American Mineralogist
Vol. 57, pp. 169-187 (1972)
COMPOSITIONAL VARIATIONS IN SYNTHETIC TOPAZ
Departmentof Geologg
P. E. RosnNrlrlrrc,
W ashingt on State Uniu ersitY
I I 16I
Pullman, W ashi'ngton
Assrnecr
series containing excessAI and tr'.
In the natural series, stable in F-deficient assemblages,topaz hydrolyzes with
XH"O decreasing temperature according to the equation AtSiO-F, *
the fugaon
dependent
are
AI,SiO.F,-,(OH)" + XIIF. Thus, topaz F/OH ratios
cities of I{F and Hro which a,rebuffered by assemblagesof four phases including
vapor,
In the solid solution series stable in F-rich assemblages,Al"* and tro- substitute
for Sia* and O* respectively. Where AI'* is not readily available anion substitutions may be eompensatedfor by tetrahedral vacancies.
Isobarically univariant curves have been determined for topaz in assemblages
with 1) quartz (or corundum), mullite (or pyrophyllite), vapor;2) quartz, AlFs'
,rapor; 3l corundum, AIFa (or ralstonite), vapor' ZtnyiLe appears below 500' C
in silicatdeficient assdmblages.
Application to naturallopaz-bearing assemblagesis limited because natural
systemsare usually open with respectto the vapor phase.
INrnoouctrox
Until recentlyfew attemptshad beenmadeto study F/OH exchange
in the silicatesexperimentally.In the past severalyears a number of
studiesinvolving micas have appeared(e'9.,Noda and Ushio, 1964;
Munoz, 1968; Munoz and Eugster, 1969; Rieder, 1971) but micas
are structurally and compositionally complex and therefore, the
results of these studies have been difrcult to interpret, and in some
casesconflicting.The mineral I'opaz,AlzSiO+(F,OH)z is relatively
simple both compositionallyand structurally and is well suited to
serveas a model for F/O'H exchangein the silicates.
Topazis a diagnosticproductof so-called"pneumatolytic"processes
which are believedto be due to crystallizationfrom and reactionswith
F-rich fluids derived from late-stage,residual magmas or from Fbearing minerals by F/OH exchangeduring regional metamorphism
(Deer, Howie, and Zussman,1962)' Topaz occurs in greisensand
with quartz
pegmatitesassociatedwith granitic rocks in assemblages
169
r70
PHILIP E. NOSENBERG
and other minerals including micas, tourmaline, etc. A knowledgeof
the stability relations of topaz would help to define the range of
physicalconditionsduringthe formationof theseassemblages.
Furthermore, since a wide range in F/OH ratios has been observed(Deer,
Howie, and Zussman,7g62; Ribbe and Rosenberg,1971), the composition of topaz may be a sensitiveindicator of environmentalconditions during its formation.
In 1887Freidel and Sarasin (in Morey and Ingerson, 1gB7) synthesizedLopazby reacting a fluorosilicicacid solutionwith a mixture
of silica and alumina at 500oc. since this first synthesistopaz has
been preparedmany times under a wide range of experimentalconditions using a variety of starting materials. To date these studies
have beenconcernedmainly with the synthesisof topaz (e.g.,MichelLeqy and Wyart, 1946; Schoberand Thilo, 1940; Coess,lgb5) or,
more recently,with other minerals,the synthesisol lopaz beingpurely
incidentalto the main purposeof the study (e.g.,Yoder and Eugsrer,
1955; Althaus, 1966). Despite theseexperimentalstudiesno attempt
has yet beenmade to determinethe stability relations and compositional variations of topaz under a rangeof conditionssimulatingthose
in natural environments.
As a contribution to this problem topaz-bearingassemblages
have
been synthesizedin the quaternary system AlFs-Al2OB-SiOrHzOin
the presenceof excesswater between350o and gb0oClargely at total
pressuresof 2000 bars. Topaz equilibria under these conditions are
believedto provide an adequatemodel for crystallization in some
natural environments.
This paper concernsvariations in the chemicalcompositionof synthetie topaz and their signifieanee.
Detailed discussionof the thermal
stability of topaz-bearingassemblages
in the above system will be
presentedin a later papernow in preparation.
Expngtuowrel Dorerr,s
Procedure
Conventional hydrothermal equipment and techniques were used throughout this study. Most of the experiments were carried out in sealed gold tubes
and cold-seal pr€ssure vessels; gold-lined Morey vessels were used in a few
of the earlier experiments.
fn order to test the influence of starting materials in experimental products,
bulk compositions (Fig. l) were prepared in duplicate, as mechanical mixtures
of reagent grade AI,O" or recrystallized A1(OH)". natural ouartz (Minas Gerais,
Brazil) or Cab-O-Sil (amorphous silica) and AlF" which was used as the source
of F in all mixtures. several compositionswere also prepared by mixing kaolinite
(API No. 9, Mesa Alta, Calif.) and AlF" with enough quartz or ALO", where
necessary,to give the required b,ulk composition. A series of early experiments
t7r
VANIATIONS IN TOPAZ
ArFS
At2o3
mullite
p y r op h y l l i t e
si02
Frc. 1. Anhydrous bulk compositions (mole per cent) of starting materials.
Each composition represents two or more mixtures. Composition triangles show
phase compatibilities in the system with ILO above 600'c. Note that topaz is
represented only by end-member composition.
172
PHILIP E. ROSENBERG
clusion. As a further check on equilibrium the F/OH ratio of topaz produced by
direct synthesis was approached from two directions above 600"C using natural
topaz having hieh aad low F/OH ratios as starting materials. These experiments
will be discussedin detail in a later section.
A series of experiments involving water-free, "dry" compositions resulted in
the encrustation of the samples against the walls of the gold capsules; the capsules were etched and some gold was deposited within the samples. Although
this phenomenon was not investigated, it appears to be due to the formation of
a corrosive fluid which adheres to gold and quenches to an imperfect glassJike
substance. These relations were never observed in the presence of water.
P hase C h,aracteri,zation
After quenching, products were identified by means of their X-ray powder
diffraction patterns using Ni4ltered, Cu-radiation and where possible, by their
optical properties. Topaz and AlF" were characterized by X-ray and optical
measurements and by infra-red speetrophotometry.
Tapaz (021) and (211) X-ray reflections, two of the three invariably observed,
were measured against an internal standard, CdFr, which has a conveniently
located reflectiou at 28.70' Cu Ka. Ralstonite (311)-spacingswere also measured
against this peak. A20 values were obtained by averaging four or more measurements (scan rate, *',/min.) per reflection and are known within 0.005' 24. The
(220)+pacing of AIFg was measured (scan rate, B'/min.) against a CdFs reflection at 56.7' Cu Ka; L20 values are based on averages of two or more measurements and are known to within 0.01' 20.
Since individual topaz crystals could not be resolved under the polarizing microscope, mean refractive indicies ol topaz aggregates were measured in Nalight. Mean
values are significant becauseof the rather low birefringence of topaz (and AIF) and
the relative sensitivity of refractive index to variations in F/OII ratios. Mean values
were also obtained for AlFa refractive indicies, although in many casesit was possible
to resolve individual crystals optically. Measured values have a precision of +.002.
For purposes of comparison with synthetic topaz mean refractive indicies were calculated for a seriesof natural samplesof known composition (Ribbe and Rosenberg,
1971) using the formula ,L : z\/ag-t (Van Valkenburg, 1961).
The relationship of lu and of refractive index to the F-content in natural
topaz have been determined by Ribbe and Rosenberg (1971). Variations in As
and mean refractive index of AlF" as well as Aor of ralstonite with F/OII ratios
are implied from data given in this paper.
Infra-red spectrophotometry was used to study (OH)--stretching bands in
topaz and AIFs. Samples were prepared using conventional KBr pellet techniques
(Lyon, 1967) and run on a Beckman IR-8-infra-red spectrophotometer with a
blank KBr pellet in the reference beam.
ExlnnrusNrAl RESur,TS
ANDTHErRIlrpr,tcerror.rs
AlFs-Vapor Equilibria
AlF3-vaporequilibria can serveas a simplifiedmodel of topaz-vapor
equilibriain the systemAlFa-Al2O3-SiOz-HzO.
X-ray, refractive index, and infra-red data indicate that AlFa hydrolyzeswith decreasingtemperatureby pariial replacementof F- by
I73
VARIATIONSIN TOPAZ
(oH)-. Direct evidenceof hydrolysis is provided by the infra-red
absorptionpattern of AlFg after equilibration with water at 600"c
and zbOobarswhich clearly showsthe presenceof an (OH)--stretching
c)
o
,c,
c,
-o
.,L
o
a
-o'
I
3.O
3.5
Wovelength( ltl
Frc. 2. Infra-red absorption pattern of hydrolyzed AlF"'
4.O
t74
PHILIP E. ROSENBERG
:-
E
7OO
@
J
Y
o
o
c,._.
600
E
,o
o\-i
500
@
4.70
468
466
484
a4r
a
462
- 20s16
L22s(20717r122s1
)
| 56
^l
t52
I 48
t44
-28s16)
A3x(28,o1r1on;1g
13111
Frc. 3. Variations in AtFe AD and ralstonite A-, X_ray measurements witb
temperature. Open circles; topaz with AIF+"(OH)", quartz, vapor (Fig. 1, B and
D). Filled circles; topaz with AIFF,(OE)", alumina, vapor Fig. l, E)-Triangles;
ralstonite replares AIF*,(OH)".
whenAlFs coexistswith topaz, quartz,and vapor (Fig. 3, opencircles).
The maximum hydrolysisof AIFB in the former assemblage
occursat
about,600'C; at lower temperatures(OH)--bearing AlFs inverts to
yalstonite,Al (OH, F) s (pyrochlorestructure), (Fig. B,triangles)which
also hydrolyzeswith decreasingtemperature,as shown by Aerr measurements.The interventionof an additional phase,AlFr(OH) (P. H.
Ribbe, personal communication),over a short temperatureinterval
between600o and 575"C inclusive is neglectedhere but will be discussedin a later paper.
Predictably, mean refractive indicies of (OH)--bearing AlFa increaseprogressivelywith decreasingA22evalues and temperatures,
ranging from 1.379:t .001 for end-memberAlFs to 1.40b* .002 for
hydroxyl-richAlFs.
The hydrolysis of AlF3 may be expressedby the equation,
AlF3 + XII"O:
AIF,-,(OH)"+ XHF.
Topaz, an aluminum fluoride,may be expectedto undergoa similar
hydrolysisgivenby the equation.
Al,SiOnF,+ X H,O : AlzSiO+Fz-"(OH)"
+ X HF.
Thus, the F/oH ratios of AlFa and topaz dependon the fugacitiesof
Htr'and Hso. These fugacitiesare fixed at a given temperatureand
pressuredue to the bufreringcapacity of four-phaseassemblages
(in-
175
VANIATIONS IN TOPAZ
cluding vapor) in the quaternary system'The greaterextent of AlFs
hydrolysis in the presenceof excessalumina (Fig. 3, filled symbols)
than in the presenceof excesssilica (Fig. 3, open symbols) suggests
The reaction
that lnr is lower in alumina-rich assemblages.
SiO,+4HF:SiFn+2H,O
buffersthe fugacity of SiFein the system.
Compositional Variabikty o! T opaz
The relationshipbetweer Aozr and the F-content of topaz (Ribbe
and Rosenberg,1971) can be assumedto hold only for synthetic
samplesapproximatingnatural compositions.Early in this study, it
was observedthat many synthetic topaz sampleshave mean refractive indicieslower than that of the F-rich end-member.Two distinct
seriesof topaz compositionscan be distinguishedby comparingmean
refractive indicies wth Ae21values (Fig. a). Synthetic topaz coexisting with quartz,mullite, and vapor (Fig. 4, open circles) approaches
of natural topaz (Fig. 4, small filled
the line basedon measurernents
circles). For these samplesthe linear relationshipbetweenAozr and
F-content,(Ribbe and Rosenberg,1971) is believedto hold; they are
referredto hereasthe t'natural" lopaz series'
In samples synthesized "dty" from end-member composition,
3 roz
,o
H
E
160
c
3 rse
''-olq
o7B
o8z
086
o9o
o94
098
l,oe
106
ll8
tzo
tz4
tza
- 2eg6)
Ae2, (20e21
Frc. 4. Variation
of mea.n refra,ctive indices rvith Am X-ray
measurements
of
176
PHILIP
E. ROSENBERG
Al2SiO4F2,(Fig. 4, small, filled squares)quartz coexistswith topaz
which is characterizedby slightly expandedlattice d.imensionsand
Iowet mean refractive indicies than that of the end-member.This
topaz is thought to representa solid solution seriesresulting from
the coupledsubstitution of Al3* and F1- for Sia*and Or- and having
the generalformula
Al, (sir_"Al"on_"F,)
Fr.
An extrapolationof the line drawn through thesedata points (Fig. 4,
lower line) intersectsthe natural topaz seriesat fluorotopazcomposition, AI2SiOaF2being the end-memberof both series.
Topaz synthesizedin the presenceof excessAlFe and water (Fig
4, open squaresand triangles) lies closeto the lower line representing
hydroxyl-free topaz solid solutions and, therefore, belongs to this
series.Ae21valuesfor thesetopaz samplesare a measureof the exten.t
of substitutionof Als* and F1- for Sia*and Or- rather than (OH)- for
F-.
The two seriesin Figure 4 representextremesin the meanrefractive
indicies of topaz; numerous intermediateshave been synthesized.
Where AlFs or its hydrolysis products (e.g.,ralstonite) are absent,
topaz compositionsin zunyite-bearing assemblages(Fig. 4, filled
circles) are believedto representhydroxyl-bearingintermediatesolid
solutionswhile in alumina-rich zunyite-freeassemblages
topaz compositions (Fig. 4, filled triangles) are hydrolyzedmembersof the solid
solution series (see Fig. 6) . The extent of the latter seriesis much
greaterthan that of the end-membersdue to the combinedeffectsof
coupledsubstitutionand hydrolysis.
Infra-red absorptionpatterns of typical samplesfrom each series
are shown in Figure 5. Sharp (OH)--stretching bands are observed
for syntheticsamplesapproximatingnatural topaz compositions(Fig.
5, 1315); these bands are indistinguishablefrorn those obtained for
natural Lopaz.Broadeningis observedin the (OH)--stretching band
of hydroxyl-bearingtopaz solid solutions (Fig. 5, 1321; Fig. 4, filled
triangles) suggestingan unresolveddoublet which may reflect the
substitution of (OH)- for F- in two 'dissimilar structural sites.
Although Figure 4 clearly defi.nesthe two compositionalseries,refractive index measurementswere not possible for a great many
samplesparticularly those synthesizedat lower temperatures.The
two seriescan.alsobe distinguishedon the basisof 4621and 4211X-ray
measurements(Fig. 6), which, in general,confirm the relationships
observedin Figure 4.'Since these parameterswere measurablefor
virtually every topaz sample,the entire range of compositioris
synthe-
177
IN TOPAZ
VANIATIONS
sized in this study can be illustrated. samples 1315 and l32l arc
locatedon this diagram.
The natural topaz series(Fig. 6) is definedby the solid line fepresenting A values (correlation coefficient -.963) calculated from
unit-cell dimensionsof fourteen natural samples (Rosenberg,1967)
and, assumingthat the linear relationshipis maintainedfor topaz with
lower F/OH ratios, by its dashed extension.A values obtained for
o
()
C,
o
-o
o
U'
-o
I
2.5
3.O
3.5
4.O
Wovelength( t.l-)
Frc. 5. Infra-red absorption patterns of synthetic topaz. $ample !315, topaz
rrith quartz, mullite, vapor (Fig. 1, A); sample 1321,topaz with corundum'
vapor (Fig. 1, C).
r78
PHILIP
E. ROSENBERG
%o^'
A-
Ar
AA
A
z
A
A
A
88
92
96
tOO
lO4
I08
|2
[6
120
t?4
t28
t32
Lo? (20a2t-?e4d)
synthetictopaz coexistingwith quartz,mullite (or pyrophyllite), and
vapor (tr'ig.6, open circles) eloselyapproachthis line. The gap in the
synthetic seriesis due to the relatively constant A values obtained
for topaz in pyrophyllite-bearingassemblages.
Equilibria involving
pyrophyllite are not fully understoodat present and are currently
underfurther investigation.
A valuesfor topaz synthesizedin the absenceof water (Fig. 6, filled
Their higher refractive indicies (Fig. 4, filled triangles) suggeststhat
they are hydroxyl-bearing.In agreementwith relationshipsshown in
VARIATIONS IN TOPAZ
17s
Figure 4, A valuesof topaz in quartz-freezunyite-bearingassemblages
(Fig.6, filled circlesand triangles) appearto representhydroxyl-bearing intermediatesolid solutions.
The topaz solid solutionseriescrossesthe natural topaz series(Fig.
6) ratherthan meetingit at end-memb
er Lopazcomposition(Al2SiOrFz).
A values lying abovethose of the natural topaz seriesand representing decreasedlattice dimensionsare believedto be due to topaz solid
solutionsin which the substitution of F'- for O'- is compensatedby
vacanciesin Sia. lattice sites. These defect solid solutions have the
generalformula
Fr.
Alr(sil _"/4o4_,F,)
Similar coupledsubstitutionshave beenreportedin the humites (Jones,
Ribbe, and Gibbs, 1969) which are structurally analogousto topaz.
Topaz synthesizedin the presenceof excessA1F3is apparently always a solid solution.Defect solid solutionsare crystallizedwhen insufficient alumina is available (SiOz-rich compositions) to form
"normal" Al-rich topaz solid solutions.A valuesfor stabledefectsolid
solutionsplot closeto the intersectionof the natural and solid solution seriesin Figure 6. The extent of this solid solutionmay be metastably enhancedby use of starting materials containingan unreactive
form of alumina, *-Al2O3,to give A values lying above those of the
naturaltopazseries(Fig.6).
Tnn Ennncr op Tpltpnnerunn
Natwal Topaz Series
For the natural topaz series, synthesized from compositions low in
AlF3, Ae21values are a measure of the hydrolysis of. lopaz which is
shown by the progressive change in these values with temperature
(Fig. 7). For assemblagesof four phases including vapor the relationship is isobarically univariant, all values lying close to a single curve.
Silica-rich compositions (Fig. 7, open circles) produce the assemblage
Lopaz, quartz, mullite, vapor above 500'C and topaz, quartz, pyrophyllite, vapor at and below 500'C. Alumina-rich compositions (Fig.
7, filled circles) yield the assemblaget'opaz, corundum, mullite, vapor
above 650oC; mullite is weak below 700'C and absent below 650'C.
corundum is replaced by AIzOs-form KIL (Torkar, 1960) below 600'C
except where o-Al2Os was used as a starting material. Zunyile appears
below 500'C in assemblageswithout cluartz.
' SameasAS(H)-l (Arama^ki
andRoy,1963).
180
PHILIP
E. NOSENBERG
Ae21rtr€&surements
for topaz synthesizedfrom alumina-rich and
from silica-rich compositionsare quite similat aL any given temperature. Although there is no reason to expect the temperature dependenceof F/OH ratios to be the samein both cases,presentdata cannot
resolve a difference.Predictable changesin slope with the appearanceof pyrophyllite and zunyite are also unresolved.Topaz in several
pyrophyllite-bearingassemblages
lies below the trend in Figure 7,
apparently due to disequilibriumbetweenpyrophyllite and the vapor
phase.Due to the paucity of data, the curve for silica-rich compositions is difficult to trace below 450oC.Topaz in zunyite-bearingassemblagesis an intermediatesolid solution (seeFigs. 4 and O).
The F/OH ratio of topaz proved to be highly refractory and attempts to approachthe curve (Fig. 7) from both directionsmet with
limited success.Significantchangesin Ae21(and a211)were observed
using natural lopaz mixed with a small amount of kaolinite and
reactedatr 750",700o,and 625' (Fig. 7, arrows), to give the assemblage topaz, quartz, mullite, and vapor. Natural topaz from Topaz
Mountain, Utah (aor, - .T6E)which is nearly (OH)--free (penfield
and Minor, 1894; Ribbe and Rosenberg,1971) and ChesterfieldCo.,
800
I
too
o
:
ooo
fi
a
E
o
F
5oo
400
300
o14
078
082
086
090
094
098
ro2
106
ilo
4
- 20.16)
aor,(26e2,
,trlc. 7. Variation of Am. with temperature for the natural topaz series. Open
circles; topaz with qttarLz,mullite (or pyrophyllite), vapor (Fig. 1, A and F).
Filled circles; topaz with corundum, mullite (or zunyite) vapor (bulk composition; mole per cent; AlF8 6, AlaOa53, SiO" 41). Arrows; reversal experiments,see
text.
VARIATIONSIN TOPAZ
181
South Carolina (4621= .890), the most hydroxyl-rich topaz ever reported (Pardee,Glass,and Stevens,1937) were used as starting maierials. .4621(and 4211)for thesesampleswas found to changein the
direction of the values obtained by direct synthesisunder the same
experimentalconditions (Fig. 7, arrows) suggestingthat a closeapproach to equilibrium has been attained at least at higher temperatures. Similar attempts to reversethe hydrolysis of topaz at lower
ul.
temperatureswereunsuccessf
In view of theseresults,the hydrolysis of the natural topaz series,
as shown by the curve in Figure 7 is regarded as well-established
above 600"C and as an approximationbelow 500"C. The extent of
(Offl- substitution for F- in synthetic samplesmay be estimated by
extrapolatingthe linear relationship between4621and F-eontent established by Ribbe and Rosenberg(1971) for natural samples.The synthetic series extends beyond the maximum subsiitution reported in
nature, (Azor= .890,15.7wb. percentF; Ribbe and Rosenberg,1971)
to a 4621value of 1.130 (7.5 wt. percentF) at 350"C with no indication that this representsthe maximum substitution possible in the
topaz structure.
TopazSokd SoluttonSeries
For the topaz solid solution series,synthesizedin the presenceof
excessAlF3, Ae21values are a measureof the extent of the coupled
substitution of AI3* for Sia*and F'- for O'-. The temperature dependenceof A621values observedin "dry" experimentsand in experiments
with excessalumina, AlF3, and water are shownin Figure 8 (squares
and circles respectively).These measurementslie close to a single
curve which representsincreasingsolid solution with decreasingtemperature.Topaz solid solutionssynthesizedin the presenceof excess
AlFe and water are hydroxyl-free at temperatures at least as low as
500.C since Ae21and refractive index measurementsfor these solid
solutions eoincidewith those of the "dry" seriesat and above this
temperature(Fig. a).
"Dry" experimentsusing end-membertopaz composition(AlzS'iO+Fz)
yielded topaz and q'dtartz;thus topaz compositionlies in the triangle
Al2sio4F2-Alzoa-AlFs(Fig. 1). In the presenceof excesswater, bulk
compositionscontainingexcessAl2O3and AlFs yield the assemblage
topaz solid solution,corundum,AlFs-,(OH), and vapor above570oC,
the extent of solid solution in topaz and the degreeof hydrolysis of
AlFa (Fig. 3) increasingwith decreasingtemperature.At lower temperaturesralstonite appearsin place of AlFa*(OH)' and hydrolyzes
with decreasingtemperature (Fig. 3). The presenceof zunyite below
182
PHILIP E. ROSENBERG
O
o
7OO
E
o3.
ii. 3.
400
086
090
094
098
tO?
| 06
il4
il8
t22
t26
Aq21(20621- 20516)
Frc. 8. Variation of Aq with temperature for the topaz solid solution series in
the presence of excessalumina. Filled squares; topaz "dry" experiments (Fig. 1,
topaz comp.). Open circles; topaz with alumnia AIF*,(OH)", vapor (Fig 1, E).
Filled circles; topaz with alumina, ralstonite, vapor (Fig. l,E). Z; zunyite present. Arrows; reversal experiments, see text.
500'C is dependenton the nature of starting materials. Bulk compositionsmadewith o-AlzOsgavethe assemblage
topaz solid solution,
ralstonite,corundum,and vapor (Fig. 8, filled circles) whereastopaz
solid solution, ralstonite, and zunyite were synthesizedfrom the
samebulk compositions
usingAl (OH)s (Fig. 8, filled circleswilh "Z") .
The refractory nature of c-Al2O3probably accountsfor this discrepancy which does affect the compositionof coexistingtopaz to some
degree (see Fig. 6). Diaspore replacedcorundum in several experimentsat 400"Cwhich arenot shownin Figure8.
Topaz solid solutions coexisting with corundum and vapor are
partially hydrolyzed; they appear as intermediatesbetweenthe two
compositionalseriesin Figure 4 (filled triangles) but lie closeto the
hydroxyl-freeLopazsolid solutionseriesin Figures6 and 8. The curve
in Figure 8 was approachedfrom two directionsat 700"C by starting
with mixtures of natural topaz (Topaz Mountain, Utah; As21= .Z6b
and ChesterfieldCo. S.C.; Aozr= .890) alumina,AlF3, and excess
water, (Fig. 8, arrows). Similar attempts at lower temperatureswere
not successful.
The temperature dependenceof Lopaz As21values for the assemblages topaz solid solution, quafi,z,AlFa_,(OH)a7 ydpot and topaz
solid solution,AlFs
vapor is shownin Figure g (open circles
"(OH),,
1&3
VABIATIONS IN TOPAZ
also lies
and fllled circlesrespectively).Topaz in these assemblages
(Fig.
triangles)
4,
open
on the curve for anhydrous solid solutions
and is, therefore,hydroxyl-free.Although a similar temperaturedependenceexists,the extent of topaz solid solution and the degreeof
topaz
hydrolysisof AlFs (Fig. 2) are both lessthan for the assemblage
(Fig.
solution
8). Solid
solid solution,corundum,AIFB-,(OH),, vapor
is probably limited by the absenceof free alumina; AIs* is not readily
available to replaceSin*.This explanationis in accordwith the tendency.tocompensate
for anion substitutionsby creationof tetrahedral
vacanciessuggestedby ,Azrrmeasurements.
Solid solution is slightly
more extensivewhen topaz coexistsonly with AlFg-"(OH)" and vapor
(Fig. 9, filled circles); topaz in this assemblageis hydroxyl-free to
temperaturesat least as low as 425"C. Due to the lesserdegreeof
AlF3 hydrolysis,ralstonite is presentonly at 400'C.
Attempts were madeto approachthe curve for topaz solid solutions
coexistingwith AlFs-,(OH), and vapor from two directionsby starting with natural topaz, AlF3, and excesswater. Although this procedurewas partially successful
usingtopaz from ChesterfieldCo., S. G.
o
an)
it
o
7nn
-@l
600
l+.t\
p
f
o
&
)
(D
o
s00
H
o
400
o.74
078
082
086
090
094
098
toz
106
llo
A.ozt(20o"1-20116)
Frc. 9. Variation of Aa with temperature for the topaz solid solution series
with and without excessquartz. Open circles; topaz with quartz AIF*"(OH),,
vapor (Fig. 1, B and D). FiIIed circles; topaz and AIF-"(OH),, vapor (Fig. l, G)'
II; ralstonite replacesAt "(OH)". Arrows; reversal experiments,see text.
184
PHILIP E. ROSENBERG
(Fig. 9, right arrows) no changesin 4621of.Lopazfrom Topaz Mquntain, Utah, were observed.t'Reversalsttwere obtained when concentrated HF was sealedquickly into gold tubescontainingthe latter material; the assemblagetopaz solid solution, AlFs-,(OH)n, vapor was
observedin every case(Fig. 9 left arrows).
THn Ernncr or Pnnssunn
Sincethe'substitution of (OH)- for F- in the natural topaz series
and the coripled substitution of Al3* for Sin. and tr'1- for O'- in the
topaz solid solution seriesare both accompaniedby an increasein
volume,the extent of thesesubstitutionsshould be somewhatgreater
at lower pressuresthan at higher pressures.
The tendencyto compensate for anion substitutionsby the creation of tetrahedral vacancies
in the topaz solid solutionseriesmay be enhancedat higher pressures
due to the smallervolumesof thesesolid solutions.
Although no attempt was made to study the effect of pressureon
compositionalvariations in topaz, the assemblage
topaz, quartz, mullite, vapor was synthesizedat 1000bars in a few experimentsbetween
800"C and 600"C. Within the precisionof the measurements,
topaz
Ao21valueswere the sameat 1000bars as at 2000bars.
Assplrsr,ecus
Rnr,errox ro NATTTRAT,
Compositionalvariations of topaz discussedin this paper apply
strictly only to alkali-free assemblages
whereasmost natural topazbearingassemblages
contain significantamountsof alkalis; mica, for
example,is an important constituentof greisens.
However,preliminary
studies (Rosenberg,1969,and unpublisheddata) suggestthat small
amountsof K (-5 wt. percentKzO) have little effecton topaz-vapor
equilibria. Since topaz compositionsin synthetic and natural assemblages are comparable,the univariant curve for the natural topaz
serieswould appearto have applicationsto geothermometry.
The compositionof topaz is determinedby the fugacitiesof HF and
H2O, pressure,and temperature.Experimental gas fugacities were
internally buffered by stoichiometricsolid phases (quartz, mullite,
etc.); these fugacitiesare fixed at a given temperatureand pressure
for assemblages
of four phases including vapor. Preliminary data
suggestthat a changein pressureby a factor of two has little effect
on topaz-vapor equilibria. However, natural topaz-bearing assemblagestypically lack internal buffer assemblages
as indicated by the
predominanceof solid phaseswith variable F/OH ratios in greisen
(e.9.,Lopaz,mica, tourmaline,apatite, etc.) (Rieder,1971).Thus, natural greisen assemblages(e.g., qlafiz, muscovite, topaz) usually
VARIATIONS IN TOPAZ
1&5
crystallizein a systemthat is open with respectto the vapor phase,
the fugacity of HF being imposedby an externalsource.Under these
conditionsno simplerelationshipbetweentopaz compositionand temperature is to be expectedand the experimentallydeterminedunivariant curve for the natural topaz seriesis not applicable.
Despite this seriouslimitation temperatureestimatesmay be poswith bulk compositions
sible in some cases.F-buffered assemblages
(lopaz;
quartz;
andalusite,silliapproachingthe quaternary system
manite, or kyanite) have been found in naturel for example,Lopaz,
quarLz,sillimanite assemblages
have been reported from Tanzania
(Kempe, 1967) and Evergreen,Colorado (Sheridanet aI., 1968).
Neglectingthe probableeffectsof pressure,topazF/OH ratios in these
assemblages
suggesttemperaturesof 850'C (Aozr= .789) and 750'C
(Aozr: .826) respectively'Thesetemperaturesmust be regardedas
approximationsbecausepressuresduring the,crystallizalton of. Lopaz
may have greatly exceeded
2000bars.While riiost natural systemsappear to have beenopenwith respectto the vapor phaseduring fluorine
may have
topazin someassemblages
metasomatismand greisenization,
crystallizedunder closedsystemconditions.
It would be of considerableirrterestto know the experimentalgas
fugacitiesin order to reconstructthe compositionof the natural vapor
phasein equilibriumwith F-bufferedassemblages.
Unfortunately,this
information cannot be obtainedfrom the experimentaldata reported
here. The buffer method (Munoz and Eugster, 1969) offers the best
potential approach,although the range of HF fugacities'established
by buffersthat have beencalibratedto date is probably too low to be
of value in studyingtopaz-vaporequilibria.
Topaz solid sdhitionshave not been reported in nature but they
may exist whdrebulk compositionsare unusuallyhigh in fluorine and'
Iow in alkalis. For example,l,opaz coexistingwith ralstoniteshouldbe
a memberof the solid solution series.This associationhas never been
report-edin nature to the author'sknowledgebut both minerals have
been found in the vicinicity of St. Peter'sDome in Colorado (Cross
and Hillebrand, 1885). It is possiblethat natural intermediatesbetween the two serieshave been describedbut not recognizedin the
literature because,typically, insufficieirtdata are provided for their
characterization.
Acr<Nowr,nDcunNrs
Financial support was provided by National Science Foundation Grants GA1161and GA-4483. The technical assistanceof Mrs. Albina. Mellott was a.valuable
contribution to this study. D. R. C. Kempe of the British Museum of Na'tural
History kindly provided a sample of topa,zfrom Tanzania.
I86
PHILIP E. ROSENBERG
RnrpnnNcns
Ar,rneus, EcoN (1966) Stability range of pyrophyllite under the effect of acids.
I. Experimental study. aontrib. Mineral. Petrologg 13, 31-50.
An.trrenr, S., axr R. Ror (1963) A new polymorph of AI,SiOu and further studies
in the system ALOrSiOrH,O. Amer. Mineral. 48, 1322-1347.
Conss,L. Jn. (1955) High Pressureminerals.J. Amer. Ceratn. Soc.38,298.
Cnoss,W., eltn W. F. Ilrr,r,ssRANo(1885) Contributions to the mineralogy of the
Rocky Mountains. U. S. GeoI.&uru. Bull.20,40-74.
Dnnn, W. A., R. A. Howre, exn J. Zussverv (1962)Rock-t,ormingMinerals, VoI. I.
Wiley, New York, L45-150.
Jor.rns, N. W., P. tr. Rrnnn, eNn G. V. Grsss (1969) Crystal chemistry of the
humite minerals. Amer. M'ineral. 54,391-411.
Knrrrn, D. R. C. (1967) Some topaz-, sillimanite-, and kyanite-bea.ringrocks from
Tanzania. M ineral. M ag. 36, 515-521.
Lrow, R. J. P. (1967) Infrared absorption spectroscopy.J. Zuusman, ed. Phgsical
Method,s in Determi,natiue M'i,neralogy, 371403. Acamedic Press, New York.
Mrcuor,-Lnvr, A., eNn J, Wyenr (1946) Reproduction synthetique de la topaze, de
la cryolite et du corindon. BulI. 9oc. France Mi,neral.69, 156-161.
Monw, G. W., eNo Eenr, INcnnsox (1937) The pneumatolytic and hydrothermal
alteration and synthesis of silicates. Econ. GeoX.32, ffi7-760.
Muxoz, J. L. (1968) Physical properties of synthetic lepidolites. Amer. Mi,neral.
s3, 1490-1512.
aND E. P. Eucsrnn (1979) Control of fugacities in fluorine-bearing hydrothermal systems.Amer. Mineral. 54, 943-959.
Noue, T., exn M. Usuro (1964) Hydrothermal synthesis of fluorine-hydroxylphlogopite; Part II. Relationship between the fluorine content, lattice constants and the conditions of synthesis of fluorine-hydroxyl-phlogopite. "/.
Chem. Soc.Japan, Ind. Chem. Sec. 67,292-297(Transl.: Geochem.Internat.
1,96-104(1964)).
Pennno,D. T., J. J. Gr,ess,ewn R. E. SnnvnNs(1937) Massive low fluorine topaz
from the Brewer Mine, S. Carolina.Amer. Mineral. 22,105&-1064.
PnNrrnr,n, S. L., eNo J. C. Mrwon, Jn., (1894) On the chemical composition and
related physical properties of topaz. Amer. J. Sci.,9er.3.47,387-396.
Rrnnn, P. II., eNo P. E. Rosnwnanc (1971) Optical and X-ray determinative
methods for fluorine in topaz. Am,er.Minerar. 56, 1812-1821.
Rnmnn, Mu,eN (1971) Stability and physical properties of synthetic lithium-iron
micas.Amer. M i,ner.al. 56,256-l2n.
RosnNnonc,P. E. (1967) Variations in the unit-cell dimensions ol topaz and their
significance.Amer. Minerar. 52, 1890-1895.
(1969) Topaz-vapor equilibria in the presenceof excesssilica a,nd water.
GeoI. Soc.Amer. Ann. Meet., Prog. Abstr.,192-193.
Scuonnn,R., eNn E. Tnrr,o (1940) ChemischelJntersuchungenvon Silicaten, X
Mitteil, Uber den iopaz, ALSiO.(F,OII,O), und seine synthese iiber ein
neues fluorhaligesAluminum Oxyd. Ber. Deutsch. Chem. Gesell. 37B, L2lgL24:0.
Snnnoex, D. M, R. B. Tevr,on,eNo S. P. Mensrr (1968) Rutile and topaz in
Precambrian gneissJefferson and Clear Creek Counties, Colorado, U. S. Geol.
Suru.Circ.567.
VARIATIONS
IN TOPAZ
187
Torrn, K. (1960) Untersuchungen uber Aluminumhyroxyde und Oxyde 4. Mitt.
M onatsh..C hem. 9L, 658-668.
VaN VelrcnNsunc,A. (1961) Synthesis of the humites, nMgrSiOr ' Me(F,OH)e.
J. Res.Nat. Bur. Stand. Phus. Clt'em.65A,415-428.
Yonnn, E. S., eNn II. P. Eucsmn (1955) Synthetic and natural muscovites. Geochem. Cosmochim. Acta 8, 225-2&.
Mantncri,pt recei,uedJune 7, 1911; accepted lor publi'cation, Jufu 30, lgll-