American Mineralogist
Vol. 57, pp. 130-136 (1972)
MONATOMIC METAL VAPOR PRESSURESFROM MINERALS
Crrrnr,nsA. Selorrr,l Departmentol GeologicalSciences,
Uniuersity of Wisconsin,,Milwaukee, Wisconsin53201
AsstR{cr
Rellative concentrations of monatomic metal vapors emitted from minerals
at geologically significant temperatures can be measured by using flameless
atomic absorption spectrophotometry. Essentially, the method involves enclosing
the mineral in an evacuated heated quartz cylinder, and measuring the absorbed
portion of a suitable resonance line as it passes through the mineral vapor.
Measurements of monatomic zinc vapors from franklinite ((Fe, Zn) Fe"On),
zincite (ZnO), and willemite (Zn,SiOn), are used to establish their paragenesis.
The method is directly applicable to phase equilibria studies involving elenients with meirsurable monatomic vapors.
The basis for using flamelessatomic absorptionspectrophotometric
minenalvapor pressuremeasurements
in phase equilibria studieslies
in the nature of the measurements
and in the consequences
of chemical
equilibrium.Briefly stated theseare as follows:
1) The chemicalpotential (p) of an elementis identicalin all stable
coexistingphasesat equilibrium.
2) For an iddal gasthe relation betweenchemicalpotential (p) and
vapor,pressure
is as follows:
p
p:polRflnra
where p, and pr.oare the chemical potentials of a pure gas at pressures
P and Po respectively,and at the same temperature T.If Po is chosen
as unity in the same system of units as P, then the above equation
becomesas follows:
p:po*RTInp
It should be noted that P in this equation is dimensionless.
3) Vapor pressuresmeasured by this method are extremely low and
can be treated as "ideal" gases.
4) If more than one vapor component is present, the proportion of
each vapor component is specified at a constant temperature. If this
distribution holds over the temperature interval measured, by monitoring one vapor substance it is possible to get a relative comparison of
' Presentaddress
: ProgramDirector-Geochemistry,
NationalScience
Foundation, Washington,D. C. 20550.
130
131
METAL VAPOR PR.E&SUTES
the chemicalpotential of the speciesin several.minerals-atvanous
All but the alkali elementslisted in Table t have essentemperatures.
tially monatomic vapors. For example,Mann and Tichner (1960)
that zinc vapor aL 520"C
showedby massspectrometricmeasurements
containslessthan 2 x l0-a percentdiatomic rnolecules.At the presmade by the
suresand temperaturesof the zinc vapor measurements
(1963),
percent
less.
Nesmeyanov
diatomic
zinc
is
even
writer, the
of
indicates
pressure
metals,
vapor
data
of
the
alkali
in summarizingthe
that for Li, Na, K, Rb, and Cs, the diatomic speciesare very subordinate (10+ for Li to 10-3for K and Rb), and that these ratios
hold over large temperatureintervals.
5) Monatomic vapors of geologicallyimportant elementsexist at
geologicallysignificant temperatures,and are usable for absorption
measurements(Table 1).
6) The proportionof vapor is so small relative to the solid that the
compositionof the solid can be regardedas constantover the measured
temperatureinterval. I have experimentallydeterminedthe temperaTABI,E
Monatomic
Vapor
Pressures
].
of
Selected
Elements
227oc (N.sm.yanov, 1963)
ELement
Zn
trm IIq
z.ot.to-s
t.sl.to-3
H9
TI
1.oo.to-fo
M9
g.rz-10-7
Ca
z.za.to-Lo
Sr
4.13.10-
Ba
t.zs.ro-1o
Ei
s.eg.to-9
NA
rc
-q
.-4
1ri, s.ez.to-131
1 N a 2z . s o . t o - 6 1
z.oo.!o-2
(r2 z.zt.to-s1
t . 1 0. r o - r
( R b z3 . o o . r o - 4 )
z . + 0 .r o - 1
(cs2 6.50.I0
-A ' )
at
132
CHARLESA. SALOTTI
ture at which the percentadsorptionof solid zinc (99.999percentpurity) equaledthe percent absorptionof zincite measuredal 457"C,
with identical optical instrumentalsettings.The vapor concentrations
can be assumedto be similar.The data of McKinley and Vance (1954)
for the absolutevapor pressureof solid zinc as a function of temperature can be used to estimate the zinc vapor pressureof zincite at
457"C. This value is approximately 10-?mm of mercury. The zinc
vapor pressure,the cell volume, and the temperatureare all known.
Ideal gas conditionscan be assumed,and the amount of zinc present
as vapor relative to that presentin zincite can be approximatelycalculated (stoichiometriczincite is assumed).The ratio of zinc present
as vapor relative to solid zincin zincite (0.100gm sample)is between
1:105and 1:106.It is only becauseof the extremesensitivity in measuring changesin the vapor concentrationsthat any variation from
stoiehiometrycan be sensed.
Vapor pressuremeasurer'nents
of solid ilhasescan be usedto determine metastablerelations betweensolids,especiallyin systemswith
low reaction rates-the vapor pressureof a metastableunivariant
system is always greater than that of the stable univariant system
at the sametemperature.Becausesmall variations in vapor pressure
can be sensitively measuredby absorption, the measurementsoffer a
means of determining phase boundariesquite precisely. Just as a
discontinuity in a cooling curve indicatesa changein the system,a
discontinuity in the vapor pressure curve indicates a change in the
system.If two contemporaneously
formed phasescontain a mutual
element, assumedto represent an independentcomponent,the chemical potential of that elementmust be equalin both phasesat the formational temperature.Becausevapor pressuresare proportional to
chemiealpotentials,the temperatureat which the vapor pressureof
the mutual elementis equal should, if the formational relations are
preserved,indicate the last equilibrium temperature.The data presentedhere for franklinite ((Fe, Zn)FezOE)
/zincite (ZnO)/willemite
(ZnzSiOn)illustrate this potential use.
A custom-built, highly sensitive atomic absorption spectrophotometer (Salotti, 1967) was used to measurethe monatomic zinc vapor
releasedduring heating from franklinite, willemite, and zincite from
SterlingHill, New Jersey.Electronprobeanalysisof all three minerals
showed them to be homogeneouswith respect to zinc and iron. A
tubular furnace with quartz end plates contained the sample.The
mineral was enclosedin a quartz cylinder, flushedwith argon,evacuated, and sealedat approximately lQd atm. A schematic diagram of
ieshownin Figrrre1,
thesystem
133
METAL VAPOR PNE&SUNES
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o
o
0o
.o
>t
a
dO
o5
h
CE
OG
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0
o
3
F
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d
o
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o
N
b0
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o c
o
o
3
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E
a
N
a,
@
o
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sl
oi
a
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d
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6
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c
o
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-o
T.J
ni
r34
CHARLES A. SALOTTI
Using the 2138A zinc line, readingswere taken in percent absorption of the basesignalfor all threemineralsbetween176"C and 464oC.
Percentabsorptionof the basesignal is convertedto absorbance(2 log (I/P) x 100).If measurements
are madeunder similar conditions,
the value of the absorbanceis dependentupon the concentrationof
monatomiczinc in the absorptioncell. The data obtainedfrom these
measurernentsare presented in Table 2. Each temperature datup
point in Table 2 is obtained by approachingthe listed temperature
from two directions at least twice. The readingsso obtained agree
well within the tolerancesimposed by the experimentalsystem.
The log of the absorbanceplotted against the reciprocal of the
absolutetemperaturedefines a straight line for each mineral. The
lines and fheir extensionsdefinedfor franklinite. willemite. and zincihe
TABLE 2
Data Used in
Construction
of Fiqure
2
Zincite
Abs
I
2
3
4
5
6
I
3
4
28
27
29
65
68
80
04
c
311
315
408
409
411
457
458
464
s84
589
681
682
684
730
73L
737
L7.I
17.0
14.7
L4.7
1 4. 6
L3.7
1 3. 7
r3.6
Absorbance
0.0132
o.0177
0.t427
0.1357
0.L487
0. 45s9
o . 4 94 9
0 . 6 99 0
-1
-1
-0
-0
-0
-0
-0
-U
88
75
85
84
83
36
31
16
Franklinite
I
2
3
4
5
L2
2I
34
176
234
28r
341
449
507
554
614
22.3
t9.7
r8.1
1 6. 3
0
0
0
0
0223
0 55 5
r024
1 80 5
-1.65
-L.26
-1.00
-0.74
0
0
0
0
0555
0969
t249
1938
-L.26
-1. 01
-0. 90
-0.71
Willemite
I
2
3
4
T2
20
25
36
319
352
372
446
592
625
645
7]-9
16.9
16.0
r5.5
13.9
135
METAL VAPOR PBE&SUBES
have dissimilar slopesand crosswithin the investigatedtemperature
interval.Theserelationsare shownin Figure 2.
,
The followinginterpretationsare possiblefrom thesemeasurements:
1) Above 445oC,the stability sequenceis willemite-franklinitezincite.
2) At - 445"C,the chemicalpotential for zinc is similar in zincite
and franklinite.
3) Between400'C and 445oC the stability sequenceis willemitezincite-franklinite.
4) At - 400oCthe chemicalpotential for zinc is similar in willemite and zincite.
5) Below 400'C the stability sequenceis zincite-willemite-franklinite.
6) Disregardingpressure,the lowesttemperaturefor zincite (Sterling Hill composition)and silica to react to form willemite (Sterling
Hill composition)is -400oC.
7) With available silica, willemite is stable relative to franklinite
in the temperature interval investigated.The sequenceof mineral
9
13
17
zl
tol/ ro
Frc. 2. PIot of the reciprocal absolute temperature against the log of the
absorbance for franklinite, willemite, and zincite from Sterling Hill, New
Jersey.
136
CHARLES A. SALOTTI
stabilities indicated by their vapor pressurecurves is in agreement
with the parageneticschemededucedby Ries and Bowen (L922)
from a textural study of the SussexCounty ores.
Flamelessatomic absorptionspectrophotometric
mineral vapor pressurestudieshave an immediateusein geochemical
investigationsof ore
deposits,igneousand metamorphicrocks.Applicationsalso exist in the
fieldsof ceramicsand metallurgy.
AcrNowr,nocrunrrrrs
Funds to constmct the custom atomic absorption spectrophotometer were
obtained from the National Science Foundation.
RernnpNcns
MnrrN, K, H., eNn A. W. TrcrrNnn (1960) Measurements of the heats of sublimation of zinc and cadmium. J. Phgs. Chenx. 64,251.
McKrwr,nv, J. D., Sn., eNo J. E. VeNce (1954) Vapor pressureof zinc. J. Ch,em,
Phgs.22, lL20.
NnsrtnreNov, A. N. (1963) Vapor Pressure ol the Chemical Elernents. Elsevier
Publ. Co., New York.
Rrus, f[., awo W. C. Bownr (19]2) Origin of the zinc ores of Sussex County,
N. J. Econ. GeoL tT, 517.
Sar,ortr, C. A. (1967) A high-resolution versatile atomic absorption spectrophotometer. Ga. Acad,.9ci. B'u11.2s,43.
Mantncrtpt recei,ued,,May 17, 1971; accepted lor phlicati,on,
August g, 1m1,