653
Canodian Minerologist
Vol. 30, pp.653-662 (1992)
THE CRYSTALSTRUCTUREOF ALUMOTANTITE:
ITS RELATIONTO THE STRUCTURESOF SIMPSONITE
COMPOUNDS
AND THE (Al,Ga)(Ta,Nb)O.
T. SCOTTERCITI, FRANK C. HAWTHORNEENNPETRTERNY
Department of Geological Sciences,University of Manitobo, Winnipeg, Manitoba R3T 2N2
AssrRAct
Two new occurrencesof alumotantite, AlTaOa, have been found: the Bikita pegmatite,Zimbabwe and the Alto
do Giz pegmatite, Brazil. Alumotantite is biaxial negative witha2V of 66(2)', {r(calc) = 2.21, A new cell with a
gluingi D(calc)of 1 .47 g/cm3.The
4.473(l),, I 1.308(4),c 4J75(l) A,, Z = 4, spacegroip Pbcn has beenassigned,
crystal structure of alumotantite from Brazil has been solved by Patterson and Fourier techniques and refined by a
full-matrix least-squaresmethod to an R index of 4.4t/o for 284 observed(3o) reflections.Oxygenatoms are nearly
hexagonallyclosest-packed;cations occupy octahedralinterstices.TaO6 octahedracorner-link to form corrugated,
perforated sheetsnormal to ts Sheetsare linked via edgesto ag-zag chainsof edge-sharingAlO5 octahedrarunning
parallel to Z. A topotaxic relationship betweenalumotantite and parent simpsoniteis observedfor samplesfrom
Sikita. this reaction involves cation migxation through an undisturbedhexagonallyclosest-packedarray of oxygen
of late pegmatitic fluids. A comparisonof the structure and inferred
atoms and is stimulated by an increasein p1u
'chemically
similar .ABOacompoundssuggeststhat alumotantite is the
field of stability of alumotantite to those of
stablelow-temperaturepolymorph of AlTaOa.
Keywords: alumotantite, crystal stnrcture, mineral data, simpsonite,topotttxy, tantalum oxide, granitic pegmatite,
Zimbabwe. Brazil.
SOMMAIRE
Nous avons d€couvertdeux nouveauxexemplesd'alumotantite, AlTaOa, dansles pegmatitesgranitiquesde Bikita,
au Zimbabwe, et Alto do Giz, au Br6sil. Ce min6ral est biaxe n6gatif; l'angle 2V est^de66Q)', et I'indice r calcul€
est de 2.21.Nous proposonsune nouvellemaille:a 4.473(l),, ll.30S(4),c 4.775(L)A,Z = 4, groupespatialPbcn,
ce qui mbnei une densit6calculdede 7.47. La structuregd5lalringde I'alumotantite brdsiliennea 6t6 d6chiffr6eavec
les techniquesde Patl'ersonet de Fourier et affinde par moindrescarr€savec matrice entibre,jusqu'ir un r6sidu R de
4.4o/o pour 284 reflexions observdes(3o). Les atomes d'oxygene d6finissent un agencementcompact quasiment
hexagonal,avec les cations dans les intersticesoctaddriques.Les octaddresTaO5 se joignent d leurs sommetspour
d6finir des feuillets ondulants, et perfords perpendiculaires d ). Les feuillets sont connectespar les arOteslaterales
pour former des chalnesen zig-zag d'octaddres AlO5 i arOtespartag6esparallbles d Z. Une relation topotaxique existe
entrealumotantiteet simpsonite,son prdcurseur,dansles dchantillonsde Bikita. Cetterdactionimplique une migration
des cations d travers un agencementcompact hexagonal intact d'atomes d'oxygtne, et serait stimulde par une
augmentationen pTadans la phasefluide tardive. D'aprds une comparaisonde la structure et du champ de stabilit6
de l'alumotantite avec ceux de compos€s.4BOa semblables,l'alumotantite serait la forme polymorphique de basse
temp€rature de AlTaOa.
(Traduit par la Rddaction)
Mots-clds: alumotantite, structure cristalline, donn6es min6ralogiques,simpsonite, topotaxie, oxyde de tantale,
pegmatitegranitique, Zimbabwe, Br6sil.
INTRoDUCTIoN
Alumotantite, AlTaOa, was first describedas a
new mineral speciesby Voloshin et al. (1981); it
occursin undssignatedgranitic pegmatitesfrom the
lPresentaddress:ResearchDivision, Canadian Museum
of Nature, Ottawa, Ontario KIP 6P4.
Kola Peninsula. At about this time, during a
systematicstudy of simpsonite,we encountereda
mineral resembling alumotantite from the Bikita
pegmatite, Zimbabwe. A literature review reveals
that this mineral had actually been encountered35
years earlier by Macgregor (1946). Our electronmicroprobe data and X-ray-diffraction studies
confirm it as alumotantite. Close inspection of
other samplesprocured for the simpsonite study
revealsa third locality for alumotantite, Soledad,
6s4
TTiE CANADIAN MINERALOCIST
Brazil. On the basis of geographic,mineralogical
and geochemicalinferences,we suggestthat this
samplemust have come from the nearby Alto do
Giz pegmatite,Rio Grande do Norte, Brazil.
OccunnnNcr ANDPARAGENEsTs
Alumotantite occurs exclusively in extremely
fractionated rare-element-enrichedgranitic pegmatites.At two of the occurrences,alumotantiteis
a constituentof albite-rich assemblages
of the host
pegmatites.At the other occurrence,Alto do Giz,
the silicatematrix (to the alumotantite)is completely kaolinized; thus the original assemblageis
unknown.
The Bikita occurrence consists of a single
albite-rich band 0.3 to 0.5 m thick, underlying a
quartz pod in the Mdara mine workings(Macgregor
1946).In our sample,the alumotantiteis associated
with simpsonite, manganoan tapiolite, manganotantalite,zirconianhafnon, apatite, albite and
muscovite. Simpsonite, the most abundant oxide
mineral of this association, was the first oxide
mineral to crystallize.Alumotantite and the other
oxide minerals occur as replacementsof, and
overgrofihs on, the simpsonite.
The Alto do Giz pegmatiteundoubtedly representsthe best and perhapsalso the most abundant
occurrenceof alumotantite; however,it is also the
leastunderstoodone from a parageneticviewpoint.
In addition to problems associated with the
extensivealteration, the geneticinterpretationsare
further obscured by the fact that at the Alto do
Giz pegmatite,Ta rnineralswere never observedjn
situ by qualified persons(Pough 1945).However,
becauseof the good preservation and intimate
association of the oxide minerals, paragenetic
relations amongst these minerals can still be
inferred. Alumotantite is in direct associationwith
simpsonite, manganotantalite, microlite, parabariomicrolite, and probable lithiowodginite.
Simpsonite,manganotantalite,microlite and lithiowodginite mark an earlier generation of oxide
minerals; alumotantite and parabariomicroliteappear later and replacecertain of theseminerals.
The Kola peninsula assemblage has been
describedelsewhere(Voloshin et al. l98l), hence
shall only be mentionedbriefly here. Alumotantite
in the Kola pegmatites is associated with
simpsonite, natrotantite, microlite, cesstibtantite
and albite. Simpsonitewas the first oxide mineral
of the associationto form; all of the others rim
and replacethe simpsonite.
PHysrcALallo Oprrcer Pnopenrrps
Alumotantite ranges from colorless (Bikita,
Kola) to translucentwhite (Alto do Giz). Similarly,
the luster rangesfrom adamantineto greasywith
increasing opacity. Crystals are subhedral to
euhedral, elongateand rectangularto'rhombic in
cross-section.The crystals are always very small.
Alumotantite from Bikita is typically lessthan 0.1
mm in length, much like the type material (Voloshin
et al. l98l); that from Alto do Giz is up to I mm
in length. Crystalsof the Bikita material commonly
occur as oriented parallel grofihs on simpsonite,
with the axis of elongation of the alumotantite
parallel to Z of simpsonite(Macgregor1946).None
of the samples examined shows any cleavage.
Because of the small size of the crystals, no
determinationsof densitywere possible.The X-ray
study, however, gives a calculateddensity of 7,48
g/cm3.
In reflected light, alumotantite has higher
reflectivity than simpsoniteand is also markedly
more bireflectant. In transmitted light, it is biaxial
negarivewith a2v of 66(2)" (Alto do Giz sample).
None of the indices of refraction of alumotantite
is lessthan 2.00. This is consistentwith GladstoneDale calculations(mean n = 2.21, constantsfrom
Mandarino 1976) ard Macgregor's assignmentof
a birefringenceof 0.12.
Curursrnv
The samples were analyzed with a MAC 5
electron microprobe operating in the wavelengthdispersion mode. The acceleratingpotential was
chosenas l0 kV for AL(cuto minimize absorption
tfk) = 0.7881and as 20 kV for TaLot to minimize
the atomic number effect; all other lines (NbZa,
SnZo) were analyzedat 15 kV. A sample current
of 40 nA measured on brass was used. The
following standards were used: YAG (Al), cassiterite (Sn), stibiotantalite (Nb) and manganotantalite (Ta). Manganese,iron and titanium also were
sought but not detected.Data were reducedby a
modified version of EMPADR VII (Rucklidge &
Gasparrini 1969) that incorporates the atomic
number and absorption correctionof Love & Scott
(1978) and the characteristicfluorescencecoirection of Reed(1965).
Table I gives all available compositions of
alumotantite; for comparative purposes,its ideal
composition also is given. Voloshin et al. (1981)
proposedthe formula AITaOa for alumotantite; all
aralyzed specimens conform well to this
stoichiometry. Alumotantite may possessminor
amountsof Sn (up to 1.0 wt.Vo SnO2,Bikita) and
Nb (up to 0.8 wt.Vo Nb2Oj, Kola) in addition to
Ta and Al. Although the data are not sufficiently
preciseto be conclusive,Sna+probably substitutes
for equal amounts of Al3+ and Ta5+, as chargebalance considerationswould dictate. Niobiumtantalum isomorphism is very minor to undetec-
655
THE CRYSTAL STRUCTURE OF ALUMOTANTITE
ALUMOTANITG
TABI.E1. CHEMIOAL@MPOSMON OF ALUMOTANIM
ALUMOTANITIE NMNH 104738
18.9
't.0
0.0
8(I9
t@.8
16.6
02
o0
81.6
't00.4
tt%
Mq
gnq
NQos
Ta?Og
CdoE
as
AI
Sn
Nb
Ta
0.01
0.@
7.98
tgs
0.8
81.1
t@.4
per 16 (O)
3.99
o.g7
0.00
3.€[t
E01
3.g}
0.67
3,98
7.9
1. Alto do Ab, BE I (NMNH 104rc8)i 2 Blkita, Zmbabwe (NMNH 105760);
d eluno€nie.
Penlmla, Ugsni Vd6hh 9ta/. (19S1); 4. ldeal mpciton
'0.o: m ddscl€d:
'-':
14.7
0.0
0.0
81,3
1@.0
4.(x)
0.@
0.m
4.00
8.ql
3. l(ola
nol sqlt
table, most probably owing to a geochemical,not
a crystal-chemicalcontrol (Ercit 1986).
Stnucruns ANALYSIS
X-ray powder-diffraction patterns of crystal
fragments of alumotantite were recorded by the
Gandolfi method. The pattern obtained for the
sample from Alto do Giz indicated single-phase
alumotantite; but patterns for the sample from the
Bikita pegrnatite persistently showed evidenceof
contaminationby simpsonite.Initially, an attempt
was made to index the pattern for the Alto do Giz
sample on the cell proposed by Voloshin el a/.
(1981) for rhe type marerial, but this failed. In
addition, severalof the linesreportedin the pattern
of the type material were not observed in pattern
of the Alto do Giz sample,but were found to be
present in the pattern of the Bikita sample,
suggestingthat the type material is contaminated
with simpsonite.Examination of the Alto do Giz
crystal fragment by the precessionmethod shows
that the cell proposedby Yoloshin et ol. (1981)is
not truly orthogonal. The diffraction pattern
possessesmmm spmelLry; from the diffraction
symmetry, a new set of axeswas located, and the
diffraction patterns were indexed on this cell.
Systematicabsencesof the types Okl with k : 2-n
+ l, hOlwith / : 2n + | and hk0wilh h + k :
2n + | uniquely determined the spacegroup as
Pbcn, The data for the type material were
re-indexedon the new cell, which resuhed in cell
parameters in good agreement with Brazilian
material and standard errors an order of rnagnitude
better than before. The indexed patterns of the
Brazilian and type alumotantite are given in Table
2. Cell parameters for all three specimensof
alumotantite are given in Table 3.
As a search of the literature showed no
compounds isostructural with alumotantite, a
structure analysis was undertaken. An equant
fragmentof dimensions0.10 x 0.12 x 0.14 mm
@17
10
110
42
e1
l(p
fil
47
130
4
040
r
t3t
20
u1
0elt
7
200
@,4
4
141
1@.
2
3
220
4
112
5
150
14
21
5
060
11
151
lgu
c/'23
2
240
21
241
12
82
2,2
4
752
w3
1m2
624
310
171
N3
242
@2
3tt
04{t
330
@1
172
2z33
t58
350
2F24
002
J90
351
395
r
8
2
6
7
5
3
2
2
1
r
2
2
4.t6
s.65
3.14
28
28
2@
2433
2388
za7
21e€
2137
21(,J
2@0
zo71
20t8
1.*7
1.885
1.8!t9
'i.&!9
1.@4
'1.79
1.646
1.6t2
1.568
1.541
1.59
1.87
1.479
1,478
1.M
1,41
1.414
1.414
1.4'12
1,97
1.384
1.3!*t
1.&.
7.M
'1.250
1,24s
1.9
1.216
1,no
t.205
r.t99
30
x
50
100
@28
10
5.63
4.10
3.64
3.13
2p.
50
s€o
70
'lG)
80
ro
3,64
3.13
aF
283
&
30
10
10
t0
'10
zg7
z3ai
a&
z1@.
zl&'
z1@.
60
30
g)
l0
m
24g)
2389
zm
2196
2142
10
10
30
10
20
40
'f0
ll0
so
't0
10
10
10
zq72
zu2
1.904
1.841
1.81t5
1.9
'10
30
40
m
30
50
z@
z@.
1.9{X
1.884
1.861
1,87
1.7*.
1.U4
1.6@
1.566
1.S
1.56
,.516
10
50
6
10
n
n
1.79
1.649
l.qts
1.5€8
1.543
1.58
m
1.475
30
l.4an
30
1.444
30
1.&
n
m
1.410
13e
4
40
1.413
lsm
10
t0
10
'lo
1354
1278
1262
124,J
m
'10
m
t0
t.35/
'1.41
1.&
1249
10
1,81
g)
10
1.47
10
10
1.197
m
t.200
t.
198'l) totrs
nci ab€orp'lbn
a. votchh ir ar, (1981). I 14.6 m
porder @era"
unfihs€d Fe Edldon
IABLEA UMT€ETI PARAMETES(A) FOF ALUMOTANME
a
b
c
4.4i6(l)
|.308{4)
4,n5$)
4,8
17.8
4,76
4,47t('t,
1r3@(3)
4,767(11
(trra{im€id,
MotG taddoI\
1. Atto do Gts, B@il (NMNH l(X738). Fu{de
gEphi€|llmhmnstq
canBta, Z-fbeted Mor6 radldon
B[d8, Zmbabf,e (NMNH I 05/60). PrGlq
etar. (lgEl).
ddadvdcffi
Kote Ps*rEJ4 tjSSR R€ftEdtrmths
2
A
was used for the collection of intensity data. Data
were collected with a Syntex P2r automatic
four-circle diffractometer over one asymmetric
unit, to a maximum 7.0 of 60o using MorKcr
radiation. A total of 446reflectionsweremeasured,
of which 284 were consideredobserved (greater
than 3o). Of these,four very weak reflections(only
slightly greater that 3o) violating Pbcn group
symmetry were removed from Jhe set. The highly
absorbing characteristicof the material (p = 480
cm-l) necessitatesan absorption correction; both
spherical and f-scan corrections were applied to
the data. Cell dimensionswere obtained from the
least-squaresrefinement of a subsetof reflections
656
THE CANADIAN MINERALOGIST
number of Ta atoms = 4 per unit cell, which is in
agreement with results of microprobe analyses
(Table l). The model convergedon a fully ordered
scheme(0 Al atoms in the Ta site and vice versa),
which indicatesthat cation disorderis not the cause
Temp€rdure tsctor iom 6edl qp(">thrh/g/,
for the deviation in bond-valencesums. To assess
whetheran error in the absorptioncorrectioncould
R =:(lFol-lFcl)/rlFol
be responsiblefor the bond-valencedeviations,the
wR E ls4lFol"lFcl)2DwlFolzl1.,w - 1
experimentwas repeated.Collection of the second
data-set followed the same procedure as for the
first, exceptthat data were collectedwith a Nicolet
automatically aligned on the diffractometer. Mis- R3ru diffractometer accordingto the experimental
cellaneousinformation about the refinement is method of Ercit et al. (1986).Although the second
given in Table 4.
refinement converged at a marginally improved
Considerationof the proposedstoichiometryand conventional R of 3.990, the resulting positional
of possibledensitiesfor alumotantiteindicatesthat parameters are not significantly different from
the true cell of alumotantite containsfour formula those of the first refinement.From this agreement,
units. Assumingcompleteorder among cations,Al we concludethat potential errors in the absorption
and Ta were assignedto separate4c sites(Wyckoff correction do not significantly affect the positional
notation). While the diffractometer intensity data parameters,and thus are not responsiblefor the
were being collected,an hl<OPattersonprojection slight deviation in bond valence.
was made from precession camera intensities;
Observed and calculated structure-factors for
Ta-Ta vectors were located, giving y1" = 0.17.
both refinementsare availablefrom the Depository
The program RFINE of Finger (1969)was used of Unpublished Data, CISTI, Natural Research
for the structure refinement. Scatteringcurvesfor Council of Canada, Ottawa, Ontario KIA 0S2.
neutral atoms were taken from Cromer & Mann Becauseof the similarity of the two refinements,
(1968)and anomalousscatteringfactors were from we present atomic positional and thermal
Cromer & Liberman (1970). The composition of parametersfor the first refinemenl only in Table 5.
the sample used in the refinement was approximated as AlTaOa. In the first stage of the
solution, the position of the Ta atom was refined. TABLEs. POSMOML AND THEBMAL PAMMEIERS FORALUMOTANTM
-El$
A difference Fourier map ,wasthen constructed, AtOm
I
y
z
from which the Al position was located.The cation Ta
0
0.16817(9)
1t4
0.41(s)
0
positions were refined, and a ATa,Al map was N
o.4s48n
1t4
0.44(11'
ol
0.4(4,)
0.312(1)
0.086(4)
o.u(a)
constructed.The map suggestedthat oxygenatoms 02
0.266(4)
0.058(1)
0.073(4)
o.66(29
occupy four 8d sites, not the two expectedfrom
pz
gs
the stoichiometry.Nevertheless,the positionsof all Arom.
p%
hr
9tz
98
four sites were entered in the refinement. The Ta
-2(8)
m(5)
4(1)
so(s)
0
0
01
4s(66)
13(e)
-2(24)
63(5'
€(23)
€(62)
isotropic temperature-factorsof the oxygenatoms 02
-8V4
1e.F2'
2(8t
e0(66)
o(2:)
0(21)
clearly indicated that^two of the four were false, NIfuffi x 104
with mean B = 4.5 A,2: thesetwo were discarded
from the refinement.Two more cyclesof refinement with isotropic temperature-factorsreduced
DsscntprroNtoF THEStnucrunr
the conventionalR index from l3.9go to 5.090.A
difference Fourier map at this stage showed no
unexpectedanomalies. The tantalum and oxygen
The array of oxygenatoms of the alumotantite
atoms were modeled as vibrating anisotropically structureis approximatelyhexagonallyclose-pack(aluminum could not be realistically refined ed (Fig. l). The close-packedlaytjring is perpenanisotropically),and a secondaryextinction correc- dicular to X. Cationsoccupyhalf of the octahedral
tion was applied, resulting in a final R index of intersticesof the oxygenarray. Consideredin terms
4.40/0.
of polyhedra,eachlevelperpendicularto Xconsists
Bond-valencecalculations [using the constanrs of branched, zig-zag chains of edge-sharing,
of Brown (1981)and Ercit (1986)lshowedrhat the slightly distorted octahedrarunning parallel to Z.
sums to the cations differ from their ideal values: AlOu octahedra make up the backbone of these
Xs(Ta) : 4.7 v.u., Ds(Al) : 3.2 v.u., possibly chains; TaO6 octahedra are afiached laterally 1o
owing to cation disorder.An attempt was made to this backbone. The gross configuration of the
refine the degree of order in the cationic sites chains is the same for all levels.
accordingto the constraint:nunber of Al atoms :
A projection along I (Fig. 2) illustrates the
TABIE4.
CNYSTAL DATA FOR ALUMOTAN'ITG
ar 4.473(1)A
b: 11.308(4)
c: 4.775(1,
gpaF grupi Ptc,
Zt4
D (@lc)r 7.48 g/crP
Cry€nal
sbs:
p (MorG):
Toralm. ot lFol:
No ot IFol > 3d:
Frd R (ob):
FirEl&f,(obo):
1.7 x I Osrm3
480ml
M
n4
4.4%
5.5%
657
THE CRYSTAL STRUCTURE OF ALUMOTANTITE
'-'-'-'-'--=*i-]-':"
\
t
rlt
,l\
.
\
o
t
tlt
T
rO
'
t.
!
'
c
,l\
,l\
tJ
rO
,'
g.'-1:=n
t_
t
,l
/l\
arl\
r.ll
\rl
Qt
\/)
rO
ll
\
\
\o
.
,l \
,l
tJ\
I
o,
,l \
,l\
f1\
\o
,
, | |
\
|
t
t,
al
flll
]...r'..t-.
1___:b
r
t.
rO
'.\r1
,l
,t
r
,lt
.' |
\
ot
r
\
r
I
.f*.-
ri.
r
O
/t
'l
\
.
tl
-i;/.'t-.':;j.j
Frc. l. Projection of the alumotantite structurealong X. Occupiedoctahedraof one layer are in solid lines, those of
the next layer down, in broken lines. Each occupied octahedron is immediately overlain and underlain by an
unoccupiedone. Aluminum atoms are denotedby filled circles,tantalum atoms, by open circles.The unit cell is
two layers deep.
interrelationship of the TaO6 polyhedra. Each
octahedronsharesfour cornerswith two octahedra
aboveand twe below it. The result is a perforaled,
corrugated sheet of tantalum-bearing polyhedra
normal to Y. Each sheetis linked to neighboring
sheets by edge-sharing with the backbone of
aluminum-bearingpolyhedra.
Interatomic distances and angles are presented
in Table 6. Details of the two types of polyhedron
in alumotantite are given in Figure 3. The Ta-O
and Al-O distances compare very well with
calculatedvaluesfrom Shannon(1976)(Ta-Oo6,=
2.008,Ta-O"4" = 2.00; Al-Oob, = 1.893,Al-O.u1"
= 1.895 A), supporting the earlier conclusionof
full order between cations. Deviations of the
coordination polyhedra from ideal octahedral
geometry are slight. Most deviations can be
rationalized in terms of cation-cation repulsions
and number and location of shared edges. The
aluminum-bearing octahedron possessesthree
shared edges regularly disposed about the
polyhedron. Two of these edgesare shared with
adjacentaluminum-bearingoctahedraand onewith
an adjacenttantalum-bearingoctahedron.Because
658
THE'EANADIAN' MINEP.{LOCIST
l-r-l
c
Frc. 2. Projection of the sheetof tantalum-oxygenpolyhedra down Y.
of this immediateenvironmentof nearly-balanced
repulsions, the aluminum atom lies near the
geometric center of its coordination polyhedron.
Becausethe tantalum-oxygen octahedron shares
only one edgeand becausethe Ta-Al interatomic
vector lies along Y(the only positional variable for
the tantalum atom), tantalum-aluminum repulsion
distorts the tantalum octahedron more than the
aluminum octahedron. As a result, interatomic
distances and angles in the tantalum polyhedron
show gleater variability than those in the aluminum
polyhedron.
TABLEA SELECIEDINTERATOMIC
DIgTANCESAND ANGLESFOF
ALUMOTANITTE
TaO@Mrcn
Al OcialFdm
Ar-o(r)a)er.88(4
-O(zlbre,1.87C,
-oey e, 1.s@
Ta-O(1)a)e e08e)
-o(r)b)e 204(4
-oebe.
lge(a
< ra-b > I6'iT
o(1)a-o(l)b)a 2.78(a, 85.3(4)
"oF ce, 27E€l. U.714'
.o(r)dxl 2s3(4), 75.7(10)
-o(Aae 288e), szTn
o(r)b-o(4a)a 286{3).9a3O
-o(ad,e 294(2),S.9(8)
o(z)d-O(4ar
a92g). s9.0{11r
<o-o>
z8BA
< O-Tg-O>
@.7 .
<Al-o>
1.89
o(1)a-o(l)dxr
-ol2rb,Q,
-O@Ste
.O@cre,
o(qb-o(At e,
-a(ASre
a$(4), 84.6(11)
27s(3), s4.7(8)
zn(E, &.2(8't
263(3), s.7C4
256{4), 84.8(9)
272(11,91,4$)
o(4r-o(4sn2@(41.8A0tlr)
<o-o>
e68
<OTa-O>
90.0
Eqldvalql Pcfrc
t x,rz
di -x,y, 1l2-z
bt1l2-x,112t,112+z
l: 1e+x,1f2+y,1l2q
q1l2+x,112"/,-z
gt x,t,1fz+z
Frc. 3. Coordination polyhedra in alumotantife. Shared
edgesof polyhedraare shown in thick rule. SeeTable
6 for the coding of the oxygenatoms.
THE CRYSTAL STRUCTURE OF ALUMOTANTITE
Sttr,tpsoNrrseNo Ar-uuoreNtttg
Topotaxy
In addition to prismatic crystals of alumotantite,
isolatedpatchesof a "cloudy alteration" in Bikita
simpsonite can be observed (Macgregor 194Q.
Gandolfi photographs of this opaque, white
material show evidence of both simpsonite and
alumotantite. Qualitative microprobe analysis also
shows a minor third phasewith only Al detectable.
These data and paragenetic considerations suggest
the phaseto be a hydroxide, perhapsgibbsite.
On the basis of the similarity of severalof the
interplanar spacingsof simpsoniteto alumotantite,
a topotaxic relationship between these minerals is
suspected.To investigatethis, a fragment of the
opaque region was studied by the precession
method. The topotaxic relationshipwas confirmed
in the photographstaken, with Xa IZs, yA [ [210]s,
aad Zol[410]s, where the subscriptsA and-S refer
to alumotantite and simpsonite,respectively.This
alumotantite is twinned; topotaxic control on the
659
twinning has producedthreefold rotation lwins of
alumotantite (about Zr) in the partially consumed
simpsonite. No additional diffraction-maxima indicative of a third phase were observed in the
precessionphotos.
Comparison of the simpsonitestructure to that
of alumotantite in light of the above relationships
showsthat the hexagonalclosest-packingofoxygen
atoms in simpsonite is preserved through the
topotaxic reaction that generatedthe alumotantite
(Fig. 4). Hence, the reaction is marked by cation
migration through an essentially unchanged array
of oxygen atoms.
Optical examination of the Bikita sample shows
that both the prismatic alumotantite and the
alumotantite in the white patchespossess(at least)
parallel X axes. By inference, the same crystallographicrelationshipQesstwinning) existsbetween
simpsoniteand both alumotantitetextural varieties.
However, important differencesbetweenthe two
varietiesdo exist:
Ftc. 4. Projection of the simpsonite structure aTongZ. Representationof the
polyhedrais asin Fig. l; Ta occupiesthe octahedrain solid rule, and Al occupies
the octahedra in broken rule. The unit cell of simpsonite outlined (fine rule) is
two layers deep. The orientation of the alumotantite cell with respect to the
hexagonal closest-packingof simpsonite is shown for reference (corners in bold
rule).
660
THE CANADIAN MINERALOGIST
(l) The prismatic variety occursas partial replacementsof, and overgrowthson, simpsonite,whereas
the patchy variety occurs as replacementsonly.
(2) The prismaticvariety occursdlong fracturesand
rims of the simpsonite,whereasthe other variety
occurs as isolated patches in the interior of
simpsonitecrystals.
(3) Aluminum silicates(muscoviteand albite)rather
than aluminum hydroxidesoccur with the prismatic
variety.
Although the two varieties differ greatly in
occurrenceand paragenesis,the rarity of alumotantite, the similar paragenetictiming of the varieties,
and the same topotaxic relationship between
simpsoniteand both alumotantite varietiessuggest
a single mechanismof reaction. The observations
outlined above suggestthe reaction of simpsonite
with minor volumesof a late fluid phasewith which
it was no longer in equilibrium. In particular, an
increasein p1. is the most feasible cause for the
breakdown of simpsonite.A generalizedequation
describingthis reactionis: Ta5* + 2AloTarO,r(OH)
: TAITaOa+ 2H* + AP=.
In the interiors of alumotantite crystals, the
aluminum releasedin this reaction is bound up as
a hydroxide (i.e., in the white patches).At the rims
of, and along fractures in the simpsonitecrystal,
pSi (and pNa, pK) is sufficiently great to result in
the incorporation of the liberated Al3+ in
aluminum silicates. Quite clearly, the phases
formed from the liberated Al3* do not play an
active role in simpsonite-alumotantitetopotaxy;
the sametopotaxic relationshipbetweensimpsonite
and alumotantite is shown by both parageneses
despitetheir differencesin mineralogy.
Pedersen1962)and partly ordered (e.9., Efremov
et al. 1981)forms havebeensynthesized.The fully
orderedform hasedge-sharingdimersof lld+-bearing octahedra;dimers are interconnectedaloug I
by zig-zagchains of ilF*-bearing octahedra and
ilong Z by corner-sharingwith these chains. The
structureis stableup to at least l700oC for AlTaOa
and up to l400oC for AlNbOa and GaNbOa at
atmosphericpressrue.
GaMOa and GaTaOacan have the wolframite
structure(Fig. 5c). A cation-disorderedform of the
structure also has been found for GaTaOa(Bayer
1962). The wolframite structure is the stable
modification for GaTaOaup to at least l580oC and
at atmosphericpressure(Bayer 1962),but is only
stable for GaNbOaabove 1.5 kbar (Tamura et al.
1980). It is another hexagonal closest-packed
structurewith octahedrallycoordinatedcations.All
cations of one type occupy a given level parallel to
the hexagonalclosestpacking, Octahedra of any
such level are connectedby edgesto form zig-zag
chainsparallel to Z and are linked by corner-sharing to chains of adjacent levels.
One other conpound has been synthesizedfor
these compositions(Burdese& Borlera 1963),but
the structure is unknown. It has no immediate
structural similarities with other synthetic .4BOa
compounds or alumotantite in terms of its
powder-diffraction properties.Unfortunately, conditions of its stability are poorly known.
From a structural viewpoint, alumotantite (Fig.
5d) is similar to all of thesecompoundsin that it
has only octahedrally coordinated cations, and
similar to most in that the aluminum polyhedraare
presentin edge-sharingchains. However, it differs
in severalimportant aspects.The total edge-sharing
STRUCTURAL
RrLerroNs AtvtoNcsr rne
chain in alumotantite is much more intricately
(Al,Ga)(Ta,Nb)OnOxrous
branched than in the other structures. Also, the
polyhedrathat contain the pentavalentcation show
Severalaluminum, gallium, tantalum, niobium a greaterdegreeof polymerization in alumotantite
oxides of z4BOa stoichiometry have been syn- (sheets)than in the other structures (disordered,
thesized; none of these have the alumotantite dimers or chains).
structure.A review of thesecompoundsand of the
All of the syntheseshave been anhydrous, and,
conditions of their synthesis reveals important except for GaNbOa, performed at atmospheric
information about the stability of alumotantite.
pressure.The tantalum-bearingcompoundsdo not
The rutile-structured polymorphs of the com- form below 1000-2000"Cunder such conditions;
poundsAlTaOa and GaTaOaare stableonly at very similarly, the niobium-bearing ones do not form
high temperatures (greater than approximately below 1000'C. In contrast, alumotantite has only
l5@'C: Bayer 1962, Isupova et al. l97l), This been found in late metasomatic assemblagesof
structure (Fig. 5a) is characterized by completely rare-element-enrichedgranitic pegmatites. This
disordered cations in a hexagonally close-packed type of occurrenceimplies formation in a fluid-rich
array of oxygenatoms. Coordination polyhedraare environment with P(fluid) = {total) : 1.5 to 3
octahedra and form straight, edge-sharingchains kbar and with a temperatureof formation between
parallelto Z. Chainsof one levelare linked to those 200 and 4A0oC(e.g., London et al. 1983).
of the next by corner-sharing.
In conclusion, we contend that from structural
The monoclinicAlNbOa structure(Fig. 5b) is the and paragenetic information, alumotantite has
most common structure for synthetic ABOn-Iype never appearedin synthesisstudies becausethese
Al, Ga, Ta, Nb oxides. Both fully ordered (e.9., have been conducted under conditions too far
661
THE CRYSTAL STRUCTURE OF ALUMOTANTITE
('
x
AtTa0a
GaTaol
o
GeTaOl
GaNb0r
g,
AtItOl
AlNbOr
GrXbOl
Atimotentito
b
d
Ftc. 5. Projectionsof .ABOastructures,where,4 representsAl and Ga, and B,Ta and Nb. Structuretypes: (a) rutile,
(b) AIMO4, (c) wolframite, (d) alumotantite. Representationof the polyhedra is as in Fig. l. For the ordered
polyhedra are stippled, ild+-bearing polyhedra are unshaded.
structures,ll,y'p*-bearing
662
THE CANADIAN MINERALOGIST
removed from its stability range. We propose that
alumotantite
is the stable low-temperature
polymorph of AlTaOa.
AcKNowLEDGEMENTS
We are indebted to J.S. White of the U.S.
National Museum of Natural History, Washington,
for providing the samples for the study. We thank
the Materials Research Institute, McMaster University, for the collection of the X-ray intensity data.
Financial support for this work was provided by
NSERC grants to FCH and E, and NSERC
Research Fellowship to FCH and an NSERC
Postgraduate Scholarship to TSE.
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lM5-1469 (in Russ.).
LoNDoN,D.L., Sroourn, E.T.C. & Roroorn, E. (1983):
Fluid-solid inclusionsin spodumenefrom the Tanco
pegmatite, Bernic Lake, Manitoba. Carnegie Inst.
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Love, G. & Scorr, V.D. (1978): Evaluation of a new
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MecGnecon, A.M. (1946):Simpsoniteand other ranralates from Bikita, southern Rhodesia.Mineral. Moo
27, t57-165.
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ReceivedNovember 3, 1983,revised manuscript accepted
February 12, 1992.