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Au alloys
Journal of Electron Spectroscopy and Related Phenomena, 24 (1981) 11-17 Elsevler Sclentlflc Pubhshmg Company, Amsterdam - Printed m The Netherlands
SURFACE
COMPOSITION
V I NEFEDOV, Institute
YA
OF NATIVE
V SALYN,
GOLD
V A MAKEEV
of General and Inorganrc
Chemzstry,
AND
Ag/Au ALLOYS
and V I ZELENOV
Academy
of Sciences
of the US S R ,
~MOSCOW (US S R ) (Received 16 February 1981)
ABSTRACT The surface composltlon of Ag/Gu alloys has been studled by X-ray photoelectron spectroscopy (XPS) Surface segregation of Ag IS observed, parbcularly for alloys having low Ag content The content of Ag m the first surface monolayer 1s m accordance with data from ion scattering and with a theoretical model for segregation m ordered solutions The surface of native gold was found to be enrlched wrth silver in the case of mine gold, and the degree of segregation was slgmficantly higher than for an alloy having similar bulk composltlon The surface of mine gold taken from an oxldlzed zone contained less silver than did that of usual mine gold The surface of placer gold was depleted of silver as compared to the bulk These data show the surface composition of native gold to differ markedly from that of an alloy having the same bulk Ag content, and to depend on the genesis of the native gold sample
THE SURFACE
COMPOSITION
OF Ag/Au ALLOYS
In the first stage of our study devoted to the surface composltlon of native gold, which 1s known to contam an appreciable amount of silver, the surface composltlon of Ag/Au alloys has been investigated The surfaces of Ag/Au alloys have been studied previously by Auger electron spectroscopy (AES) [l-3] and ion scattering spectroscopy (ISS) [4,5] AES did not reveal any slgmflcant differences between the surface and volume composltlons of the alloys studied, although slight segregation of Ag at the surface was not excluded ISS showed the presence of noticeable surface segregation of Ag The relative mtensltles m ESCA spectra of Ag/Au alloys have been reported m ref 6, but the problem of surface segregation was not considered there We have used ESCA to study the composltlon of the surface of Ag/Au alloys The sample alloys were obtamed by alloying the required quantities of pure Ag and Au metals The bulk alloys were then ground into fme powders Measurements were carried out on these powdered samples using a Vman VIEE-15 spectrometer unth an Mg anode The problem has been stated m
0368-2048/81/0000-0000/$02
50
01981
Elsevler Sclentdlc Pubhshmg Company
12
such a way that the composltlon of the alloy surface after contact wrth the atmosphere 1s of interest Therefore measurements were performed after the powders had been held m ar for some days We also studied powdered alloy samples annealed rn u at 400°C for 8 h The surface composltlons have been calculated using the relation
C&/CL”
=
UABI~A~
3d
AA~Z 3d )/(IAu/~Au
(1)
4fhAu4fl
where Cf 1s the surface concentration of an element, I, the intensity, oI the photolonlzatlon cross-section (taken from ref 7) and h, the photoelectron mean free path (taken from ref 8) for the correspondmg X-ray photoelectron hne Expermental data are presented m Fig 1, from which it can be seen that for small Ag concentrations surface segregation of Ag takes place which 1s increased by heating In prmclple, two reasons for this segregation can be considered For the first, Ag segregation 1s to be antlclpated according to theories [9, lo] of segregation at the alloy surface (Fig 2) For the second, oxldatlon of Ag by oxygen present m the atmosphere also assists Ag segregation to the alloy surface Since the Ag 3d energy value for Ag,O 1s very close to the value for metalhc Ag, the ESCA spectra do not allow determlnation of the state m which Ag 1s present at the surface An increase m the degree of Ag segregation on heat treatment may be due either to approach of the equlllbrrum condition, or to a reduction of the surface contammatlon layer thickness The last effect will cause the ratio IAg/IAU to increase since
I
2
3
c” (Ag/Au)
Fig 1 Surface atomic ratios CS(Ag/A u ) versus bulk values C”(Ag/Au) and heated (X) samples of Ag/Au alloys
for unheated
(0)
13
4
IO
3
' 2
q=
0.8
“*4
/
/'
,d'
0.6
.X Cl g
04 ,G' 02
/
P / x 1' 7" I J QZ
/
/
RI ,f'
/
/
/
0," /
/
I 0.6
I 04
I 08
I 1.0
"ig Fig 2 Dependence of mole fraction &s on x);s 1, ordered-solution theory based on surface tenslon [l] , 2, ordered-solution theory m approxlmatlon of surface bond-breaking [ 2 ] , 3, ideal-solution model [ 5 ] , 4, experunental model [9] Expernnental points unheated (e) and heated (X) samples (this work, contmuous-type segregation, ESCA), W, ISS data [4], 0, ISS data [5]
Ag 3d photoelectrons possess smaller kmetlc energy (-890 eV) compared to Au 4f photoelectrons ( hr1170 eV) Let us consider the posslblhty of explammg quantltatlvely the observed extent of Ag surface segregation wlthm the scope of exlstmg theories (Figs 2 and 3) Segregation 1s known to modify usually the first monolayer of an alloy, i e a surface film of - 2 9 A thickness Takmg an average X value of 17 a for the Ag 3d and Au 4f lines and an escape angle for photoelectrons of 45” (m our instrument), the contrlbutlon n from the uppermost monolayer of thickness d = 2 9 ii to the overall intensity 1s obtamed as n =
1 -exp(-d/hsm45”)
= 0 21
The measured atomic concentration ratio CS(Ag/Au) = C&&i, can be wntten as CS(Ag/Au)
= nC, (Ag/Au) + (1 - n)C”(Ag/Au)
(2) m eqn (1) (3)
where C, (Ag/Au) IS the ratio of atomic concentrations of Ag and Au m the first monolayer, and C’ 1s the bulk concentration ratio In Fig 2 the experimental data on the surface composltlon of Ag/Au alloys obtained by ISS [4,5] and m this work are compared with different theories In this figure the mole fraction xip of Ag at the surface 1s calculated (eqn ( l)), assumrng contmuous-type enrichment of usmg the ratio CA&i, the surface layer by Ag In Fig 3, which 1s analogous to Fig 2, our data are presented for xiAg calculated by eqn (3) assuming the segregation to occur
14
’
2
/ g
X/ x
l
X z “*
06
!/ l
/’
,/’
,,’
0 ** /i/...
1 x
0
020
/
/
&
04
/
/
/
/
/
/
//’ /
/
/’
,’
/ /-
/
/
I
1
I
I
02
04
06
08
I
10
unheated (0) and heated (X) samples Fig 3 Dependence of mole fraction X~A~ on x& (this work, segregation m first monolayer only, ESCA), for 1, 2, I and q, see legend to Fig 2
wlthm the first monolayer
XLg = C, (A&W/V
only
+ C, (&/Au)]
(4)
The data of Fig 3 show both semlquantltatlve agreement of our data with the results obtamed by ISS (ISS gives the concentration m the first monolayer) and agreement with calculations of segregation based upon models of ordered solutions It 1s unportant to note that for larger C’(Ag/Au) ratios even a small increase in P(Ag/Au), when calculated for one monolayer, gives a slgmflcant segregation value (see Figs 2 and 3) This 1s indicative of the large error associated with x i Ag and of the low sensrtlvlty of C”(Ag/Au), which m this C”(Ag/Au) range does not allow any conclusion about whether or not segregation occurs Thus we have measured values for Ag segregation m Ag/Au alloys which agree with experimental and theoretical data reported m the literature The fact of prmclpal slgrnflcance for the following dlscusslon of the results for the surface composltlon of native gold 1s that for alloys the ratio C”(Ag/Au) 1s usually close to the C’(Ag/Au) value Only m the case of small Ag concentrations can the Cs (Ag/Au) noticeably (twofold) exceed the C’ (Ag/Au) value
THE SURFACE
OF NATIVE
GOLD
Native gold of both mme and placer origin may contain up to 50% Ag It 1s known that the process of native gold formation does not involve melting
15
25
05
05
IO
15
20
c” (Ag/Au) Fig 4 Surface atomic ratios C”(Ag/Au) versus bulk values C”(Ag/Au) for native gold samples taken from various deposits ?b, sample of mine gold, 7al and 7az, two samples of placer gold (all three from one deposit), particle size, -0 4 -I- 0 31 for 7al and -0 1 -IO 08 for 7az, I, typical curve for mme gold, II, curve for Ag/Au alloys
of the metal, and therefore the dlstrlbutlon of Ag m native gold may be nonuniform In particular, the surface of gold particles may be enriched with or depleted of Ag Some indirect mdlcatlons of both Ag enrichment [ 12, 131 and depletion [11,14-161 can be found m the literature As far as we know, the present work 1s the first mvestlgatlon of the surface composltlon of native gold by means of ESCA We have used ESCA to determine the ratio C$(Ag/Au), as given by eqn (l), for gold particles taken from vmous deposits Special care was given to the sampling procedure to exclude any alteration of surface composition dunng the extraction of gold particles from the rock Measurements were performed only on manually extracted particles which did not undergo any stage of treatment with chemical reagents The results are presented m Fig 4” and suggest the followmg conclusions (a) The surface of mme gold (deposits Nos l-4) 1s enriched with Ag Segregation of Ag m mme gold occurs to a much higher degree than m Ag/Au alloys havmg the same bulk composltlon, and 1s St111present for high Ag concentrations Curve I m Fig 4 1s drawn for lllustratlve purposes only The experimental matenal avadable 1s not sufflclent to assert the presence of some umversal dependence of surface composltlon C’(Ag/Au) on bulk values CV(Ag/Au) for mine gold particles, independent of the character of a particular deposit Thus, for the mme gold sample taken from one quite spe~lfic * We have observed the segregation of a number of elements at the surface of native gold particles, but m this paper we restrict dlscusslon to the rat.10 Ag/Au only
16
(from a geologcal pomt of view) deposit, we observed a surprlsmgly high Ag segregation the ratio Cs( Ag/Au) /C” ( Ag/Au) was - 10, which slgnlflcantly exceeds the value for deposit No 1 (Fig 4) havmg approxnnately the same CV(Ag/Au) value The results obtamed provide unambiguous evidence that the mechamsm of surface enrichment of mme gold particles by Ag 1s by no means the same as m the case of Ag/Au alloys Indeed, after remelting the mme gold from deposit No 4, the surface composltlon was found to comclde with that obtained for an alloy havmg the same bulk Ag/Au ratio, 1 e the CS(Ag/Au) value had been reduced more than twofold Some indirect data mdlcatmg shght surface enrichment of mme gold particles by Ag have been reported previously [ 12, 133 , but the methods used were not sufficiently sensltlve to surface concentrations The observed effect of enrichment was relatively small (not exceeding some tens of percent), while the present results show that the Ag concentration at the surface can sometimes be considerably higher than that m the bulk (b) The surface of placer gold (deposits Nos 5, 6, 7a) 1s slightly depleted of Ag Such depletion for gold particles taken from deposits of slm1la.rkmd has been previously reported m the literature [ 11, 14-161 The small magmtude of the effect observed was due to msufflclent sensltn&y of the methods used to the uppermost surface layer It 1s znterestmg to note that the gold par&les from deposit No 5, which IS an open-type placer, exhibit a much smaller surface Ag content than the gold particles from deposit No 6, whch ISa burled-type placer In the latter case the gold particles undergo much weaker mechanical action m the course of mlgratlon This fact seems to be an mdlcatlon of possible mechanical removal of Ag from the surface of gold particles during their migration in a placer deposit (c) The surface of mme gold particles taken from oxldlzed zones (deposits Nos 7b, 8) was found to contam less Ag than the surface of the usual mine gold havmg the same bulk composltlon However, for the sample taken from the oxidized zone of deposit No 10, such a relative depletion of Ag was not observed We also carried out measurements on one sample of secondary gold (deposit No 9) the surface of which was found to be enrxched with Ag Thus, the present experimental results show that for native gold particles the ratio C’(Ag/Au) 1s slgmflcantly dependent on the deposit type, 1 e placer or mme, and m the last case It ISimportant whether or not the sample 1s taken from an oxidized zone The slgnlflcant difference m the surface composltlon of native gold and of Ag/Au alloys havmg slmllar bulk composltlon makes it possible to state defmltely that the phenomena observed cannot be explamed on the basis of simple segregation processes The specific genesis of native gold should be the most important factor determmmg the surface composltlon A dlscusslon of these data from the geological standpoint will be gven m a subsequent paper
17 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
S H Overbury and G A Somoqal, Surf Scl , 55 (1976) 209 R Bouwman, L U Toneman, M A H Boersma and R A van Sunten, Surf Scl , 59 (1976) 72 M Yabumoto, U Watanabe and T Yamasklta, Surf Scl ,77 (1978) 615 G C Nelson, Surf Scl ,59 (1976) 310 M J Helley, D G Swartzfager and V S Sundaram, J Vat Scl Technol , 16 (1979) 664 H Ebel and R Kamer, Wlss Z K -Marx-Umversltat Lelpzlg, 4 (1976) 416 J H Scofield, J Electron Spectrosc Relat Phenom , 8 (1976) 129 M P Seah and W A Dench, Surf Interface Anal , 1 (1978) 2 M Kelley, J Catal, 57 (1979) 113 M P Seah, Surf Scl , 80 (1979) 8 G A Desborough, Econ Geol , 65 (1970) 304 Yu S Berman and V M Novlkov, Kolyma, 2 (1969) 41 N V Petrovskaya, Native Gold, Nauka, Moscow, 1973, p 213 (in Russian) S V Yablokova, Cent Inst Geol Res Works, 79 (1968) 153 (m Russian) A A Saprykm and S V Yablokova, Izv Tomsk Polytekh Inst ,239 (1970) 390 A S Ivoylov, L P Zayalova, V I Llpskaya and V G Barankevlch, Proc 7th AllUnion Conf on Local X-Ray Spectral Analysis and Its Apphcations, Chernogolovka, U S S R , 1979, p 143 (In Russian)
SURFACE
COMPOSITION
V I NEFEDOV, Institute
YA
OF NATIVE
V SALYN,
GOLD
V A MAKEEV
of General and Inorganrc
Chemzstry,
AND
Ag/Au ALLOYS
and V I ZELENOV
Academy
of Sciences
of the US S R ,
~MOSCOW (US S R ) (Received 16 February 1981)
ABSTRACT The surface composltlon of Ag/Gu alloys has been studled by X-ray photoelectron spectroscopy (XPS) Surface segregation of Ag IS observed, parbcularly for alloys having low Ag content The content of Ag m the first surface monolayer 1s m accordance with data from ion scattering and with a theoretical model for segregation m ordered solutions The surface of native gold was found to be enrlched wrth silver in the case of mine gold, and the degree of segregation was slgmficantly higher than for an alloy having similar bulk composltlon The surface of mine gold taken from an oxldlzed zone contained less silver than did that of usual mine gold The surface of placer gold was depleted of silver as compared to the bulk These data show the surface composition of native gold to differ markedly from that of an alloy having the same bulk Ag content, and to depend on the genesis of the native gold sample
THE SURFACE
COMPOSITION
OF Ag/Au ALLOYS
In the first stage of our study devoted to the surface composltlon of native gold, which 1s known to contam an appreciable amount of silver, the surface composltlon of Ag/Au alloys has been investigated The surfaces of Ag/Au alloys have been studied previously by Auger electron spectroscopy (AES) [l-3] and ion scattering spectroscopy (ISS) [4,5] AES did not reveal any slgmflcant differences between the surface and volume composltlons of the alloys studied, although slight segregation of Ag at the surface was not excluded ISS showed the presence of noticeable surface segregation of Ag The relative mtensltles m ESCA spectra of Ag/Au alloys have been reported m ref 6, but the problem of surface segregation was not considered there We have used ESCA to study the composltlon of the surface of Ag/Au alloys The sample alloys were obtamed by alloying the required quantities of pure Ag and Au metals The bulk alloys were then ground into fme powders Measurements were carried out on these powdered samples using a Vman VIEE-15 spectrometer unth an Mg anode The problem has been stated m
0368-2048/81/0000-0000/$02
50
01981
Elsevler Sclentdlc Pubhshmg Company
12
such a way that the composltlon of the alloy surface after contact wrth the atmosphere 1s of interest Therefore measurements were performed after the powders had been held m ar for some days We also studied powdered alloy samples annealed rn u at 400°C for 8 h The surface composltlons have been calculated using the relation
C&/CL”
=
UABI~A~
3d
AA~Z 3d )/(IAu/~Au
(1)
4fhAu4fl
where Cf 1s the surface concentration of an element, I, the intensity, oI the photolonlzatlon cross-section (taken from ref 7) and h, the photoelectron mean free path (taken from ref 8) for the correspondmg X-ray photoelectron hne Expermental data are presented m Fig 1, from which it can be seen that for small Ag concentrations surface segregation of Ag takes place which 1s increased by heating In prmclple, two reasons for this segregation can be considered For the first, Ag segregation 1s to be antlclpated according to theories [9, lo] of segregation at the alloy surface (Fig 2) For the second, oxldatlon of Ag by oxygen present m the atmosphere also assists Ag segregation to the alloy surface Since the Ag 3d energy value for Ag,O 1s very close to the value for metalhc Ag, the ESCA spectra do not allow determlnation of the state m which Ag 1s present at the surface An increase m the degree of Ag segregation on heat treatment may be due either to approach of the equlllbrrum condition, or to a reduction of the surface contammatlon layer thickness The last effect will cause the ratio IAg/IAU to increase since
I
2
3
c” (Ag/Au)
Fig 1 Surface atomic ratios CS(Ag/A u ) versus bulk values C”(Ag/Au) and heated (X) samples of Ag/Au alloys
for unheated
(0)
13
4
IO
3
' 2
q=
0.8
“*4
/
/'
,d'
0.6
.X Cl g
04 ,G' 02
/
P / x 1' 7" I J QZ
/
/
RI ,f'
/
/
/
0," /
/
I 0.6
I 04
I 08
I 1.0
"ig Fig 2 Dependence of mole fraction &s on x);s 1, ordered-solution theory based on surface tenslon [l] , 2, ordered-solution theory m approxlmatlon of surface bond-breaking [ 2 ] , 3, ideal-solution model [ 5 ] , 4, experunental model [9] Expernnental points unheated (e) and heated (X) samples (this work, contmuous-type segregation, ESCA), W, ISS data [4], 0, ISS data [5]
Ag 3d photoelectrons possess smaller kmetlc energy (-890 eV) compared to Au 4f photoelectrons ( hr1170 eV) Let us consider the posslblhty of explammg quantltatlvely the observed extent of Ag surface segregation wlthm the scope of exlstmg theories (Figs 2 and 3) Segregation 1s known to modify usually the first monolayer of an alloy, i e a surface film of - 2 9 A thickness Takmg an average X value of 17 a for the Ag 3d and Au 4f lines and an escape angle for photoelectrons of 45” (m our instrument), the contrlbutlon n from the uppermost monolayer of thickness d = 2 9 ii to the overall intensity 1s obtamed as n =
1 -exp(-d/hsm45”)
= 0 21
The measured atomic concentration ratio CS(Ag/Au) = C&&i, can be wntten as CS(Ag/Au)
= nC, (Ag/Au) + (1 - n)C”(Ag/Au)
(2) m eqn (1) (3)
where C, (Ag/Au) IS the ratio of atomic concentrations of Ag and Au m the first monolayer, and C’ 1s the bulk concentration ratio In Fig 2 the experimental data on the surface composltlon of Ag/Au alloys obtained by ISS [4,5] and m this work are compared with different theories In this figure the mole fraction xip of Ag at the surface 1s calculated (eqn ( l)), assumrng contmuous-type enrichment of usmg the ratio CA&i, the surface layer by Ag In Fig 3, which 1s analogous to Fig 2, our data are presented for xiAg calculated by eqn (3) assuming the segregation to occur
14
’
2
/ g
X/ x
l
X z “*
06
!/ l
/’
,/’
,,’
0 ** /i/...
1 x
0
020
/
/
&
04
/
/
/
/
/
/
//’ /
/
/’
,’
/ /-
/
/
I
1
I
I
02
04
06
08
I
10
unheated (0) and heated (X) samples Fig 3 Dependence of mole fraction X~A~ on x& (this work, segregation m first monolayer only, ESCA), for 1, 2, I and q, see legend to Fig 2
wlthm the first monolayer
XLg = C, (A&W/V
only
+ C, (&/Au)]
(4)
The data of Fig 3 show both semlquantltatlve agreement of our data with the results obtamed by ISS (ISS gives the concentration m the first monolayer) and agreement with calculations of segregation based upon models of ordered solutions It 1s unportant to note that for larger C’(Ag/Au) ratios even a small increase in P(Ag/Au), when calculated for one monolayer, gives a slgmflcant segregation value (see Figs 2 and 3) This 1s indicative of the large error associated with x i Ag and of the low sensrtlvlty of C”(Ag/Au), which m this C”(Ag/Au) range does not allow any conclusion about whether or not segregation occurs Thus we have measured values for Ag segregation m Ag/Au alloys which agree with experimental and theoretical data reported m the literature The fact of prmclpal slgrnflcance for the following dlscusslon of the results for the surface composltlon of native gold 1s that for alloys the ratio C”(Ag/Au) 1s usually close to the C’(Ag/Au) value Only m the case of small Ag concentrations can the Cs (Ag/Au) noticeably (twofold) exceed the C’ (Ag/Au) value
THE SURFACE
OF NATIVE
GOLD
Native gold of both mme and placer origin may contain up to 50% Ag It 1s known that the process of native gold formation does not involve melting
15
25
05
05
IO
15
20
c” (Ag/Au) Fig 4 Surface atomic ratios C”(Ag/Au) versus bulk values C”(Ag/Au) for native gold samples taken from various deposits ?b, sample of mine gold, 7al and 7az, two samples of placer gold (all three from one deposit), particle size, -0 4 -I- 0 31 for 7al and -0 1 -IO 08 for 7az, I, typical curve for mme gold, II, curve for Ag/Au alloys
of the metal, and therefore the dlstrlbutlon of Ag m native gold may be nonuniform In particular, the surface of gold particles may be enriched with or depleted of Ag Some indirect mdlcatlons of both Ag enrichment [ 12, 131 and depletion [11,14-161 can be found m the literature As far as we know, the present work 1s the first mvestlgatlon of the surface composltlon of native gold by means of ESCA We have used ESCA to determine the ratio C$(Ag/Au), as given by eqn (l), for gold particles taken from vmous deposits Special care was given to the sampling procedure to exclude any alteration of surface composition dunng the extraction of gold particles from the rock Measurements were performed only on manually extracted particles which did not undergo any stage of treatment with chemical reagents The results are presented m Fig 4” and suggest the followmg conclusions (a) The surface of mme gold (deposits Nos l-4) 1s enriched with Ag Segregation of Ag m mme gold occurs to a much higher degree than m Ag/Au alloys havmg the same bulk composltlon, and 1s St111present for high Ag concentrations Curve I m Fig 4 1s drawn for lllustratlve purposes only The experimental matenal avadable 1s not sufflclent to assert the presence of some umversal dependence of surface composltlon C’(Ag/Au) on bulk values CV(Ag/Au) for mine gold particles, independent of the character of a particular deposit Thus, for the mme gold sample taken from one quite spe~lfic * We have observed the segregation of a number of elements at the surface of native gold particles, but m this paper we restrict dlscusslon to the rat.10 Ag/Au only
16
(from a geologcal pomt of view) deposit, we observed a surprlsmgly high Ag segregation the ratio Cs( Ag/Au) /C” ( Ag/Au) was - 10, which slgnlflcantly exceeds the value for deposit No 1 (Fig 4) havmg approxnnately the same CV(Ag/Au) value The results obtamed provide unambiguous evidence that the mechamsm of surface enrichment of mme gold particles by Ag 1s by no means the same as m the case of Ag/Au alloys Indeed, after remelting the mme gold from deposit No 4, the surface composltlon was found to comclde with that obtained for an alloy havmg the same bulk Ag/Au ratio, 1 e the CS(Ag/Au) value had been reduced more than twofold Some indirect data mdlcatmg shght surface enrichment of mme gold particles by Ag have been reported previously [ 12, 133 , but the methods used were not sufficiently sensltlve to surface concentrations The observed effect of enrichment was relatively small (not exceeding some tens of percent), while the present results show that the Ag concentration at the surface can sometimes be considerably higher than that m the bulk (b) The surface of placer gold (deposits Nos 5, 6, 7a) 1s slightly depleted of Ag Such depletion for gold particles taken from deposits of slm1la.rkmd has been previously reported m the literature [ 11, 14-161 The small magmtude of the effect observed was due to msufflclent sensltn&y of the methods used to the uppermost surface layer It 1s znterestmg to note that the gold par&les from deposit No 5, which IS an open-type placer, exhibit a much smaller surface Ag content than the gold particles from deposit No 6, whch ISa burled-type placer In the latter case the gold particles undergo much weaker mechanical action m the course of mlgratlon This fact seems to be an mdlcatlon of possible mechanical removal of Ag from the surface of gold particles during their migration in a placer deposit (c) The surface of mme gold particles taken from oxldlzed zones (deposits Nos 7b, 8) was found to contam less Ag than the surface of the usual mine gold havmg the same bulk composltlon However, for the sample taken from the oxidized zone of deposit No 10, such a relative depletion of Ag was not observed We also carried out measurements on one sample of secondary gold (deposit No 9) the surface of which was found to be enrxched with Ag Thus, the present experimental results show that for native gold particles the ratio C’(Ag/Au) 1s slgmflcantly dependent on the deposit type, 1 e placer or mme, and m the last case It ISimportant whether or not the sample 1s taken from an oxidized zone The slgnlflcant difference m the surface composltlon of native gold and of Ag/Au alloys havmg slmllar bulk composltlon makes it possible to state defmltely that the phenomena observed cannot be explamed on the basis of simple segregation processes The specific genesis of native gold should be the most important factor determmmg the surface composltlon A dlscusslon of these data from the geological standpoint will be gven m a subsequent paper
17 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
S H Overbury and G A Somoqal, Surf Scl , 55 (1976) 209 R Bouwman, L U Toneman, M A H Boersma and R A van Sunten, Surf Scl , 59 (1976) 72 M Yabumoto, U Watanabe and T Yamasklta, Surf Scl ,77 (1978) 615 G C Nelson, Surf Scl ,59 (1976) 310 M J Helley, D G Swartzfager and V S Sundaram, J Vat Scl Technol , 16 (1979) 664 H Ebel and R Kamer, Wlss Z K -Marx-Umversltat Lelpzlg, 4 (1976) 416 J H Scofield, J Electron Spectrosc Relat Phenom , 8 (1976) 129 M P Seah and W A Dench, Surf Interface Anal , 1 (1978) 2 M Kelley, J Catal, 57 (1979) 113 M P Seah, Surf Scl , 80 (1979) 8 G A Desborough, Econ Geol , 65 (1970) 304 Yu S Berman and V M Novlkov, Kolyma, 2 (1969) 41 N V Petrovskaya, Native Gold, Nauka, Moscow, 1973, p 213 (in Russian) S V Yablokova, Cent Inst Geol Res Works, 79 (1968) 153 (m Russian) A A Saprykm and S V Yablokova, Izv Tomsk Polytekh Inst ,239 (1970) 390 A S Ivoylov, L P Zayalova, V I Llpskaya and V G Barankevlch, Proc 7th AllUnion Conf on Local X-Ray Spectral Analysis and Its Apphcations, Chernogolovka, U S S R , 1979, p 143 (In Russian)