American Mineralogist, Volume 77, pages 545-557, 1992
Conversionof perovskite to anataseand TiO2 (B): A TEM study and the use of
fundamentalbuilding blocks for understandingrelationshipsamong the TiO, minerals
Jrr.r.r.lN F. BlNrrnr,Drt Dlvro
R. VBnr,rN
Department of Earth and Planetary Sciences,The Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
Ansrucr
TiO, crystallizes in at least seven modifications encompassingseveral major structure
types including those of rutile, hollandite, and PbOr. We have adopted a fundamental
building block (FBB) approach in order to understand better the relationships between
these structures.The identification of common structural elementsprovides insights into
possiblephasetransformation mechanismsas well as the weatheringreactionsthat convert
perovskite (CaTiOr) to TiO, anataseand TiO, (B).
We have studied the weathering of perovskite from the Salitre II carbonatite, Minas
Gerais, BraziI, by transmission electron microscopy. Although the dissolution of perovskite, a proposed component of the high-level nuclear waste form Synroc, has been modeled thermodynamically and studied experimentally, its direct natural replacementby Ti
oxides during weathering has not been previously described. Our results show that the
perovskite is replaced topotactically by both anataseand TiO, (B). The reaction appears
to involve an intermediate product that can be interpreted as a half-decalcified,collapsed
derivative of perovskite. The second step, involving removal of the remaining Ca, can
occur in one of two ways, leading either to anatase or in rare casesto TiO, (B). The
orientations of the interfaces between both perovskite and anataseand perovskite and
TiO, (B) are consistent with collapse of the corner-linked Ti-O framework to form edgesharingchains. Despite the orthorhombic structure of perovskite, the resultsshow that the
anatase can develop with its c axis parallel to any of its three pseudocubic axes. The
complete destruction of perovskite produces arrays primarily of anatasethat display a
variety of textures, including porous aggregatesand recrystallized needles.REE elements
releasedfrom perovskite are immobilized by precipitation of rhabdophane that is intimately intergrown with anatase.
INrnonucrroN
is related to the rutile structure by a second-orderphase
transformation (discussed by Hyde, 1987). The highpressurepolymorph, TiO, II, has the a-PbO, structure;
the highly metastable TiO, (H) polymorph has the hollandite structure; and TiO, (B) has a structure that is
closely related to that of VO, (B). The similarities between some of these polymorphs and the related structures of anataseand brookite will be discussedbelow.
All of the polymorphs contain edge-and corner-linked,
octahedrally coordinated Ti cations. The available structure refinements(seeasterisksin Table l) show relatively
constant Ti-O distancesand quite variable O-O distances, particularly when the shared vs. unshared octahedral
edgesare compared.Pairs offace-sharing octahedra(containing trivalent Ti) are found only in crystallographic
shear structuresin reduced rutile derivatives (TiOr_.).
Previous discussionsofthe relationships between certain of these structures have focused on the presenceof
sheetsof O atoms that approach a cubic closest-packing
* Present address: Department of Geology and Geophysics, arrangement(such as in anatase),very distorted hexagoUniversity of Wisconsin-Madison, 1215 West Dayton Street, nal closest packing that approachescubic (as in rutile;
Madison,Wisconsin53706,U.S.A.
Hyde et al.,1974), or an approximately double hexagonal
There are currently four naturally occurring TiO, polymorphs [rutile, anatase,brookite, and TiO, (B)] and at
least three (plus some very high-pressureforms) polymorphs that have been produced synthetically. Details
for six of the distinctive polymorph structures are listed
in Table I . It is generallyconsideredthat at low pressures
only rutile has a true field of stability; anataseand brookite form metastably(Dachille et al., 1968; Post and Burnham, 1986).Post and Burnham (1986) used ionic modeling to show that the electrostatic energies of rutile,
anatase,and brookite are not greatly different. Navrotsky
and Kleppa (1967) measuredAH : -5.27 kJlmol for the
anataseto rutile reaction.
The structureof the most commonly occurringand best
known TiO, phase,rutile, is sharedby a number of minerals including pyrolusite, cassiterite,stishovite, and in a
distorted form, marcasite.The CaCl, polymorph of TiO,
ooo3404x/ 9210506-0545$02.00
545
546
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
TaBLE1. Data for some TiO2polymorphsand perovskite
Structure
Rutile'
Anatase*
Brookite'
Tio, (B)
Tio, il'
Tio, (H)
Perovskite"
Space group
Density
(g/cm")
P4"lmnm
Allamd
Pbca
C2lm
Pbcn
Alm
Pcmn
4.13
3.79
3.99
3.64
4.33
3.46
4.03
Unit-celldata
(nm)
a:
a:
a:
a:
a:
a:
a:
0 . 4 5 9 ,c :
0 . 3 7 9 ,c :
0 . 9 1 7 ,b :
1 . 2 ' 1 6b, :
0.452.b :
1 . 0 1 8 ,c :
0 . 5 3 7 .b :
0.296
0.951
0 . 5 4 6 ,c :
0 . 3 7 4 ,c :
0.550.c :
0.297
0 . 7 6 4 ,c :
0.514
0 . 6 5 1 ,0 : 1 O 7 . 2 V
0.494
0.544
Reference
Cromer and Herrington(1955)
Cromer and Herrington(1955)
B a u r( 1 9 6 1 )
Marchand et al. (1980)
Simonsand Dachille(1967)
Latroche et al. (1989)
Kay and Bailey(1957)
* Structures refined by X-ray ditfraction.
closest-packingarrangement (as in brookite). These descriptions emphasizedifferencesamong the structuresbut
fail to highlight their close relationships.
Andersson (1969) showed that rutile could convert to
the TiO, II structure by cation displacement.Simons and
Dachille (1970) noted that similar structural elementsare
presentin anataseand TiO, II and suggestedthat a transformation between cubic closest-packedand hexagonal
closest-packedarrangementsinvolves shear between O
Iayers.
Bursill and Hyde (1972) and Hyde et al. (1974) described a relationship betweenhollandite and rutile structures involving rotation of adjacent rutile blocks by
clockwise and counterclockwise rotation. Alternatively,
Bursill (1979) demonstratedthat the hollandite structure
can be generatedfrom the rutile structure by intersecting
antiphase boundaries or crystallographicshear.Latroche
et al. (1989) reportedthat the TiO, (H) polymorph converted to anataserather than rutile with increasingtemperature and noted that the structural relationship between TiO, (H) and anatasewas not clear.
The dissolution of perovskite and its replacement by
TiO, has been studied experimentally under a range of
hydrothermal conditions (e.g.,Myhra et al., 1984; Kastrissioset al., 1986,1987;Jostsonset al., 1990).The interest is partly due to the proposal that perovskite could
be a major component of the synthetic assemblageof
titanate minerals known as Synroc, a potential nuclear
waste form. Thermodynamic calculations have clearly
shown that perovskite is unstable with respect to both
titanite and rutile at low temperaturesand in natural waters (Nesbitt et al., l98l). Kastrissioset al. (1987) suggestedthat perovskite dissolvedunder hydrothermal conditions (between ll0 and 190 "C) and that euhedral
anatase and brookite crystals grew epitaxially from an
amorphousTi-O layer.Jostsonset al. (1990)summarized
much of the experimental work on perovskite dissolution
including data ofPham et al. (unpublished data), which
suggestthat under epithermal conditions (<80 "C) an
amorphousTi-rich layer approximately l0 nm thick forms
on the perovskite surface.This layer is believed to incorporate Ca ions and act as a protective barrier that inhibits
further dissolution.
The first part ofthis paper describesthe results ofour
attempts to find a unified way of describingthe Ti oxide
structures.The approach, basedon the use offundamen-
tal building blocks (FBB; Moore, 1986),emphasizesthe
similarities betweenthe structures.The aim is to provide
insights into possible direct mechanismsby which polymorphic phasetransformations involving TiO, might occur. We discuss the application of this view to understanding the TiO, (B)-anatase transformation. In the
secondpart of this paper we use the FBB description as
well as experimental results to describe perovskite
(CaTiOr)-TiO, weathering reactions.
AN FBB APPROACHTO THE STRUCTURAL
RELATIONSHIPS
BETWEEN
RUTILE,
ANATASE'
BRooKrrE, TiO, (B), aNo TiO' (H)
There are many ways to describe structures and the
relationships between them. For example, the utility of
the polysomatic-seriesapproach has been discussedin
detail by Veblen (1991). Although this structural rationalization works well for many minerals, it cannot be
simply applied to describethe TiO' polymorphs, primarily becauseof the absenceof slabs common to all polymorphs. As noted in the introduction, both crystallographic shear models and descriptions based upon the
geometry of the O framework have been used to describe
and relate some of the TiO, structures.In this discussion,
we take a rather different approach. Following Moore
(1986),we describethe TiO, structuresin terms of their
construction from a single structural unit, or FBB, by
either direct assembly or shear. This approach reveals
both the close relationships (some groups of TiO, polymorphs contain common structural layers) and the differencesbetweenmembers of the group (e.g.,in the stacking ofthese layers).
Choice of an FBB
The attempt to use an approach basedon the presence
of common structural elements in the TiO, polymorphs
was hampered by two problems. First, the largest structural block common to all of them is merely a pair of
edge-sharingoctahedra.Second,the group of TiO, structures falls into two categories,namely, those that contain
chains ofedge-sharingoctahedrain one orientation [TiO,
(B), anatase]and those with chains in two orientations
[rotation parallel to the chain relatesthe two orientations,
e.g.,in rutile, brookite, TiO, (H)1.
All ofthese structures except rutile can be constructed
from a unit composed of four edge-sharingoctahedra.
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
The(l0l)- and(103)ofanatase.
Fig.2. FBBrepresentation
boundedslabsareindicated.TheFBBis shownby thesoliddark
slabare
Fig. l. Representations
of the fundamentalbuildingblock. line, and the octahedraincludedin the (103)-bounded
Iflight-stippledoctahedrahavea coordinatez : O,l (z perpen- groupedby the dashedline.
dicularto the page),thendark-stippledoctahedra
arelocatedat
z:0.5.
sharing, so that light-stippled octahedraat y : 0,1 define
the cornersof the unit cell as shown. Dashedparallel lines
Becausethe basic rutile structural elementscan be created isolatea (101)-boundedslab ofanatasereferredto in the
by stacking ofthis unit and the rutile structure itselfcan discussion of the brookite structure, and those labeled
be generatedfrom this assembly by shear, we have se- (103) separategroups of octahedra (outlined) that form
lected this four-octahedron unit to be the fundamental slabs common to the TiO, (B) structure.
building block. The FBB is illustrated in four orientations
Figure 3 shows a representationofa slice through the
in Figure l.
TiO, (B) structure. The FBB is indicated by the heavy
Although we will not discussit further, this FBB also solid line, and the corner-linked FBB slab common to the
occurs in many structures other than the TiO, poly- anatasestructure (Fig. 2) is shown by the dashedoutline.
morphs.For example,it is the sameas a four-octahedron The common structure parallel to (103) anataserationalsegmentof the Ml cation chain found in the pyroxenes, izes that TiO, (B) is converted to anataseby shear inand it occurs as a fragment of dioctahedral and triocta- volving growth ofledgesalongthe (103)planesofanatase
hedral sheetsin the layer silicates, oxides, and hydrox- (Banfieldet al., l99l).
ides.
The FBB representation of the brookite structure is
shown in Figure 4a. This diagram illustrates that brookite
Anatase, brookite, TiO, (B), and TiO, II and their
relationships to rutile
There are a number of reasonsto begin this discussion
with the anatasestructure. Anatase is an important lowtemperature alteration product of many Ti-bearing minerals (biotite, pyroxene, perovskite), and a number of
thermally induced polymorphic transformations involving this mineral have been reported. The anatase-rutile
reaction that occursduring low-grademetamorphism was
studied experimentally by Shannon and Pask (1964) and
subsequently(with conflicting results) by Kang and Bao
(1986).The conversionof both TiO, (H) (Latrocheet al.,
1989)and TiO, (B) (Brohan et al.,1982) polymorphs to
anatasehas been reported. The structural rationalization
for the TiO, (B) to anatasetransformation is well understood (Brohan et al., 1982; Tournoux et al., 1986; Banfield et al., l99l). However, an explanationfor the TiO,
(H) to anatasetransformation has not been provided.
A representationof a slice of the anatasestructure constructed from the FBB (in the orientation shown on the
Fig. 3. FBB representationof TiO, (B). The FBB is shown
far right in Fig. l) is given in Figure 2. Consecutivelayers by the dark solid line, and the (103)-bounded anatase slab is
stack directly above the layer shown, linked by corner highlighted by the dashedline.
548
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
A
B
A
A
B
A
B
B
B
B
B
A
(a) Brookite
(c) Anatase
Fig. 6. The stackingofslabsshownin Figures5a and 5c (A
and B) (a) to form brookite,(b) to form TiO, II. The same
stackingof slabscontainingstraightratherthan kinked chains
producesrutile (c) to form anatase.
by shear,as indicated by the arrows. When viewed down
c of brookite, shear of this type throughout the structure
producesinfinite edge-linkedchains (out ofthe page).Alternatively, conversion ofthese chains to infinite chains
could involve cation hopping (every second pair along
the chains in Fig. 5 to form straight, edge-sharinghorizontal chains) rather than shear.Similar cation displacements relating rutile and TiO, II are discussedby Simons
can be constructedfrom structural slabsfound in anatase and Dachille (1970) and Hyde and Andersson(1988).
(the {101}-boundedslabs).In brookite, theseslabsoccur
Figure 5 showsthe arrangementof chains of FBBs (Fig.
in two alternating orientations, shown in Figure 4a as 5a) that form the (100) layers of brookite, the (101)stippled and unstippled. Figure 4b shows the orientation boundedslabsofanatase,and the (100) layersofTiO, II.
of the FBB for the unstippled slab. The first anataseslab The stacking of layers as shown in Figure 5b (A) or iden(stippled) is oriented with an anatasea axis coming out tical layers with an orientation as shown Figure 5c (B)
ofthe page,the secondwith an anatasea axis vertical.
generatesthe structural arrangementsillustrated in FigFigure 4c showsa strip ofbrookite structure that occurs ures 6a (brookite: AABBAABB . . .), 6b (TiO, II, and by
along the plane where the stippled and unstippled ana- rearrangementas discussedfor Fig. 4c, rutile: ABAB . . .),
tase-type slabs are joined. This strip can be converted and 6c (anatase:BBB . . .). Thus, ignoring the polyhedral
into the basic unit of the rutile structure (l l0 rutile slab) distortions, anatase,brookite, and TiO, II can be considered to be simply polytypes of TiO, basedon stacking of
layers illustrated in Figure 5b.
A 180'rotation ofoctahedra relatesthe A- and B-type
as noted by Simonsand Dachille (1970),
layerssuggesting,
that transformations betweenstructuresinvolve shearbetween O planes.A similar mechanism involving shearof
the O planes parallel to the planes ofoctahedra was postulated for the enstatite to clinoenstatite transformation
(Coe and Kirby, 1975).Although such mechanismsinvolving two sets of shear operations or cation hopping
plus shear would convert anataseor brookite to rutile,
we currently have no direct evidence to indicate the actual mechanismsby which the transformationsoccur, and
other mechanismsmay well operate.
Fig. 4. (a) FBBrepresentation
ofthe brookitestructure.The
dark line showsthe FBB, and the dashedline highlightsthe
(101)-bounded
anataseslab.Arrowsindicatethe brookiteaxes
(bold)andtheanatase
directions(italics)in theplaneofthe slabs,
(b) the FBB in the secondorientation,(c) structuralelement
relatedto the rutile structureby shear,as indicatedby the arrows.
Fig. 5. (a) Assembly of an FBB to form a chain, (b) arrangement ofchains to form sheetsfound in both anataseand brookite
[the (100) plane ofbrookite], (c) an FBB that forms sheetsidentical to those in b but rotated 180o.The numbers indicate how
the sheetin c may be superimposedonto that in b.
The TiO, (H) structure and its relationship to the
other polyrnorphs
An FBB representation of the TiO, (H) (hollandite)
structure is given in Figure 7. Unlike the structures de-
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
549
(a) 11l
Fig. 7. FBB representation
of the basicunit of the TiO, (H)
of chainsconstructure.The structurecontainstwo orientations
structedfrom the FBBhighlightedby the darklines(a) andin a
secondorientation(b).
scribed above, in TiO, (H) the FBBs are stackedto form
infinite chains ofedge-linked octahedralthesechains occur in two orientations. The TiO, (H) polymorph seems
structurally more closely related to rutile than to anatase,
and it has been shown that electron-beam damage can
result in the conversion of synthetic hollandite compounds to the rutile structure (Bursill, 1979). Consequently, it is interesting that Latroche et al. (1989) report
the conversion of TiO, (H) to anatase with increasing
temperature. A possible mechanism for this reaction involves displacement of cations from the chains in one
orientation into adjacent octahedral sites (Fig. 8a), followed by shear as indicated in Figure 8b. This produces
the TiO, (B) structure, which can in turn be converted to
anataseby the previously reported shear mechanism. If
this type of structural transformation occurs, we predict
that TiO, (B) developsas an intermediate phasebetween
TiO, (H) and anatase.
The structural relationships betweenperovskite and
TiO, structures
It has been previously noted that the TiO, (B) structure
can be described as a derivative of the ReO. structure
(Tournoux et al., 1986). Likewise, the perovskite structure is closely related to the ReO, structure (Hyde and
O'Keefe, 1973). In perovskite, Ca sits within a cornerlinked Ti-O framework that is identical to that found in
ReO, (i.e., perovskite is a stuffed derivative of ReOr).
Consequently,there are closesimilarities betweenthe perovskite and TiO, (B) structures.The relationships noted
Fig. 8. (a) Diagramillustratingthe conversionoftwo chain
orientationsin TiO, (H) to one by cationhopping:[l] : TiO,
(H) structure,[2] : cationdisplacement;
[3] : resultingstructure. (b) Arrows indicatethe shearneededto transform[3] to
Tio, (B).
in the precedingsectionthus imply that anatase,brookite,
TiO, il, and in a more complex way, rutile are also related to the perovskite structure.
ExpnnrprnNTAr, METHoDS AND SAMpLES usED
We studied naturally weatheredperovskite crystals from
the Salitre II carbonatites in Minas Gerais, Brazil. The
385-143,and 385-179,werecollected
samples,385-P4a,
by Anthony Mariano. Samples 385-179 and 385-143
contain lamellar-twinned perovskite embayed and veined
by anatase,whereas 385-P4a is composed primarily of
anatase,exsolved magnetite,and only minor residual perovskite.
Sampleswere examined with a Philips 420 ST transmission electron microscope(TEM) operatedat 120 keV.
Sampleswere preparedby dispersingcrushedgrains on a
holey carbon grid and by Ar-ion milling specimensremoved from petrographic thin sections. Minerals were
identified by their selected-areaelectron diffraction
(SAED) patterns and by their compositions. Analytical
electron microscope (AEM) data were obtained from the
thin edgesof samples using an EDAX Lidrifted Si de-
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, G)
o
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o
?"n","."
o _ @ t t & otobr
0 . 1 9n m
4ln
I rvD
o
oo.P,'.
o2o.nOo.r9
nt
t
danatase
bperovskite
a"n"r"a"f
a
O.
O2ooatl
o
&?.s.3
O.
@^^rlO ro-+canatase
o ct%?o
ooo
o
o
A
danatasel
'
Goo"n
oro ro
OOOO@/OOOO+canatase
0 10 p
ooo
o
bperovskite
Fig. 9. SAED patterns (a) down [001] ofanatase (a) and [001] ofperovskite (p; pseudocubicunit cell); (b), (c) down [010] of
anataseand [00 I ] of perovskite. In the SAED pattern in b the [00 I ] direction of anataseis perpendicularto and in c parallel to the
doubled axis (2x) ofperovskite (D ofthe orthorhombic cell). (d) Indexing ofa, b, c using orthorhombic perovskite cell.
tector and processedwith a Princeton Gamma-Tech System IV X-ray analyzer.
Anatase powder X-ray diffraction patterns were obtained using a Scintag automated powder diffractometer
with CuKa, radiation. Peak positions were corrected for
instrumental and physical aberrations through the use of
internal standard lines from Si (Standard ReferenceMaterial 640b). Cell dimensions were determined using Latcon, a Scintag least-squareslattice parameter refinement
program.
ExprnrprnNTAl RESULTS
SAED patterns indicate that perovskite is largely replaced by anataseand that the reciprocal lattices of the
minerals are oriented preferentially with respectto each
other. When viewed with the beam parallel to [001] of
anatase,it is apparent that the a axes ofanatase are parallel and approximately equal in length to the pseudocubic a axes of perovskite (Fig. 9a). We make reference
to the pseudocubicaxesofperovskite becausethis setting
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
551
0.381
0.380
0 379
0 378
o 377
0 376
3
sample #
Fig. 10. Comparisonbetweenthe lengthsof the a axes(in
nm) of undopedanatase:I : Cromerand Herrington(1955);
2 : JCPDSvalue;3, 4 : valuesfor undopedsyntheticanatase
determinedusingthe procedures
and equipmentdescribedhere
(Bischoftin preparation)and for anatasereplacingperovskite,
5. The 3oerrorbarsfor the standarddeviationsfrom the refined
a-axisdimensionsareshown.
permits an easiercomparison with anatase.The relationship between the pseudocubicand orthorhombic cells is
given by Megaw (1973)and White et al. (1985).For clarification, we have shown the orientation of the pseudocubic axes on the SAED patternsin Figures9a,9b,9c
and included the orthorhombic indices on the diagrams
in Figure 9d. Although electron diffraction patterns show
doubling of one axis (b of the orthorhomic perovskite
structure), the c axis of anatase shows no tendency to
preferential development parallel or perpendicularto this
direction (Figs. 9b, 9c). Thus, as far as the weathering
reactions are concerned,perovskite behavesas ifit were
cubic or pseudocubic. The origin of broad streaks and
extra reflections in SAED patterns and the associated
moir6 fringes in images will be discussedbelow.
Areas clearly showing the contact betweenanataseand
perovskite were rare. In general,either unaltered perovskite or porous aggregatesof anatasewere encountered.
Where contacts were viewed with the beam parallel to
[010] of anataseand to a pseudocubic a axis of perovskite, strips of anataseonly a few tens of nanometerswide
were preserved adjacent to the interface. No areas of
amorphous Ti oxide were detectedalong the boundaries.
AEM analysesofthe anatasetypically showedthe presence of 2-5o/oFeO (occasionallyup to l0o/oFeO associated with goethite-anataseintergrowths), l-2o/o CaO, and
< loloAlrO, and MgO. No other impurities were detected
(e.g., Nb, REE). Figure l0 illustrates that the c axes of
anataseafter perovskite are demonstrablylarger than values for pure anatase.The value determined for the c axis
was smaller at the lo level but not significantly different
at the 2o level. The larger a dimension compared with
electronmicrographdown [001]
Fig. I l. Low-magnification
ofboth anatase(A) and perovskite(P) showinglittle evidence
with the perovskitefor substantialvolume changeassociated
anatasereactron.
undoped anatasemay indicate cationic substitutions involving Fe3*,OH, and interstitial Ca within the anatase.
At low magnification with the beam parallel to [001]
of anatase (Fig. I l), anatase and perovskite were intimately intergrown and the material exhibited very low
porosity. At higher magnification (Fig. l2), most interfacesbetweenperovskite and anatasewere approximately
Fig. 12. High-resolution image down [001] of both anatase
and perovskite showing the orientation ofthe interface betrween
these minerals. Darker areaswithin the anatase(arrowed) may
be relict strips ofperovskite.
5s2
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
Fig. 13. (a) High-resolution image down [010] of anatase(A) and [001] of perovskite (P) showingthe orientation of the interface.
Strips ofa third phasewith a lattice spacinglarger than those found in anataseare indicated (I). (b) SAED pattern. (c) Enlargement
of the image in a showing the perovskite margin.
parallel to the pseudocubic a axes of perovskite and a
axesof anatase.Lattice fringes are not presentin the imageof anatasebecausethe spacingsare so small that they
were excludedby the objective aperture (spacingssmaller
than 0.22 nm were excluded). Lattice fringes associated
with the irregular strips within anataseare probably associatedwith relict perovskite.
Figure 13 illustrates a contact between perovskite and
anataseviewed down [010] of anatase.The numerous
moir6 fringes inclined (two orientations) to the interface
are a sample preparation artefacl (discussedbelow) and
should be ignored. In addition to these, there is a set of
fringes with a spacingof approximately 0.67 nm parallel
to the interfaceand (001) ofanatase.The spacingofthese
fringes cannot be readily explained as a moir6 effect from
overlapping perovskite and anatase,and they appear to
be developed in areas where perovskite is clearly not
present (Fig. l3c). These fringes were not observedat all
contacts and are not present along interfacesother than
those subparallelto (001) ofanatase.The zone between
the perovskite and all crystalline products is very narrow,
generally < I nm wide. No amorphous Ti oxide was detected in this region.
In addition to anatase,a secondTiO, mineral was identified in thesesamples.SAED patterns indicated that this
is TiO, (B) (Marchand et al., 1980),which has been reported by Banfieldet al. (1991).TiO, (B) was present(in
both ion-milled and crushedgrain-mount samples)as homogeneousareasadjacentto perovskite surfaces(Fig. la)
and intergrown with anatase.
SAED patterns from the coexisting minerals indicated
that the [0 I I ] TiO, (B) and I I I 2] perovskite zones(pseudocubic axes)were subparallel,and (200) of TiO, (B) and
(l l0) of perovskite (pseudocubicaxes) were inclined to
each other by about 20". The interface is subparallel to
( I I 0) of perovskite (pseudocubicaxes)and inclined to c*
of TiO, (B) by about 45".
When regionscomposedentirely of anatasewere viewed
down (100), the crystalswere found almost always to
contain planar gapsparallel to (001). Thesegapsgive rise
to streaking in some electron diffraction patterns (Fig.
l 5b).
The SAED patterns (Fig. 9) indicate that the c axis of
anatasedevelopsparallel to the three pseudocubicperovskite axes.Consequently,the weatheredperovskiteshould
contain crystals of anatase in three orientations. However, it is difficult to establish (statistically) whether c
anatasedevelopsparallel to each ofthese axeswith equal
probability.
Figure 16 illustrates an area composed of intergrown
anatasecrystalsin both [001] and [00] orientations. The
SAED pattern (Fig. l6b) indicates a very slight misorientation betweenthe (001) and (100) planesof the two
sets of crystals.
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
553
imagedown [010] of anatase
Fig. 15. (a) High-resolution
showingregulargapsparallelto the (002)fringes.Moir6 fringes
areinclinedto (002)fringes.(b) SAEDpattern.X indicatesadditionalreflectionsreferredto in the text.
of crystalsshow signsof recrystallizationand elimination
of very fine-scaleporosity.
The anataseis extensively intergrown with Ca-bearing
light rare earth element (LREE) phosphateminerals (Fig.
l8). Previouswork (Mariano. 1989)has shown that the
10 nm
Fig. 14. (a) Imageshowingan areaof TiO, (B) exhibiting
0.62-nm(001)fringes.TiO, (B) adjacentto perovskitehasbeen
destroyedby ion milling. (b) SAEDpatternfrom TiO, (B). Arrow indicatesthe c* direction.
Althougl some areasof anataseviewed with the beam
parallel to [001] show little porosity, some aggregatesof
needle-shapedanatase crystals with a high intercrystal
porosity were occasionallynoted (Fig. l7). The interiors
Fig. 16. (a) Low-magnification image showing an intergro\4'thof anatasecrystals oriented with their [001] directions
both perpendicular (as indexed) and parallel to the beam. (b)
SAED Datternlrom areashown in a.
554
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
Fig. 18. (a) Iow-magnification image showing intimately inanatase(A) and rhabdophane (R). (b) SAED pattern
tergrown
with a highintercrystal
Fig. 17. Aggregate
of anatase
needles
from rhabdophane.
porosrty.
LREE pattern of the minerals is identical to that of the
perovskite, indicating that the REEs were releasedfrom
the perovskite and reprecipitated without fractionation.
SAED patterns from several zone axeswere recorded from
needlessuch as those shown in Figure 18. TheseSAED
patterns were consistent with the identification of this
mineral as rhabdophane.Sample 385-P4aalso contains
abundant, complexly exsolved magnetite that shows no
sign ofalteration.
Sample preparation artefacts
Most of the specimens examined in this study were
preparedfor transmission electron microscopy by Ar-ion
milling without liquid N, cooling of the specimen. All
diffraction patterns from the anatasesamplespreparedin
this way showed extra reflections (particularly apparent
in Figs. 9c, l3b, l5b) that could be indexed to a cubic
unit cell v/rth d2oo: 0.21 nm that developedin two orientations with the ( 100) directions slightly rotated to either side of the tetragonal anataseaxes. Moir6 fringes in
high-resolutionimages(Figs. l3a, l5a) were pervasive,
occurring in small patches in two symmetry-related orientations over the entire sample. Dark-field images suggestthat the phasecausingthe moir6s was more abundant
along the sample margins. Diffraction patterns and imagesfrom crushed grains of anatasesamples showed no
evidence of extra spots or moir6 fringes. Furthermore,
specimens prepared by ion milling using a liquid-Nrcooled stagedid not show the extra reflections or moir6
fringes.We conclude that a surface-coatingphaseformed
during non-cooledion milling. Basedon the sizeand shape
of the unit cell, we tentatively suggestthat the material is
a surfacecoating ofTiO. In any case,the extra spots and
moir6 fringes should be ignored since they are not related
to the natural sample microstructure.
Drscussron
Perovskite alteration
Perovskite is a major igneous mineral in the Salitre II
carbonatite. Some altered samplesare almost completely
composedof TiO, minerals that pseudomorphthe perovskite. The coexistenceof anataseand TiO, (B) with rhabdophane,calcite, and goethite supports the currently held
view (Mariano, 1989) that the replacementof perovskite
by Ti minerals occurred as the result of weathering.
SAED patterns and images from intergrown perovskite, anatase,and TiO, (B) (Figs. 9, l3) clearly demonstrate that the TiO, minerals replacing perovskite are topotactically oriented. The anataseintergrowthscomposed
of crystals in two or more orientations (Fig. 16) may reflect initiation ofthe anatase-formingreaction at separate
sites in the perovskite. The direct overgrowth of TiO,
minerals in specificorientations onto the perovskite surface is consistent with the possibility that the reactions
involve direct structural inheritance ofparts ofthe Ti-O
framework of perovskite.
Alternative mechanisms, (l) dissolution followed by
reprecipitation or (2) dissolution followed by formation
of an amorphous Ti intermediate and crystallization of
anatase,are not inconsistentwith the topotactic relationship describedabove. However, given the extremely low
solubility of Ti under weathering conditions, a mechanism involving complete congruent dissolution of perovskite seemsunlikely. Furthermore, Jostsonset al. (1990)
report no Ti in the solutions associatedwith hydrothermal perovskite dissolution experiments.The perovskiteTiO, mechanism suggestedby Kastrissios et al. (1986,
the importance
1987)and Myhra et al. (1984)emphasizes
of Ca leachingand development of a disorderedor amorphous Ti-rich surfacelayer. Becausewe find no evidence
for an amorphous Ti phase along the interface between
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
anataseand perovskite (e.g.,Fig. l3), we suggestthat this
mechanism does not predominate during surficial weathenng.
High-resolution images indicate that the interface region between perovskite and anataseis very narrow (< I
nm wide; Fig. l3c). If this remaining perovskite is to be
converted to anatasewithout large-scalesolution transport of Ti, Ca leachingand structural reorganizationmust
occur along this planar region. In a zone as narrow as
this, distinguishing among transport of components
through a solution layer, surfacerestructuring, growth of
an amorphous intermediate film, and direct structural inheritance is extremely difrcult.
The presenceof two metastable polymorphs in place
of the thermodynamically stable polymorph rutile suggeststhe importance of kinetic and structural factors in
determining the reaction products.The formation of these
polymorphs may be controlled by the specific structural
inheritance involved in the reaction mechanisms or by
the structure of the surface onto which epitactic growth
occurs.
The growth of two TiO, minerals suggeststhat decalcification ofperovskite can occur by alternateroutes with
similar energeticrequirements.Although anataseis by far
the predominant Ti-bearing alteration product, the presence of TiO, (B) is of significance.TiO, (B) has been
reported in low-grade metamorphic rocks (Banfield et al.,
1991), but it has not been recognizedpreviously as a
weathering product.
Imagesrecordedwith the beam parallel to [001] of anatase (e.9., Fig. I l) srrggestedthat the areas occupied by
reactantand product structuresis approximatelythe same,
as observed in this orientation. Contacts between perovskite and anataseshowing the third dimension of the intergowths (beam parallel to ( 100) anatase)and preserving strips of anatasegreater than a few nanometerswide
were extremely rare. In regions where perovskite had been
completely destroyed, the anatase contained abundant
gaps (Fig. l5), suggestingthat a substantial reduction in
volume occurs parallel to [001] of anatase.
Recrystallization of the anataseto form arrays of elongate crystalsresults in the appearanceof very large holes.
The very significant volume change associatedwith the
perovskite-anatasereaction is particularly apparent where
this recrystallization has occurred.
555
Figure l9a shows an idealized (undistorted) representation of the perovskite structure viewed down (100)
(pseudocubicaxes). Orthorhombic distortions from this
idealized cubic arrangementinvolve displacementof the
O atoms with minimal disruption of the cation array
(O'Keefeand Hyde, 1977;White et al., 1985).
The conversion of perovskite to anataseand TiO, (B)
requires elimination of Ca, which is transported away
from the reaction site and reprecipitatedas calcite in other areas. The reaction also requires conversion of some
corner-linked to edgeJinked pairs of octahedra. The removal of every secondplane of Ca atoms by leachingand
the collapse of the Ti-O framework, as indicated by the
arrows in Figure l9a, generatea hypothetical structure
(Figs. l9b, l9c) with the composition Ca.rTiOrr. This
structure is closely related to shearedperovskite derivative structures in the (Na,K,Ca,Nb)TiO, system (Hyde
and Andersson, 1988). The arrangementof more darkly
stippled octahedrain Figures I 9b and I 9c is found in the
TiO, (B) structure.
The micrographsshown in Figures l3a and 13c illustrate a phase developed at the interface between anatase
and perovskite that is characteized by a 0.67-nm spacing
perpendicular to the interface, betweenthat of drooof anatase(0.475 nm) and the 0.77-nm spacingequivalentto
[020] perovskite (pseudocubicaxes).Becausethis material has the characteristicspredicted for the intermediate,
partly decalcifredweathering product (Figs. l9b, 19c), it
may have the structure shown in Figure l9c.
The secondstep to form anatasefrom the intermediate
phase may occur by removal of the remaining Ca and
collapse of the Ti-O framework, as indicated by the arrows in Figure l9c. The experimentally observed orientations of the intergrown perovskite and anatase, the
presence of what is interpreted to be the predicted
intermediate phase,the orientations of the interfacesbetweenthe three phases,the seeminglysmall volume change
associated with the intergrowths in two dimensions,
and the significant volume change associatedwith the
third dimension parallel to [001] of anataseare all consistent with the view that perovskite is converted to anatase by leaching and structural collapse as illustrated
in Figure 19. Consequently, the reaction can be written 2CaTiOr(pro")+ 2CaorTiOrnr,, + Cafrd,+ Oe) + 2Cai^[,+ 2Ol;or.The actual aqueous spe2TiO2("n","".)
cies are not known.
The perovskiteto anataseand TiO, (B) reactions
The TiO, (B) structure can be generatedfrom the inIt is not possible to use textural arguments to prove termediate, partly decalcified structure (Fig. l9c) by an
that transformations involved leaching of Ca and recon- alternative secondstep that involves Ca removal and colstruction of the existing Ti-O framework rather than oc- lapse of the Ti-O framework as indicated in Figures 19e
curring through a transient amorphous intermediate. We and l9f. This reaction can be written 2CaTiO, (prov.)+
favor the former mechanism because(l) an amorphous 2CaorTiOrr,,",, + Cafg + Oi; - zTiO, @,+ 2Ct(^ +
phase was not detected and (2) the direct mechanisms 2Offi. Consequently,we suggestthat the formation of TiO,
described below, which are a consequenceof the close minerals from perovskite during weathering can be atstructural relationships between perovskite, anatase,and tributed to stepwise decalcification, with alternative diTiO, (B), provide reasonablereaction pathways consis- rections of second-stagecollapse of the Ti-O framework
tent with the orientation relationships between reactant resulting in either anataseof TiO, (B). The conversion of
and products.
perovskite to TiO, minerals by a mechanism involving
556
(a) Perovskite:
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
structural collapse eliminates the requirement for largescaletransport of Ti in solution.
For the reaction to involve inheritance of slabs of the
perovskite Ti-O framework, it is necessaryto remove Ca.
The presenceof abundant gaps perpendicular to the directions of volume reduction and the proposed structural
collapsesuggestthat relatively narrow slabsof perovskite
react at one time. A possible mechanism involves leaching of Ca from the surface layer (as occurs to depths of
at least 3 nm in labradorite dissolution; Hochella et al.,
1988),with chargebalancemaintained by bonding of two
H* to two O atoms. This leaching opens pathways (via
the vacant Ca sites)to the interior of the crystal, through
which Ca could be transported to the surface.Collapseas
indicated by Figure l9a superimposesthe two OH groups,
so that the excessstructural O can be eliminated as HrO.
This could be written CaTiO. + 2H* + (s.z+ * HrTiOr;
H,TiO3-TiOr+H,O.
In this study we have not detectedamorphous Ti oxide
or brookite in the alteration assemblage,which distinguishesthe weatheringreaction from the reaction studied
experimentally at higher temperatures(e.g.,Kastrissioset
al., 1987).The absenceofbrookite is not surprisingif, as
we suggest,the natural weathering reactions involve a
direct structural transformation. As shown in Figure 4,
the brookite structure contains anatase-typeslabs in a
secondorientation that cannot be generatedfrom the perovskite or intermediate structure by simple collapse of
the Ti-O framework.
CoNcr,usroNs
We have presenteda way of viewing the TiO, structures that highlights their similarities and provides clues
to the mechanisms by which the polymorphic transformations between them occur. Considering these structures to be constructed from slabs or blocks related by
shear or cation displacement simplifies the study of the
perovskite to anataseand TiO, (B) weathering reactions
(this study) and the TiO, (B) to anatase(Brohan et al.,
1982; Banfield et al., l99l) and rutile-TiO' II transformation mechanisms(Hyde and Andersson,1988).
The FBB approach to interpreting other transformation mechanisms,such as brookite or anataseto rutile or
However, we
TiO, (H) to anatase,cannot yet be assessed.
have made predictions about these reactions that can be
testedwith future experimental work. If an FBB description appropriate to all the observed transformations could
be found, it would further enhanceunderstanding ofthe
relationships between theseminerals.
The results ofthe perovskite weathering study suggest
that the reactions involve direct structural inheritance of
planesofcorner-linked Ti octahedraand proceedby step-
F
Fig. 19. Diagram illustrating the stepwiseconversion ofperovskite (viewed down one pseudocubicaxis) to anatase(a), (b),
(c), (d); and Tio, (B) (a), (b), (e), (f).
BANFIELD AND VEBLEN: PEROVSKITE TO ANATASE AND TiO, (B)
wise removal of Ca and structural collapseto form edgesharingoctahedralpairs. Although some redistribution of
Ti by dissolution and reprecipitation may occur (e.9.,
during recrystallization), the mechanism described here
eliminates the need for large-scalesolution transport of
this very insoluble element.The formation of anataseand
TiO, (B), rather than other TiO, polymorphs,emphasizes
the importance of the structure of the precursor mineral
in determining the weathering product formed.
AcxNowr,BocMENTS
We thank Anthony Mariano for providing the weatheredSalitre II carbonatite samplesand initial information about their mineralogy.Timothy
White, George Guthrie, Jeffrey Post, and Dimitri Sverjensky provided
helpful comments that geatly improved this manuscript. This work was
supporredby NSF grant EAR-8903630.
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