American Mineralogist, Volume61, pages921-926, 1976
Determinationof hydrogenin silicatesusingthe ion beamspectrochemical
analyzer:
applicationto hydrolyticweakening
Icxe,nusS. T. Tsonc, ALEX.C. McLnnBN
Departmentof Physics,Monash Uniuersity
Clayton,Victoria3/,68,Austalia
eNo BRucsE. Hosss
Departmentof Earth Sciences,Monash Uniuersity
Clayton, Victoria 3168,Austalia
Abstract
The ion beamspectrochemical
analyzer(IBSCA), which utilizesthe emittedradiationfrom
sputteredatoms,has beenusedfor the quantitativeanalysisofhydrogencontentin a number
of silicatesamples.An attempt is made to interpret the role of hydrogenin the processof
hydrolyticweakeningin thesematerials.
surfacelayersare
the target surface.As successive
process
the
in
the
of
bombardment,
sputtered
away
Accurateanalysesof hydrogenin the form of water
homogeneous
solid
or
the
of
a
bulk
composition
havebeenobtainedby Wilkinsand Sabine(1973)on
solid can
nominally anhydroussilicatesusing an electrolytic concentrationprofile of an inhomogeneous
quantitative
analyses
of
and
be
analyzed.
Qualitative
method.Information on water contentin minerals
by
method
have
been
described
solids
using
this
can also be obtainedby well-established
techniques
such as differentialthermal analysis/thermogravimet-Tsongand Mclaren (1974,1975).A similarmethod
for the
ric analysis(DTA/TGA) and infrared absorption hasalsobeenusedby Whiteet al. (1972,1974)
purpose.
same
spectroscopy,
but suchinformationis generallyonly
In this paperwe show that IBSCA providesa new
approximate.The possibilityof determininghydrogen in mineralsusingthe ion microprobemassana- method for the detectionof hydrogenin a varietyof
lyzer (IMMA) has recentlybeen demonstratedby silicatesamples.Accuratemeasurementof hydrogen
Hinthornand Andersen(1975).Novelmethodsusing content will enableus to understandmore fully the
nuclearreactionsand proton scatteringwith MeV problemsconcerningthe hydrolytic weakeningof
ion beamsto analyzehydrogenhave beenreported naturaland syntheticmaterials(Griggs1967).
Introduction
by Cohenet al. (1972),Leich and Tombrello(1973),
and Ligeon and Guivarch(1974),but thesecannot
be considered
as instrumentaltechniques
which can
be usedin the laboratoryon a routine basis.Conventional in situ analytical methodssuch as X-ray
fluorescenceand electron probe micro-analysisfail
completelyfor the detectionof hydrogenand other
light elements.
A relatively inexpensiveand yet versatileinstrument, the ion beam spectrochemicalanalyzer
(IBSCA), has beendevelopedto performrapid and
accurateelementalanalysisof solid surfaces.
This is
done by bombarding the surface with a focussed
beamof medium-energy
heavyions and detectingthe
emitted radiation from excitedatoms eiectedfrom
Method
The IBSCA apparatusused for the detectionof
hydrogenin solidsis shown in Figure l. Specpure
argongaswas usedto providethe bombardingions.
ion beam
An intense,stable,and near-monoenergetic
was produced by the duoplasmatronion source.
Whenthe ion sourcewasoperatedat +12 kV, theion
currentdensityat the targetwasabout20 p.A mm-2.
The einzellensfocusedthe beamto a spotof -2 mm
in diameterat the targetsurface.The chargeaccumulation on the nonconductingsurfacewasalleviatedby
insertinga metallicmask with a 2 mm diameterhole
over the target surface.The mask under bombardment actedas a sourceof secondarvelectronswhich
921
TSONG,McLAREN AND HOBBS
GAS
I
V
+ . | 2k v
rt2 kY
LEt{S
I
MULTIPLE
TARGEI HOLDER
V
TO PUHPING
SYSTEII
OIJARTZ
wrN00w
rRrs
LEI{S
INIERFERENCE
FILTERS
Ftc. l. Schematic diagram of the IBSCA apparatus for the determination of hydrogen in solids.
neutralized the charge on the surface of the target.
The background pressurein the vacuum systemwas
-10-? torr. The partial pressureof argon was I X
l0-5 torr when the ion source was in operation.
The multiple target holder can accommodateup to
six samples.For the analysisof hydrogen,one target
was usually an inert (i.e. hydrogen-free)sample such
as pure copper or aluminum. The light collection
optical system utilized interference filters for maximum sensitivity. Two narrow band interference filters (HBW l0 A) were used to isolate the hydrogen
6563 line. If only one filter was used, the intense Li
6707 line from lithium-rich minerals would interfere
with the hydrogen line. A one-to-one image of the
target was formed on the iris by the first lens. The iris
was installed to cut out any stray light which did not
come from the bombarded target. For maximum signal-to-noiseratio, the iris had to be adjustedto about
the same size as the area under bombardment. i.e.. 2
mm in diameter.
In a previous report (Tsong and Mclaren 1975),it
has been shown that quantitative analysisof a particular element,x, can be carried out if a plot of the
intensity, I, us. AM.Z, is constructed from standard
samples.It has also been shown that the results are
independentof the matrix composition of the silicate
minerals.In the presentstudy,l is the intensityof the
H6563 line, AM is the rate of total massloss of the
sampleduring sputteringdeterminedby weighingthe
(MettlerM5/SA) before
samplewith a microbalance
and after l0 minutes'bombardment,and l4ts is the
weightpercentof hydrogenin the solid sample.Figure 2 showsa working curveof (6563) us.A.M.Ws.
The hydrogenconcentrationin thesestandardsampleslies in the rangeof 0.17-0.54weightpercentor
2-ll atomic percent.The hydrogencontentin the
form of water of the mica sampleswas estimated
from DTA/TGA measurementsor from ignition
loss. In the other samples,the hydrogenwas estimated from the chemicalformulaeassumingstoichiometry;the errors involved in thesecould therefore be substantial.Nevertheless,
the plot showsa
reasonable
straightline similar to the plots for the
major elements in feldspars reported previously
(Tsongand Mclaren 1975).
When the inert (hydrogen-free)
samplewas bombarded,a signalcorresponding
to about 0.6 on the
intensityscalein Figure 2 was observed.This was
taken as the backgroundopticalnoiseleveland was
about six timeshigherthan the levelgivenby the dark
currentlimit of the photomultiplier.The intensities
shown in Figure 2 have all been correctedfor this
923
DETERMINATION OF HYDROGEN IN SILICATES
was most striking in quartz crystalsfrom the polycrystallinequartzite.Water was used in the surface
preparation of the quartzite sample. When the
samplewas bombardedin the target chamber,the
light intensitysignalreacheda very high peak before
asseenin Figure3a.However,when
slowlydecaying,
the samplewas heatedfor 48 hours in an oven at
l30oC,bombardmentof the sampleproduceda signal which quicklyroseto an equilibriumvalue(Fig.
3b). From this, it was concludedthat the initial rise
wasdue to water adsorbedon the samplesurface.To
avoid this effect, all sampleswere heat-treatedat
130'C prior to bombardment.We were therefore
analyzingthe content of hydrogenin the bulk of the
silicatesample.
12
/c
3
€o
.:o
c
cc 6
o
o
o
0
0.5
a
A
o
.
6
o
+
X
O
Discussion
MICA
EPIOOTE
cLrNozolslTE
TREMOLITE
PREI{NITE
DATOLITE
r.0
1.5
2.0
AMWH ( pg min-lwt.c/o)
2.5
Ftc. 2. Working curve for determininghydrogenin silicates.
The intensityof the H6563line is plottedagainstthe productofthe
rate of massloss,AM, and the weightpercentof hydrogenin the
material, Ws.
background.If we considerthat the backgroundhydrogensignalwasdue to the residualhydrogenin the
vacuum systempresentas excitedatoms in the ion
beam,then this signalwouldcorrespondto a hydrogen partial pressure
of l0-1' torr, i.e.,about l0 ppm
of the residualgasin thechamberwashydrogen.This
hydrogenin the
is probablyratherhigh, considering
atmosphere
is only about 0.5 ppm.
scatterin the workingcurve(Fig.
The considerable
in hydrogen
2) is due to the unknownuncertainties
concentrationin the standardsamples.This partly
reflectsthe difficulty in making an accurateanalysis
of the hydrogenor water contentof minerals.The
scatterin the IBSCA workingcurveis similarto that
qbtainedby Hinthorn and Andersen(1975) using
IMMA.
The hydrogenbackgroundimposesa lower detection limit to the.irresentwork. If a signal-to-noise
ratio of 2 : I is definedas an acceptablelimit, then
this will correspondto about 160ppm by weightor
3000ppm of atomsof hydrogenin quartz.This compareswell with the detectionlimit of approximately
TesI-B l. Comparison of the hydrogen content in a number of
quartz and silicate samples as determined by IBSCA and IR
absorption spectroscoPY.
IBSCA
(wt.Z hydtogen)
Xo synth.
SI synth.
quartz
quartz
r.
9uarL&
rtrrrrr.
W3 synth.
Results
Using the working curve in Figure 2 the weight
concentrationof hydrogen in a number of natural
and syntheticquartzand silicatesampleswas determined by measuringthe intensityof the H6563line
and the rate of masslossduring sputtering.The resultsare shownin Table l Typicalrateof massloss
in quartz was about 5 ;rg min-l. In some quartz
samplesthe intensitysignalwas found to rise to an
initial peak beforedecayingvery rapidly with time
until an equilibrium value was reached.This effect
o.23
0 ,1 7
0.r3
o.r2
quartz
quartz
tl4 synth.
quartz
Suprasil
synth.
quartz
Vitrosil
natural
(natural)
Smoky quartz
(synth.)
Toyo quartz
Feldspar 190 (An66)
Feldspar
112 (An52)
Mlcroscope
coverslip
ln quartzlte
Gralns
Cent!a1 Australla
0.06-0.18
0.17
0.10
0.06
lR-absorPtlon*
(wt.Z hydrogen)
0.01s0
0. 0100
0.0026
0.0026
0. 0014
Not analysed
0,03
0.12
glass
from
o.22
0 . 0 4 - 0 .1 4
*These values
(1967) and Hobbs et aI.
are taken ftom crlggs
(Prlvate
comunlcation)
(1972).
hovevet, Kirby
Recently,
by a
should be corrected
polnted
out that
these values
o
f
0
.
5
,
factot
TSONG.McLAREN AND HOBBS
of largeOH banding
crystals,because
of the existence
parallelto the growth surface(Morrison-Smithet al.,
in press).
The hydrogencontentof the two feldspars(Table
I ) seemsreasonable,
consideringthat the hydrogenin
nominallyanhydroussilicateshas beenshownto be
as high as 0.08weightpercentwhen measuredby the
electrolytictechnique(Wilkinsand Sabine1973).
Application to hydrolytic weakening
tn
=
c
a
j
o
=
ul
c
a,
c
0246
Time ( mins )
Ftc. 3. Intensityof the H6563line plottedas a functionof time
for quartzite,(a) showin! adsorbedwater on the surfaceand (b)
showingno adsorbedwaterafterthe samplehas beenheat-treated
at 130'Cfor 48 hours.
20@ ppma in IMMA (Hinthorn and Andersen,
1975).Much of the backgroundhydrogencan probably be attributed to the contaminationof hydrocarbonfrom the siliconefluid usedin the oil diffusion
pump, despiteliquid nitrogen trapping. A lower
backgroundmay be expectedif a fluorocarbonfluid
is used as the pump fluid (Holland et al., 1972).
Perhapsthe bestsolutionto this problemwould beto
ion-pumpthe vacuumsystem.
The hydrogenconcentrationin two of the samples,
W4 and quartzite (Table I ), was found to vary in
differentparts of the sample.In the caseof quartzite
this is not surprising,as the quartzcrystalscontain
mica inclusions.Bombardmentof an areawith mica
yields a higher hydrogenconcentrationthan bombardmentof an area free of theseinclusions.In the
case of W4, however,the heterogeneous
hydrogen
contentis thought to be due to inhomogeneityin the
Many workers (Carter et al., 1964;Griggs and
Blacic,1965;Griggs1967;Hobbs,1968;Hobbset al.,
1972: Blacic 1972: and Morrison-Smith et al., in
press)have demonstrateda correlationbetweenthe
mechanicaland recrystallisationbehaviorof silicates
and the hydrogencontentincorporatedin the structure. In general,the higherthe hydrogencontent,the
weakerthe silicateand the easierthe materialrecrystallizesat a particular temperature.All of the work
mentionedhas relied upon infrared absorptionspectroscopyfor the determinationof 'the hydrogencontent. The absorptionspectrumin the vicinity of 3300
wasmeasured,
wavenumbers
and theresultsof Brunner et al. (1961)and Kats et al. (1962)were usedto
interpretthe hydrogencontentin termsof hydrolyzed
silicon-oxygenbridges;the total hydrogen content
was assumedto be incorporatedinto the silicate
structurein the form of Si-OH.HO-Si groups.
Two observationsnow seemto detract from such
(1) Wilkinsand Sabine(1973)have
an interpretation:
made an intensivestudy of hydrogenin nominally
anhydroussilicates.They point out that the infrared
techniqueis not suitablefor quantitativedetermination of hydrogenbecausethe amountof absorptionis
dependentnot only on the crystalplate thicknessbut
alsoon bond orientationand otherstructuralfactors.
Moreover,the presence
of OH bondsmust be confirmed by deuteriumsubstitution,otherwiseother intbrpretationsmay be placed on the structural site
occupiedby the hydrogen.It should perhapsbe emphasizedthat the infraredmethod only measuresthe
concentrationof OH bonds,from which the hydrogen contentis calculated.Thereare other ways,such
as interstitial sites and complicatedpoint defects,
which may be importantmeansof incorporatingthe
hydrogenin quartz.
(2) The IBSCA resultsreported in Table I show
from the infraredresults.Between
largediscrepancies
l0 and 100timesmore hydrogenwasdetectedin the
syntheticqvartz crystalsusing IBSCA than was calculated from the infrared data. This discrepancy
could be due to the problemsin the infrareddetermi-
DETERMINATION OF HYDROGEN IN SILICATES
xo
l0
-'
W4
r0-'
I
t03tT.
of
FIc. 4. Plot of log(H,zSi)versusl/7" (thecriticaltemperature
weakening)for 4 syntheticquartz crystals.The valuesof T" are
taken from Griggs (1967)and Hobbs et al. (1972).An average
valueof 0.12 weightpercentor 7.0 X lO-'zH/Si is taken for W4.
cate)ratherthan water.Thereforeit appearsunlikely
that the hydrogendeterminedby IBSCA originates
from submicronsize bubbles.Moreover, if bubbles
did existin the estimateddensity,the syntheticquartz
would havea milky appearance.
If the IBSCA resultsare taken at facevalueand a
plot is madeto examinethe relation betweenhydrogencontentand the criticaltemperatureof hydrolytic
(similarto Fig.9 in Griggs,1967,and Fig.
weakening
16 in Hobbs et al., 1972),a linear relationshipbetweenthe logarithmichydrogencontent and the reciprocalof the critical temperaturefor weakeningis
(Fig. ). Thusthegenerallinesof argument
preserved
seemnot to
usedin hydrolyticweakeningdiscussions
be alteredby the IBSCA results,but the precisestructural role playedby hydrogenin silicatesnow needs
carefulstudy.
Conclusion
The IBSCA techniquehasprovideda new way of
measuringthe hydrogencontent in quartz and silicates.Whetherthe hydrogenoccursas OH, HzO, or
it with similar effiany other form, IBSCA measures
ciency. The absoluteconcentrationof hydrogen in
syntheticquartz crystalsdeterminedby IBSCA appearsto be much higherthan previousmeasurements
by infrared absorption spectroscopy,and this has
openedup the questionconcerningthe precisestructural role of hydrogenin thesematerials.
Acknowledgments
We thank Messrs.R. G. Turner,A. Vas and R. Horan for many
on the designof IBSCA. We also thank Dr.
useful discussions
K. Norrishof C.S.I.R.O.Divisionof Soilsand Mr. W. Birchof
the National Museum of Victoria for the supplyof the standard
silicatesamples.We are gratefulto Dr. C. Donaldsonfor helpful
commentson the manuscript.
nationmentionedaboveor to unknownerrorsin the
IBSCA technique.Sincefluid inclusionsareknown to
exist in thesematerials,the possibilityremainsthat
much of the excesshydrogenrecordedby the IBSCA
techniquecould be free water in the form of small
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for publication,February27, 1976