of the small thre(:·
-phase field: L + ach + Fe•.•., dose to the Fe-Se border present at 900" C, which
is substituted by a two-solid-phase field: ach.... + Fe•.•. (dr. run n. 720 and fig. 13).
PHASE llELATIONS IN nlE C_ F....S. SYST EM AT
900"
AND
7000 C
87
The inner boundaries of the two extreme two-solid-solution fidds: bz..•. + Cu•.•.
and ach.... + Fe.... are dictated by the Fe- and Cu-content that Cu and Fe can
rt:Spt:ctivdy dissolve at 700" C (HANSEN & ANDERKo, 1958 and supplements) and by
the results of the experiments performed in the region bt:low the bz..•. + ach •.•.
divariant fidd (efr. e.g. runs nn. 744 and 749).
No variation in the position of the boundary between the two three-solid-phase
fidds: Cu•.•. + bz...•. + Fe.... and bz..•. + ach•.•. + Fe•.•. was observed with respect
Fig. 13. - Ph(l(omicrograph of charge n. 720 (oil immersion, p"ralld nkols, 300 x). Simple intergrowth
of ach •.•. grains (whithi,h grey)' with prominent twin Iamc1be (grey), and Fe grains (white).
to the one portrayed between Cu .... + bz..... + Fe.... and bz..•. + L + Fe.... at
C (dr. e.g. run n. 747). The appearance at 700" C of the divariant fidd:
bz..•. + ach.... , in fact. has only prohibited the L-liquid coexisting with Fe.....
9()()0
ConclusioD/!I
The phase rd ations in the Cu-Fe-St: system at 900" and 7()()o C have been defined
accuratdy by quenching experi ments. At 900" C a large ternary liquid fidd dominates
the central ponion of the system and only two solid phases, apart from those on
the Cu-Fe border. were detected. At 700" C the only difference with the tie-l.ines
pattern depicted for the 9()()0 C isotherma! section is the appearance. in the central
region of the system. of an univarial), fidd: bz...•. + L + ach •.•.• with the liquid
apex at almost 50 70 at. Se. No ternary phases were detected either at 900" or at
7()()O C in contrast with the situation in the Cu-Fe-S system at the same temperatures.
88
C. P . BERNARD INI, C. MAZZETT I
According to YUND & KULLERUD (1966) and to KUl.Lf.RUD et al. (1969), in fact, at
both these temperatures the central area of the Cu-Fe-S system is characterized
respectively by a small and a larger solid solution field, sllccessively indicated as
i.... field (BARTON, 1973; C .. SRI, 1973). While the boundaries of this field arc very
variable both in the Cu/Fe ratio and in the S-content, the latter is always lower
than that of stoichiometric CuFeS2.
In the Cu-Fe-Se system even for compositions corresponding to those of
rnooihockite and haycockite (CABR I & Hu.L, 1972), or between the two, no solid
ternary phase was detected. Owing to the much lower temperature limits of the
stability fields of the different phases in the two binary systems Cu-Se and Fe-Se
with respect to the corresponding sulfur systems, a situation similar to that of the
Cu-Fe-S system at 900° and 700" C, should occur in the Cu-Fe-Se system at lower
temperatures. More complicated phase relations, therefore, are to be expected in
this system at 5000 and 300° C where more changes in the tie-lines configuration
should occur owing to the appearance of new solid phases and separation of high-temperature ~olid solutions.
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