Topotaxial reactions during oxidation of ilmenite single crystal

Abstract

The mechanism of ilmenite–rutile transformation during oxidation of natural ilmenite crystal was studied at elevated temperatures in air. The progress of oxidation with annealing time was studied in the temperature range between 600 and 900 C. 2.5 mm cubes were cut from the single Mn-ilmenite crystal in two special orientations, [001]ILM and [1-10]ILM, that allowed determination of crystallographic relations among the reaction products. Using X-ray diffractometry, energy-dispersive spectroscopy, and electron microscopy (SEM, TEM) techniques, we determined that the ilmenite to rutile and hematite transformation is triggered by surface oxidation of divalent cations (Fe, Mn) from the starting ilmenite and their crystallization in the form of hematite and bixbyite on the surface of the single crystal. Surface oxidation and outdiffusion of Fe2+ and Mn2+ ions opens paths for exsolution of rutile within the pseudo-hexagonal oxygen sublattice of the parent ilmenite, following simple topotaxial orientation relationship [001]RUT {010}RUT || [210]ILM {001}ILM. With this transformation, new channels for fast out-diffusion of divalent cations to the oxidation surface are opened along the c-axis of the rutile structure. The volume difference of the reaction products causes cracking of the single crystal, which opens additional free surfaces for accelerated oxidation. The results of this study contribute to better understanding of the recrystallization processes during pre-oxidation of ilmenite.

Full text

Topotaxial reactions during oxidation of ilmenite single crystal
Nadez
ˇda Stankovic
´
1
•Aleksander Rec
ˇnik
1
•Nina Daneu
1
Received: 28 May 2015 / Accepted: 8 September 2015 / Published online: 21 September 2015
ÓSpringer Science+Business Media New York 2015
Abstract The mechanism of ilmenite–rutile transforma-
tion during oxidation of natural ilmenite crystal was stud-
ied at elevated temperatures in air. The progress of
oxidation with annealing time was studied in the temper-
ature range between 600 and 900 °C. 2.5 mm cubes were
cut from the single Mn-ilmenite crystal in two special
orientations, [001]
ILM
and 1
10½
ILM, that allowed determi-
nation of crystallographic relations among the reaction
products. Using X-ray diffractometry, energy-dispersive
spectroscopy, and electron microscopy (SEM, TEM)
techniques, we determined that the ilmenite to rutile and
hematite transformation is triggered by surface oxidation of
divalent cations (Fe, Mn) from the starting ilmenite and
their crystallization in the form of hematite and bixbyite on
the surface of the single crystal. Surface oxidation and out-
diffusion of Fe
2?
and Mn
2?
ions opens paths for exsolution
of rutile within the pseudo-hexagonal oxygen sublattice of
the parent ilmenite, following simple topotaxial orientation
relationship h001iRUT 010
fg
RUT jj h210iILM 001
fg
ILM. With
this transformation, new channels for fast out-diffusion of
divalent cations to the oxidation surface are opened along
the c-axis of the rutile structure. The volume difference of
the reaction products causes cracking of the single crystal,
which opens additional free surfaces for accelerated
oxidation. The results of this study contribute to better
understanding of the recrystallization processes during
pre-oxidation of ilmenite.
Introduction
Rutile (TiO
2
) is one of the most important industrial
materials used for the production of white pigments and
elementary titanium [1]. Worldwide there are only few
economic alluvial deposits of rutile and these cannot sup-
ply enough material for the market demands. Therefore,
most of the titania production nowadays is based on the
conversion of ilmenite (FeTiO
3
)[1,2].
Different processes like acid-leaching or reduction of
ilmenite are used to remove Fe [3] and to extract TiO
2
from
ilmenite [2,4]. These processes are more efficient if they are
combined with pre-oxidization of ilmenite [2,5]. Many
studies on the oxidation of natural [5–10] and synthetic [3,6,
11–13] ilmenite are available, and it is generally accepted that
the oxidation process consists of two major steps: (i) con-
version of ferrous (Fe
2?
) to ferric form of iron (Fe
3?
)and(ii)
formation of different Fe- and Ti-containing phases depend-
ing on the temperature of oxidation. Below *800 °C, the
oxidation products are hematite (Fe
2
O
3
) and rutile (TiO
2
):
2Fe2þTiO3þ1
2O2!
\800 CFe3þ
2O3þ2TiO2:ð1Þ
Depending on the characteristics of the starting ilmenite
(particle size, degree of alteration) and processing condi-
tions (temperature, time, oxygen partial pressure), forma-
tion of intermediate crystallographic shear (CS) phases like
Fe
2
Ti
2
O
7
(Fe
2
O
3
2TiO
2
)[5,7] and Fe
2
Ti
3
O
9
(Fe
2
O
3

3TiO
2
)[3,6,11–15] is reported in the low-temperature
range. Above *800 °C, however, ilmenite decomposes
&Aleksander Rec
ˇnik
aleksander.recnik@ijs.si
Nadez
ˇda Stankovic
´
nadezda.stankovic@ijs.si
Nina Daneu
nina.daneu@ijs.si
1
Department for Nanostructured Materials & Joz
ˇef Stefan
International Postgraduate School, Joz
ˇef Stefan Institute,
Jamova cesta 39, 1000 Ljubljana, Slovenia
123
J Mater Sci (2016) 51:958–968
DOI 10.1007/s10853-015-9425-y

into the high-temperature mixed oxide, pseudobrookite
(Fe
2
TiO
5
) that forms together with rutile, according to the
following equation:
2Fe2þTiO3þ1
2O2!
[800 CFe3þ
2TiO5þTiO2:ð2Þ
In order to most effectively extract TiO
2
from the
starting ilmenite, it is especially important to understand
the ilmenite oxidation in the low-temperature regime
(\800 °C).
The most critical process during ilmenite oxidation is the
ferrous (Fe
2?
)toferric(Fe
3?
) iron transition. It has been
shown that fine-grained ilmenite starts to oxidize already at
500 °Corevenlower[6,8,9] and that ion migration (dif-
fusion) processes become enhanced at higher temperatures
[10]. The oxidation of iron starts at surface by diffusion fol-
lowing the logarithmic rate law and continues with the slower
lattice diffusion according to the parabolic rate law [12]. In
the process of oxidation, iron diffuses out of the structure of
ilmenite and produces a surface hematite layer, while needle-
like rutile texture is developed within the sample [10–12,14].
Beyond simple chemistry, the oxidation process, there
are complex diffusion-controlled topotaxial reactions of
which the structural aspects are poorly understood. These
structural transformations are possible because ilmenite,
rutile, and hematite are closely related through a common
hexagonal close-packed oxygen sublattice, which enables
complex intergrowths of these phases [16]. There are only
few studies dealing with the structural aspect of the
transformation process, e.g., which are the favorable
crystallographic directions for the diffusion of different ion
species within more or less rigid oxygen sublattice. In
nature, rutile often occurs in the form of oriented inter-
growths, which are a consequence of topotaxial transfor-
mations and exsolutions from structurally related
precursors in special crystallographic directions [17–21].
Especially common are rutile/hematite intergrowths that
form as topotactic replacement after ilmenite [18]. Two
principal orientation relationships have been reported
between the rutile and corundum-type precursor structures,
OR-1: h001iRUT 010
fg
RUT jj h210iCOR 001
fg
COR and OR-2:
h101iRUT 010
fg
RUT jj h210iCOR 001
fg
COR [16,20]. Rutile
exsolutions according to the OR-1 follow six directions
within ilmenite/hematite matrix, where the exsolved rutiles
coincide at 120°, producing non-crystallographic junctions.
On the other hand, exsolutions according to OR-2 are more
complex and follow 12 different directions in ilmenite/
hematite matrix. Rutiles exsolved according to this law
produce {101} and {301} twin boundaries in addition to
three other types of non-crystallographic contacts.
According to Rec
ˇnik et al. [21], development of a certain
OR entirely depends on the kinetics of transformation that
is controlled by the surface oxidation rate.
All previous studies on oxidation of ilmenite for the
production of titania were performed on fine crystalline
ilmenite, where diffusion processes are fast and structural
transitions are difficult to observe. To study these effects,
we used single crystal of ilmenite, where the kinetics of
reactions is expected to be slower, enabling observation of
the transformation process along specific crystallographic
orientations. The natural ilmenite crystal used in our study
was Mn-rich, which was helpful to understand the transport
of divalent cations (Mn, Fe) during oxidation process.
Materials and methods
In our study, we used a crystal of natural ilmenite from Mt.
Zagi (Khyber Pakhtunkhwa province, Pakistan). The
crystal measured about 3 cm in diameter and was 0.5-cm
thick. It had a characteristic thick-tabular {001} habit with
well-developed basal and pyramidal faces (Fig. 1). Prior to
oxidation experiments, the crystal orientation and chemical
composition of the starting sample were determined by
XRD and SEM/EDS. While the orientation of the c-axis
was evident from the crystal’s morphology, the aand
baxes were determined from an XRD pattern recorded on a
slice of a pyramidal face, identified as (012) (see Fig. 1).
Based on this information, we were able to determine the
crystallographic orientation of the initial crystal.
Starting sample composition
According to EDS analysis of bulk sample, it has been
confirmed that the initial crystal contains a significant
Fig. 1 Tabular Mn-ilmenite crystal from Zagi mountain, NW
Pakistan. aPhotograph of original Mn-ilmenite crystal. bIts shape
is defined by well-developed basal and prismatic faces. Basal surface
shows 60°striations resulting from alternation of growth ledges
parallel to prism faces. The prism face, marked in a, was used to
determine the crystallographic orientation of the crystal. cXRD
recorded on this facet shows only the {0, k,2k} reflections, indicating
that the index of this facet is (012). The grid in billustrates the cutting
of the crystal into 2.5 92.5 mm single-crystal cubes used for
oxidation studies
J Mater Sci (2016) 51:958–968 959
123

amount of manganese (Mn), replacing for iron (Fe) in an
equimolar ratio corresponding to (Fe
1–x
,Mn
x
)TiO
3
solid-
solution series between ilmenite (FeTiO
3
) and pyrophanite
(MnTiO
3
), with an empirical composition Fe
0.58
Mn
0.42
TiO
3
. Equimolar exchange of A-site cations indicates that
Ti
4?
ions are compensated by divalent Mn and Fe [21].
The sample is homogeneous in composition from center to
rim. Powder XRD analysis of the starting sample shows a
systematic shift of reflections towards lower Bragg angles,
consistent with the chemical composition of the sample.
The unit-cell parameters of natural Mn-ilmenite are a,
b=5.110 A
˚, and c=14.168 A
˚(determined from exper-
imental XRD; Fig. 2), placing the mineral on the tie-line
between ilmenite (a,b=5.0884 A
˚and c=14.0855 A
˚;
Wechsler and Prewitt [22]) and pyrophanite (a,b=5.1395
A
˚and c=14.2829 A
˚; Kidoh et al. [23]). Both measure-
ments thus place the sample at the mid of the solid-solution
series, which makes it ideal for studying ilmenite-to-rutile
transformation, as well as the mobility of Fe and Mn during
the oxidation process.
Sample preparation and heat treatment experiments
To study the structural aspects of Mn-ilmenite oxidation,
the crystal was cut into 2.5-mm cubes with the edges
parallel to [001]
ILM
, [110]
ILM
, and 1
10½
ILM axes (ILM:
Mn-ilmenite). After cutting, the samples were heat-treated
in air in a temperature range between 600 and 900 °C for 1,
12, and 100 h with heating and cooling rates of 10 °C/min.
We kept care of the relative orientation of the samples
during handling. The oxidation rate, R
ox
, of heat-treated
samples was determined from the mass difference of the
samples Dm(in wt%; measured to 0.1 mg accuracy) before
and after the heat treatment, according to the following
equation:
Rox ¼Dm
Dm0
100 %;ð3Þ
where Dm
0
stands for a theoretical mass gain of 5.28 wt%
that would correspond to completely oxidized sample
(taking into account the empirical Fe:Mn ratio in the
starting sample), where divalent cations Fe
2?
and Mn
3?
in
Mn-ilmenite are converted into trivalent oxides Fe
2
O
3
and
Mn
2
O
3
, following a similar oxidation pathway, as descri-
bed in Eq. 1.
Sample characterization
Phase composition of starting ilmenite and the composi-
tions of the heat-treated samples were determined by
powder X-ray diffractometry (XRD; PW1710, Philips,
Germany) using Ni-filtered Cu-Karadiation in the range
2h=10°–60°with a step of 0.017°and recording time of
1 s/step. Microstructural investigations were performed
Fig. 2 Powder X-ray
diffractogram of starting Mn-
ilmenite and the samples heated
at 600–900 °C for 12 h.
Reflections of the starting
sample fall between ilmenite
and pyrophanite reflections,
indicating a member of FeTiO
3
–
MnTiO
3
solid-solution series.
(110) rutile reflection first
appears at 700 °C(asterisk). At
800 °C, in addition to rutile,
(104) hematite reflection and
(222) bixbyite reflection start to
emerge at 33.15°and 32.96°,
respectively (down-pointing
triangle). At 900 °C,
pseudobrookite phase starts to
form. Colored lines represent
major reflections of ilmenite
(blue), pyrophanite (green),
rutile (red), pseudobrookite
(black), and hematite (brown)
(Color figure online)
960 J Mater Sci (2016) 51:958–968
123

using a scanning electron microscope with a field-emission
electron source (FEG-SEM; JSM-7600F, Jeol, Japan) and
equipped with an energy-dispersive X-ray spectrometer
(EDS; INCA 350, Oxford Instruments, England).
Microstructure evolution of oxidized samples was studied
in basal [001]
ILM
and 1
10½
ILM prism cross-sections. Same
two orientations were used to prepare the samples for
studying the orientation relationships of the product phases
by transmission electron microscopy (TEM), using con-
ventional 200 kV microscope (TEM; JEM-2100, Jeol,
Japan) with LaB
6
electron source. For TEM investigations,
3-mm disks were cut from the regions of interest in the two
selected orientations using an ultrasonic cutter. The disks
were thinned to *100 lm and dimpled to *15 lm at the
disk center. Electron transparency was obtained by ion
milling (PIPS, Precision Ion Polishing System, Gatan Inc.,
USA), utilizing 4.5 kV Ar
?
ions at an incidence angle of 8°
until perforation.
Results
XRD analyses of starting Mn-ilmenite reference and
samples fired for 12 h at different temperatures between
600 and 900 °C are shown in Fig. 2. Untreated sample
shows reflections that correspond to a member of solid-
solution series between ilmenite and pyrophanite [24]. No
splitting of the reflections is observed. The sample fired at
600 °C shows no products, which is in accordance with
previous HT oxidation studies of ilmenite [11,12]. Faint
reflections of rutile first appear in the sample annealed at
700 °C. After heating at 800 °C, these reflections become
more intense and a shoulder after the (104)
ILM
reflection
starts to grow due to the presence of hematite. At 900 °C,
additional reflections start to emerge, as the oxidation
reaction begins to follow the Eq. 2. With a decrease of
Mn-ilmenite reflections, rutile reflections start to gain in
intensity. In addition, reflections of the HT pseudo-
brookite (Fe
2
TiO
5
) phase are observed. The conversion
rate (R
ox
) increases with the annealing time. This suggests
that oxidation kinetics of the single-crystal samples is
slow, as it is mainly limited by bulk (crystal) diffusion of
cations.
SEM images of [001]
ILM
cross-sections of the samples
heat-treated for 12 h in the temperature range of
600–900 °C are shown in Fig. 3. The sample fired at
600 °C (Fig. 3a) shows no evident transformation, only
Mn-ilmenite, which is consistent with XRD data recorded
on this sample (Fig. 2). At 700 °C (Fig. 3b), about 1-lm-
thick Fe-oxide rich layer appears on the surface of the
sample, which is consistent with the appearance of hema-
tite reflections in XRD spectra. This is accompanied by a
20–25-lm-deep recrystallization zone, characterized by
thin rutile lamellae, intersecting at 60°. Formation of oxide
layer at the crystal surface and rutile lamellae in the
recrystallization zone indicates that at this temperature, the
onset of the oxidation process might be driven by out-
diffusion of Fe
2?
ions from ilmenite crystal and its surface
oxidation to Fe
2
O
3
. As a result, rutile lamellae start to form
in the Fe-depleted ilmenite. At 800 °C (Fig. 3c), both the
hematite layer become thicker and the recrystallization
zone extends deeper into the crystal, indicating faster
oxidation kinetics. Along with the sagenitic rutile in the
recrystallization zone cracks start to appear. The cracks
broaden from the interior towards the surface of the crystal,
forming numerous side branches. Quantitative SEM/EDS
analyses of the matrix phase between the rutile lamellae
revealed that its composition is similar to that of the
starting ilmenite. In addition to fine-grained hematite
(Fe
2
O
3
), the surface oxide layer contains bixbyite (Mn
2
O
3
),
indicating that both Fe
2?
and Mn
2?
diffuse out of the
parent Mn-ilmenite during oxidation. SEM/EDS mappings
showed that their distribution is inhomogeneous. Gener-
ally, near the ilmenite interface, the layer is enriched in Fe,
whereas its surface shows more Mn. The presence of bix-
byite is in accordance with the study of Grant et al. [25]
who reported that bixbyite is the most stable low-temper-
ature Mn-oxide phase when Fe
3?
ions are available in
abundance. The 900 °C sample (Fig. 3d) is completely
recrystallized; sagenitic rutile extends throughout the
original ilmenite crystal. Here, rutile starts to react with the
ilmenite matrix to form pseudobrookite.
Let us take a look on the time dependence of the oxi-
dation process at 800 °C, where the reaction follows Eq. 1.
According to XRD analyses, the fraction of the reaction
product rutile increases with the increasing oxidation time,
while major reflections belong to the starting Mn-ilmenite
(Fig. 4). Minor weak and broad reflections of hematite
(Fe
2
O
3
) and bixbyite (Mn
2
O
3
) appear after longer anneal-
ing times in the diffractogram, indicating their low amount
and fine grain size.
The comparison of microstructures at 800 °C clearly
shows a progress of oxidation with time, starting from the
surface towards the interior of the samples (Fig. 5). After
annealing for 1 h, the oxide (hematite/bixbyite) layer is
*3-lm thick, and the recrystallization zone with sagenitic
rutile extends *160-lm deep into the ilmenite crystal
(Fig. 5b). After 12 h, the recrystallization zone reaches a
depth of only *220 lm (Fig. 5c), which suggests expo-
nential dependence of oxidation rate related to increasingly
longer diffusion paths for cations that are leaving the
crystal. Within the ilmenite matrix, rutile lamellae extend
along three principal directions, intersecting at 120°sug-
gesting that the exsolution of rutile is structurally con-
trolled by the parent Mn-ilmenite matrix. Exsolution of
rutile is accompanied by the formation of cracks, which
J Mater Sci (2016) 51:958–968 961
123

broaden with oxidation time. In large cracks, the formation
of hematite is observed. The depths of recrystallization
zones with corresponding thicknesses of the surface oxide
layers and mass gains for different heat treatment condi-
tions are summarized in Table 1.
A close-up of rutile exsolutions in parent ilmenite viewed
in basal orientation (Fig. 6a) suggests a simple orientation
relationship OR-1, where the c-axes of exsolved rutile
domains intersect at 60°[17,21]. To determine a precise
orientation relationship between rutile and hosting Mn-il-
menite, the 800 °C/12 h sample was further studied by
TEM. Figure 6b shows a low-magnification bright-field
TEM image of coinciding rutile lamellae viewed in [001]
ILM
projection of ilmenite. Apparently, rutile is exsolved in three
directions within the Mn-ilmenite matrix. In this sample, the
average width of rutile lamellae ranges from 50 to 25 nm.
The angles measured between individual rutile lamellae
roughly correspond to 60°. Electron diffraction pattern
(Fig. 6c) recorded across the selected rutile/Mn-ilmenite
area, shown in Fig. 6b, shows that Mn-ilmenite is oriented in
[001]
ILM
projection, while the rutile domains are in
[010]
RUT
projection, and (33-0)
ILM
plane of ilmenite coin-
cides with (002)
RUT
plane of rutile, or alternatively,
(110)
ILM
is parallel to (200)
RUT
. Converting planes into
directions, the orientation relationship exactly corresponds
to OR-1: h001iRUT 010
fg
RUT jj h210iILM 001
fg
ILM, with
rutile–ilmenite interface (200)
RUT
|(110)
ILM
. It appears that
the rutile’s c-axis runs parallel to the elongation of the rutile
lamellae, which coincides with the most rapid diffusion
pathway reported for rutile [26].
While in the mid of the recrystallization zone rutile has a
well-defined structure, we observe an unusual transitional
rutile-type phase at the recrystallization front, where
ilmenite crystal is being transformed into a rutile. Here,
some rutile lamellae exhibit an extra superperiodicity,
characteristic for modulated structures. Figure 7a shows a
HRTEM image of such transition zone between ilmenite
and rutile. While the upper part of rutile domain shows a
Fig. 3 Back-scattered SEM images of samples heat-treated at
different temperatures for 12 h. aAt 600 °C, the sample is
homogeneous without any oxidation products and appears identical
to untreated sample (Fig. 5a). bAt 700 °C, a thin oxide layer appears
at the surface of the sample, indicated by bright BSE contrast,
together with *20-lm deep recrystallization zone (indicated by
dashed white line) extending from the surface into the interior of the
crystal. cAt 800 °C, the oxide layer and the recrystallization zone
proportionally increase. Along with sagenitic rutile cross-hatching,
the ilmenite matrix deep cracks start to appear in this zone. dAt
900 °C, the sample is completely recrystallized
Fig. 4 XRD diagram of untreated Mn-ilmenite and the samples
heated at 800 °C for 1, 12, and 100 h. Major reflections belong to Mn-
ilmenite. Rutile (marked by asterisk) is present already after 1 h of
heating and its amount increases with prolonged annealing time.
Broad heap at 2H&33°, that becomes evident after 12 h, belongs to
hematite and bixbyite (down-pointing full triangle)
962 J Mater Sci (2016) 51:958–968
123

regular pattern for the rutile structure in [010]
RUT
projec-
tion, the lower part shows superperiodic pattern along one
set of lattice planes. The boundary between these two
domains is diffuse, showing smooth transition from one to
another pattern. Fourier transforms from regular (Fig. 7b)
and superperiodic (Fig. 7c) rutile indicate triple periodicity
along one set of (101)
RUT
diffraction spots. Similar tripling
in this direction was reported by Putnis [27] and Rec
ˇnik
et al. [21] for Fe-rich rutile, where the authors assigned this
modulation to the so-called Guinier–Preston (GP) zones.
Further information on exsolution mechanism was
obtained in prism orientation of the sample. Figure 8shows
the 800 °C/100 h sample in [1 
10]
ILM
projection. Here, the
exsolution texture appears very different compared to that
in basal projection. In [1
10]
ILM
projection, the c-axis of
parent Mn-ilmenite is pointing upwards. SEM image in
Fig. 8a shows that rutile exsolution lamellae are lined
parallel to the [110]
ILM
prism surface of ilmenite. Vertical
orientation of the exsolution lamellae indicates that they
chose the direction of fast cation diffusion reported for
oxides with the corundum-type structure [28]. The length
of individual rutile columns well exceeds 100 lm, while
their widths are generally below 250 nm. A close-up into
the fine texture of exsolution lamellae shows that rutile
columns are interrupted by perpendicular nano-sized nodes
extending across the width of the column (Fig. 8b). This
interesting feature was further analyzed by TEM. Rutile
columns have an unusual bamboo-like appearance where
the rutile lamella is randomly interrupted by joint-like
nodes (Fig. 8c). The main feature of bamboo-like rutile
columns is that the rutile sections are concave with respect
to hosting ilmenite and the nodes are thicker and have a
convex shape. Chemical composition (EDS) of these nodes
indicates that they consist of ilmenite, with a slightly
altered Fe/Mn ratio compared to that of the parent ilmenite.
While the composition of the parent ilmenite is Fe
0.60
Mn
0.40
TiO
3
(TEM/EDS analysis), the composition of the
ilmenite within the rutile nodes corresponds to Fe
0.67
Mn
0.33
TiO
3
(see Table 2). Interestingly, the Fe/Mn ratio in
the rutile lamella is comparable to the ilmenite in the node
sections of the lamella, whereas a slight increase of this
ratio is also observed in parent ilmenite. Figure 8d shows a
typical joint section at higher magnification. At first glance,
Fig. 5 Progress of oxidation process with annealing time at 800 °C
viewed along [001]
ILM
orientation. aPolished Mn-ilmenite crystal in
its pristine condition. The crystal is homogenous without any apparent
chemical variations, pores, or inclusions. bAfter 1 h of annealing, a
surface hematite/bixbyite layer starts to form along with deep
recrystallization zone with sagenitic rutile. A close-up in lower-right
corner shows directions of exsolved rutile near the sample surface
covered by a layer of oxide (bright contrast). cAfter 12 h,
recrystallization accompanied by the formation of deep cracks further
advances from the crystal surface towards its interior. dAfter 100 h,
the sample is completely recrystallized
Table 1 Measured weight gain (Dm), oxidation rate (R
ox
), thickness
of hematite/bixbyite surface layer (d
HB
), and depth of recrystallization
zone (d
RT
) for samples annealed at different T–tconditions
T(°C) t(h) Dm(%) R
ox
(%) d
HB
(lm) d
RT
(lm)
700 1 n/a n/a n/a n/a
12 n/a n/a 1–2 20
100 0.56 11.2 5 120
800 1 0.29 5.6 3 160
12 0.79 15.8 10 200
100 1.34 26.8 25 compl.
900 1 0.42 8.4 5 270
12 1.04 20.8 22 compl.
100 2.17 43.4 25–100
a
compl.
a
Irregular thickness
J Mater Sci (2016) 51:958–968 963
123

a complex situation carries a valuable information on the
dynamics of diffusion-controlled exsolution processes. In
addition to different chemical composition, ilmenite in
rutile nodes has different orientation than the parent
ilmenite indicating its different origin. While the c-axis in
parent ilmenite is pointing upwards, that in the nodes
points sideways. Both compositional and structural features
strongly indicate a different origin of nodular ilmenite,
which will be termed as ilmenite’. FFT transform of the
ilmenite–rutile–ilmenite’ section (Fig. 8e) clearly shows
that the ilmenite domains are rotated by 90°with respect to
each other. Similar Fe/Mn ratio to that measured in rutile
lamellae and its perpendicular orientation suggests that
ilmenite’ exsolved from rutile on cooling, rather than from
parent ilmenite. Another evidence of this process is a
concave shape of rutile sections that indicate reverse
exsolution of ilmenite that resulted in a volume reduction
of rutile.
The reverse exsolution of ilmenite’ is indicative for
intense cation diffusion dynamics through rutile. Figure 9
shows a HRTEM image of ilmenite–rutile–ilmenite’ sec-
tion with proposed model of structurally controlled exso-
lution processes that lead to the formation of bamboo-like
rutile exsolution lamellae. The sequence of exsolution can
be deduced based on structural coherency across the
ilmenite–rutile and rutile–ilmenite’ interfaces. hcp stacking
of the parent ilmenite is easily translated into the a-plane of
rutile through reordering of Ti
4?
ions in the common
oxygen sublattice. Formation of rutile structure involves
minor tetragonal distortion that produces a hcp-like stack-
ing also in the perpendicular, b-plane of rutile. During the
heat treatment, rutile structure serves as the easy transport
pathway for out-diffusion of cations (Fe
2?
and Mn
2?
) that
are leaving the structure. During annealing, the rutile is
thus ‘saturated’ by Fe
2?
and Mn
2?
. However, when the
temperature drops, rutile throws out the excess cations
down to a given limit when they are ‘frozen’ in the
structure. Exsolution of Fe
2?
and Mn
2?
can take place only
Fig. 6 Rutile exsolution lamellae in the 800 °C/12 h sample observed
in [001]
ILM
projection. aSEM/BSE image of exsoluted rutile lamellae
intersecting at 60°.bBright-field TEM image of coinciding rutile
lamellae. cElectron diffraction pattern of Mn-ilmenite and rutile from
an area indicated in b. Indexing the EDP shows that rutile and ilmenite
are met in OR-1. Black and red dots on the right part of EDP are
simulated diffraction patterns for ilmenite and rutile, showing coinci-
dence of the two lattices (Color figure online)
Fig. 7 Transitional rutile-type phase at the recrystallization front
observed in the 800 °C/12 h sample. aHRTEM image of recrystal-
lization front, where ilmenite (upper right corner) is being trans-
formed into a rutile (below) through an intermediate phase with
evident superperiodicity. bFFT of already transformed rutile and
cFFT of transitional GP phase that shows tripling of (101)
RUT
reflection spots. For a reference, simulated EDP of rutile (red dots)is
overlaid over the experimental SAED pattern (Color figure online)
964 J Mater Sci (2016) 51:958–968
123

within the rutile, which now becomes the parent structure
in this secondary topotaxial process. Rutile and precipitated
ilmenite’ form another low-energy interface with b-plane
of rutile parallel to hcp sequence of ilmenite’. This inter-
face is structurally identical to the primary ilmenite–rutile
interface, but translated by 90°through rutile structure (see
Fig. 9b). The resulting ilmenite–ilmenite’ (110)
ILM
|(003)
ILM’
interface is fully incoherent and reflects the exsolution
sequence.
Discussion
The ilmenite oxidation is a typical example of topotaxial
recrystallization, where the original single crystal is
transformed into structurally related phases along the
crystallographically best matching directions [16]. Ilmenite
transformation occurs within the almost unaffected oxygen
sublattice, only the cations are rearranged within the
O-sublattice to form more stable products. In the present
study, we show that oxidation of ilmenite is structurally
controlled topotaxial reaction process where rutile TiO
2
is
exsolved from the parent Mn-ilmenite phase following
previously described OR-1: h001iR010
fg
Rjj h210iI001
fg
I
orientation relationship [21,27]. This complex structural
transformation is driven by surface oxidation and lattice
diffusion. The only species that can be oxidized in the
system are the Fe
2?
and Mn
2?
ions, while Ti
4?
ions are
already in their fully oxidized state. This triggers a constant
diffusion of divalent cations to the crystal surface, where
Fig. 8 Rutile exsolution lamellae in the 800 °C/100 h sample viewed
in ½1
10ILM projection. aLow-magnification SEM/BSE image of
recrystallized ilmenite with *25-lm-thick surface oxide layer
(white). The lengths of rutile exsolution lamellae, lined along the c-
axis of parent ilmenite, exceed 100 lm. bClose-up showing tiny
light-gray notches interrupting the vertical rutile columns (dark-
gray). cBright-field TEM image of a single rutile column showing a
bamboo-like morphology. EDS analysis shows that these nodes have
an ilmenite-like composition, slightly different from the parent Mn-
ilmenite. dClose-up of an ilmenite node within a rutile column
showing the evidence of complex exsolution processes. eFFT of the
ilmenite–rutile–ilmenite’ section showing that the c-axes of parent
ilmenite and ilmenite’ in node sections of rutile are rotated by 90°to
each other, visible by e.g., {116} diffraction spots
Table 2 TEM/EDS analysis of compositions in parent ilmenite,
exsolved rutile, and secondary ilmenite’ precipitated from rutile
Ti (at.%) Fe (at.%) Mn (at.%) Fe/Mn
Ilmenite 50.2 ±2.7 30.3 ±2.6 19.5 ±2.1 3:2
Rutile 95.5 ±0.8 3.1 ±0.5 1.5 ±0.4 2:1
Ilmenite’ 48.2 ±2.4 34.6 ±3.0 17.2 ±1.6 2:1
J Mater Sci (2016) 51:958–968 965
123

they are oxidized into their trivalent state and form poly-
crystalline (Fe, Mn)-oxide layer, which is consistent with
the previous studies on fine crystalline ilmenite [6,11].
No different than in fine crystalline ilmenite [15], oxi-
dation of Fe
2?
and Mn
2?
begins on the surface of the
crystal, while the reaction is driven by the chemical
potential [6,10]. As a consequence, the host becomes Mn-
and Fe-deficient, which triggers structural rearrangement
and the formation of rutile lamellae within the so-called
recrystallization zone. In this (Fe, Mn)-depleted Mn-il-
menite, Ti
4?
ions start to rearrange into closely related
rutile structure within the ilmenite host. This is possible
because ilmenite and rutile have a compatible hexagonally
close-packed oxygen sublattice (tetragonally distorted in
rutile), where cations partially occupy the available octa-
hedral sites. The transformation is accomplished by out-
diffusion of Fe
2?
(and Mn
2?
) followed by rearrangement
of remaining Ti
4?
ions to form a rutile structure. During
rearrangement of Ti within the O-sublattice, a transition
zone is formed in the recrystallization front, where we
observe rutile with an unusual modulation. In rutile, similar
structures can be produced by crystallographic shear (CS)
mechanisms [14,29], or indicate the presence of transi-
tional Guinier–Preston (GP) zones [27]. CS structures are
more characteristic of non-stoichiometric rutile (reduced;
Ti
3?
present in the CS-planes), whereas GP zones have
been reported for Fe-rich rutiles [21,27] similar to that in
our samples. Transitional GP zones were found only at the
recrystallization front of rutile and it seems that the tran-
sition phase is short-living and rapidly recrystallizes to
normal rutile structure by out-diffusion of the Fe ions.
The main question is how divalent cations diffuse from
the interior of ilmenite crystal to its surface where they are
oxidized. Solid-state diffusion studies showed that the
mobility of Fe along the c-axis of the hematite is several
orders of magnitude faster than in prism directions [28].
This opens channels for rearrangement of the Ti
4?
ions,
forming topotaxial rutile exsolutions along the c-axis of
parent ilmenite [21]. It can be expected that the diffusion
rates are similar in the Mn-ilmenite, which is isostructural
with hematite. This is supported by our observations, which
indicate that the c-axis of parent Mn-ilmenite provides the
main diffusion path for Fe
2?
and Mn
2?
cations, leaving a
free space for rearrangement of Ti
4?
to form rutile along
this direction (see general alignment of rutile columns in
Fig. 8). As soon as rutile is formed, this accelerates the out-
diffusion of divalent cations, as their diffusion (Fe) along
the c-axis of rutile is faster by several orders of magnitude
compared to that in hematite [24,26,28]. The channel-like
structure of rutile serves as an easy pathway for fast out-
diffusion of Fe
2?
(and Mn
2?
) towards the crystal surface,
where they are oxidized. As a consequence, rutile is
heavily enriched with Fe and Mn (EDS analyses, Table 2).
On cooling, the solid solubility of these cations is
Fig. 9 The mechanism of exsolution processes during oxidation of
ilmenite under 800 °C. aHRTEM image showing parent ilmenite
(left), exsolved rutile (center), and secondary ilmenite’ (above).
bStructural relations between the exsolved phases. First, rutile is
exsolved from parent ilmenite forming a coherent (110)
ILM
| (100)
RUT
interface. On cooling, Fe
2?
and Mn
2?
cations that diffused through
rutile are exsolved and form ilmenite’ with secondary coherent
(010)
RUT
| (110)
ILM
0
interface. With this operation, the two ilmenites
are brought into non-crystallographic (110)
ILM
| (003)
ILM
0
orientation
966 J Mater Sci (2016) 51:958–968
123

exceeded, and therefore, they are exsolved from rutile in
the form of ilmenite’ in orientation perpendicular to matrix
ilmenite. The observed difference between the Fe/Mn ratio
compared to the primary Mn-ilmenite suggests different
diffusion rates of the two cations, which is consistent with
diffussion data reported by Sasaki et al. [26]. Higher Fe/Mn
ratio in rutile (and in secondary ilmenite exsoluted from
rutile on cooling) thus suggests faster out-diffusion of Fe
2?
ions compared to Mn
2?
, which is driven by a preferential
oxidation of Fe
2?
on the surface. Owing to negative vol-
ume difference, the host ilmenite crystal starts to crack,
which further boosts the transformation (see Figs. 3and 5),
and in this way, oxidation process progresses further into
the single crystal.
Conclusions
We have shown that oxidation of ilmenite (in our case Mn-
ilmenite) in air is a topotaxial transformation caused by
surface oxidation of divalent cations from the ilmenite
structure. This triggers progressive recrystallization from
the surface towards the interior of the crystal, according to
the following mechanism:
(1) Surface oxidation generates out-diffusion of divalent
cations (Mn and Fe) and formation of polycrystalline
hematite/bixbyite layer at the crystal surface.
(2) Depletion in Mn and Fe in the interior of the
ilmenite crystal leads to structurally controlled
exsolution of rutile lamellae, following a simple
crystallographic relationship h001iRUT 010
fg
RUT jj
h210iILM 001fg
ILM, producing networks of sageni-
tic rutile within parent ilmenite.
(3) Formation of rutile lamellae further boosts out-
diffusion of divalent cations (Mn and Fe) along the
c-axis of the rutile structure and accelerates the
oxidation process.
(4) Volume difference between the starting ilmenite and
the exsolved rutile causes cracking of the parent
ilmenite crystal, and opens further paths for oxidation.
The described recrystallization mechanism studied on
the scale of a single ilmenite crystal helps to understand the
oxidation processes in fine crystalline ilmenite, where the
diffusion paths are much shorter, and thus, ilmenite-to-ru-
tile transformation is incomparably faster.
Acknowledgements This work was supported by the Slovenian
Research Agency under the Project No. J1-6742 »Atomic-scale
studies of initial stages of phase transformations in minerals« and
PhD Grant No. 1000-11-310225. The research leading to these results
has received funding from the European Union Seventh Framework
Programme [FP7] under Grant agreement no. 312483 (ESTEEM2).
References
1. Murphy P, Frick L (2006) Titanium. In: Kogel JE et al (eds)
Industrial minerals and rocks—commodities, markets and uses.
Society for Mining Metallurgy, and Exploration, Inc., Colorado,
pp 987–1003
2. Zhang W, Zhu Z, Cheng CY (2011) A literature review of titanium
metallurgical processes. Hydrometallurgy 108(3–4):177–188
3. Xiao W, Lu X, Zou X, Wei X, Ding W (2013) Phase transitions,
micro-morphology and its oxidation mechanism in oxidation of
ilmenite (FeTiO
3
) powder. Trans Nonferrous Met Soc China
23(8):2439–2445
4. Janssen A, Putnis A (2011) Processes of oxidation and HCl-
leaching of Tellnes ilmenite. Hydrometallurgy 109(3–4):194–201
5. Zhang G, Ostrovski O (2002) Effect of preoxidation and sintering
on properties of ilmenite concentrates. Int J Miner Process 64(4):
201–218
6. Fu X, Wang Y, Wei F (2010) Phase transitions and reaction
mechanism of ilmenite oxidation. Metall Mater Trans A
41(5):1338–1348
7. Gupta S, Rajakumar V, Grieveson P (1991) Phase transforma-
tions during heating of ilmenite concentrates. Metall Trans B
22(5):711–716
8. Jabłonski M, Przepiera A (2001) Estimation of kinetic parameters
of thermal oxidation of ilmenite. J Therm Anal Calorim 66(2):
617–622
9. Karkhanavala MD, Momin AC (1959) The alteration of ilmenite.
Econ Geol 54(6):1095–1102
10. Zhang J, Zhu Q, Xie Z, Lei C, Li H (2013) Morphological
changes of panzhihua ilmenite during oxidation treatment. Metall
Mater Trans B 44(4):897–905
11. Bhogeswara Rao D, Rigaud M (1974) Oxidation of ilmenite and
the product morphology. High Temp Sci 6:323–341
12. Bhogeswara Rao D, Rigaud M (1975) Kinetics of the oxidation of
ilmenite. Oxid Met 9(1):99–116
13. Briggs R, Sacco A (1993) The oxidation of ilmenite and its
relationship to the FeO–Fe
2
O
3
–TiO
2
phase diagram at 1073 and
1140 K. Metall Trans A 24(6):1257–1264
14. Grey IE, Reid AF (1972) Shear structure compounds (Cr,Fe)
2
Ti
n-2
O
2n-1
derived from the a-PbO
2
structural type. J Solid State
Chem 4(2):186–194
15. Grey IE, Li C (2001) Low temperature roasting of ilmenite—
phase chemistry and applications. AusIMM Proc 306(2):35–42
16. Dent Glasser LS, Glasser FP, Taylor HFW (1962) Topotactic reac-
tions in inorganic oxy-compounds. Q Rev Chem Soc 16(4):343–360
17. Armbruster T (1981) On the origin of sagenites: structural
coherency of rutile with hematite and spinel structures types.
Neues Jahrb Mineral 7:328–334
18. Force E, Richards P, Scott K, Valentine P, Fishman N (1996)
Mineral intergrowths replaced by ‘elbow-twinned’ rutile in
altered rocks. Can Miner 34(3):605–614
19. Daneu N, Schmid H, Rec
ˇnik A, Mader W (2007) Atomic struc-
ture and formation mechanism of (301) rutile twins from Dia-
mantina (Brazil). Am Miner 92:1789–1799
20. Daneu N, Rec
ˇnik A, Mader W (2014) Atomic structure and
formation mechanism of (101) rutile twins from Diamantina
(Brazil). Am Mineral 99:612–624
21. Rec
ˇnik A, Stankovic
´N, Daneu N (2015) Topotaxial reactions
during the genesis of oriented rutile/hematite intergrowths from
Mwinilunga (Zambia). Contributions to Mineralogy and Petrol-
ogy 169(2): 19/1-22
22. Wechsler BA, Prewitt CT (1984) Crystal structure of ilmenite
(FeTiO
3
) at high temperature and at high pressure. Am Miner
69:176–185
J Mater Sci (2016) 51:958–968 967
123

23. Kidoh K, Tanaka K, Marumo F (1984) Electron density distri-
bution in ilmenite-type crystals. II. manganese(II) titanium(IV)
trioxide. Acta Crystallogr B 40:329–332
24. Wu X, Qin S, Dubrovinsky L (2010) Structural characterization
of the FeTiO
3
–MnTiO
3
solid solution. J Solid State Chem
183:2483–2489
25. Grant RW, Geller S, Cape JA, Espinosa GP (1968) Magnetic and
crystallographic transitions in the a-Mn
2
O
3
–Fe
2
O
3
system. Phys
Rev 175(2):686–695
26. Sasaki J, Peterson NL, Hoshino K (1985) Tracer impurity dif-
fusion in single-crystal rutile (TiO
2-x
). J Phys Chem Solids
46(11):1267–1283
27. Putnis A (1978) The mechanism of exsolution of hematite from
iron-bearing rutile. Phys Chem Miner 3(2):183–197
28. Sabioni ACS, Huntz AM, Daniel AMJM, Macedo WAA (2005)
Measurement of iron self-diffusion in hematite single crystals by
secondary ion-mass spectrometry (SIMS) and comparison with
cation self-diffusion in corundum-structure oxides. Phil Mag
85(31):3643–3658
29. Reece M, Morrell R (1991) Electron microscope study of non-
stoichiometric titania. J Mater Sci 26:5566–5574. doi:10.1007/
BF00553660
968 J Mater Sci (2016) 51:958–968
123