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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.
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Topotaxial reactions during oxidation of ilmenite single crystal
ˇda Stankovic
Aleksander Rec
Nina Daneu
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]
and 1
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
and Mn
ions opens paths for exsolution
of rutile within the pseudo-hexagonal oxygen sublattice of
the parent ilmenite, following simple topotaxial orientation
relationship h001iRUT 010
RUT jj h210iILM 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.
Rutile (TiO
) 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
Different processes like acid-leaching or reduction of
ilmenite are used to remove Fe [3] and to extract TiO
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 [510] and synthetic [3,6,
1113] ilmenite are available, and it is generally accepted that
the oxidation process consists of two major steps: (i) con-
version of ferrous (Fe
) to ferric form of iron (Fe
formation of different Fe- and Ti-containing phases depend-
ing on the temperature of oxidation. Below *800 °C, the
oxidation products are hematite (Fe
) and rutile (TiO
\800 CFe3þ
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
)[5,7] and Fe
)[3,6,1115] is reported in the low-temperature
range. Above *800 °C, however, ilmenite decomposes
&Aleksander Rec
ˇda Stankovic
Nina Daneu
Department for Nanostructured Materials & Joz
ˇef Stefan
International Postgraduate School, Joz
ˇef Stefan Institute,
Jamova cesta 39, 1000 Ljubljana, Slovenia
J Mater Sci (2016) 51:958–968
DOI 10.1007/s10853-015-9425-y
into the high-temperature mixed oxide, pseudobrookite
) that forms together with rutile, according to the
following equation:
[800 CFe3þ
In order to most effectively extract TiO
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
) 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 [1012,14].
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 [1721].
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
RUT jj h210iCOR 001
COR and OR-2:
h101iRUT 010
RUT jj h210iCOR 001
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
amount of manganese (Mn), replacing for iron (Fe) in an
equimolar ratio corresponding to (Fe
solution series between ilmenite (FeTiO
) and pyrophanite
), with an empirical composition Fe
. Equimolar exchange of A-site cations indicates that
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
˚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]
, [110]
, and 1
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
, 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
Rox ¼Dm
100 %;ð3Þ
where Dm
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
and Mn
Mn-ilmenite are converted into trivalent oxides Fe
, 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
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
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]
and 1
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
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.
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)
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
) phase are observed. The conversion
rate (R
) 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
SEM images of [001]
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
ions from ilmenite crystal and its surface
oxidation to Fe
. 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
), the surface oxide layer contains bixbyite (Mn
indicating that both Fe
and Mn
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
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
) and bixbyite (Mn
) 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
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]
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
projection, while the rutile domains are in
projection, and (33-0)
plane of ilmenite coin-
cides with (002)
plane of rutile, or alternatively,
is parallel to (200)
. Converting planes into
directions, the orientation relationship exactly corresponds
to OR-1: h001iRUT 010
RUT jj h210iILM 001
ILM, with
rutile–ilmenite interface (200)
. 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
regular pattern for the rutile structure in [010]
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)
diffraction spots. Similar tripling
in this direction was reported by Putnis [27] and Rec
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
projection. Here, the
exsolution texture appears very different compared to that
in basal projection. In [1
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]
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
(TEM/EDS analysis), the composition of the
ilmenite within the rutile nodes corresponds to Fe
(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]
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
), thickness
of hematite/bixbyite surface layer (d
), and depth of recrystallization
zone (d
) for samples annealed at different Ttconditions
T(°C) t(h) Dm(%) R
(%) d
(lm) d
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
Irregular thickness
J Mater Sci (2016) 51:958–968 963
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
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
and Mn
) that
are leaving the structure. During annealing, the rutile is
thus ‘saturated’ by Fe
and Mn
. 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
and Mn
can take place only
Fig. 6 Rutile exsolution lamellae in the 800 °C/12 h sample observed
in [001]
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)
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
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)
interface is fully incoherent and reflects the exsolution
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
exsolved from the parent Mn-ilmenite phase following
previously described OR-1: h001iR010
Rjj h210iI001
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
and Mn
ions, while Ti
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
10ILM 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
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
and Mn
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
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
(and Mn
) followed by rearrangement
of remaining Ti
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;
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
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
and Mn
cations, leaving a
free space for rearrangement of Ti
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
(and Mn
) 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)
| (100)
interface. On cooling, Fe
and Mn
cations that diffused through
rutile are exsolved and form ilmenite’ with secondary coherent
| (110)
interface. With this operation, the two ilmenites
are brought into non-crystallographic (110)
| (003)
966 J Mater Sci (2016) 51:958–968
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
ions compared to Mn
, which is driven by a preferential
oxidation of Fe
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.
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
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).
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... Penn & Banfield (1998) have shown that oriented attachment of polymorphic TiO 2 nanoparticles can result in twinning and oriented intergrowths. Another mechanism leading to rutile crystals/domains in twinned orientation is oriented (topotaxial) recrystallization and/or epitaxial growth of/on structurally related precursor minerals (Armbruster, 1981;Force et al., 1996) (Rečnik et al., 2015;Stanković et al., 2016). In principle, oriented recrystallization can result in the development of coplanar cyclic twins of rutile, which can be theoretically composed of up to six rutile domains, separated by five {101} TBs and an additional non-crystallographic contact (Hahn & Klapper, 2006;Padró n-Navarta et al., 2020). ...
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Contact and multiple cyclic twins of cassiterite commonly form in SnO 2 -based ceramics when SnO 2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO 2 on CoNb 2 O 6 and Co 4 Nb 2 O 9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO 2 grain growth; CoNb 2 O 6 forms at ∼700°C and Co 4 Nb 2 O 9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO 2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb 2 O 6 grains can only result in the formation of contact twins, the Co 4 Nb 2 O 9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO 2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO 2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb ⁵⁺ and Co ²⁺ ions into the SnO 2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.
... Similar effect can be obtained in the case of the rutile {301} twin formation ( Figure S10). Rutile twin can be grown on some types of hexagonal substrates, such as sapphire (Al 2 O 3 ) (Gao et al., 1992;Lee et al., 2006) and hematite (α-Fe 2 O 3 ) , or formed from other hexagonal precursors, e.g., ilmenite (FeTiO 3 ) (Janssen et al., 2010;Stanković et al., 2015;Daneu et al., 2014). ...
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Nanotwin structures in materials engender fascinating exotic properties. However, twinning usually alter the crystal orientation, resulting in random orientation and limited performances. Here, we report a well-aligned rutile TiO2 nano-twin film with superior preferential orientation than its isostructural substrate. By means of the synchrotron X-ray Laue nano-diffraction technique, the crystal orientation, twin boundaries, and deviatoric stresses of the film were quantitatively imaged at unprecedented spatial resolution to unravel the underlying mechanism of this anomalous alignment. Massive {101}-type rutile nanotwins were observed and a crystallographic relationship of the heteroepitaxy was proposed. The rapid twinning and twin-controlled heteroepitaxy are responsible for the texture improvement. This work would open up opportunities for rational design of better twin-based functional materials, and implies the powerful capabilities of X-ray nanodiffraction technique for multidisciplinary applications.
... relict reactant phase present, independent knowledge of replacement relationships). In the case of the twinned ilmenite from Productora, the ilmenite-to-rutile relationships have been established by previous studies (Stanković et al. 2016;Plavsa et al. 2018) and verified here (Fig. 1) and so we can infer the twin relationship based on studies of ilmenite and other corundum structured minerals (Minkin and Chao 1971;Wang et al. 1990). In the case of the rutile replacing the cubic mineral (Fig. 3), the obvious candidate for the precursor phase is titanomagnetite. ...
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Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility. Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed phases can be inferred in ore deposits. An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following well-documented orientation relationship to the ilmenite [0001]ilmenite // < 100 > rutile and < 101¯0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10{\bar{1}}0$$\end{document} > ilmenite // [001]rutile The anatase is a single crystal and shows a potential orientation relationship [0001]ilmenite = (0001)ilmenite // {211}anatase and < 101¯0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10{\bar{1}}0$$\end{document} > ilmenite // < 01¯1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0{\bar{1}}1$$\end{document} > anatase The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase production. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is important in dolomite nucleation. Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related to the ilmenite orientation by the following relationship: [0001]ilmenite // < 100 > titanite and {101¯0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10{\bar{1}}0$$\end{document}}ilmenite // (001)titanite The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite needle leading to the formation of substructure in the absence of deformation. Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from Productora tourmaline breccia complex shows misorientation distributions consistent with < 001 > Rutile // < 110 > cubic and < 100 > Rutile // < 111 > cubic Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to document the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common < 100 > axis is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the aggregate is consistent with the above relationship replacing ilmenite with a {101¯2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10{\bar{1}}2$$\end{document}} twin.
... This is due to a different origin of their formation. In the structurally related rutile (TiO 2 ) it has been shown that twinning is a consequence of complex topotaxial replacement reactions after structurally related bulk precursors [49], and is driven by the changes in oxidation conditions that consequently trigger diffusion controlled remobilization of cations [50]. A similar mechanism could be responsible for twinning of cassiterite. ...
We investigated the effects of dual doping of SnO2 varistor ceramics with 1 mol% CoO and different amounts of Nb2O5 (0.1 - 2 mol%) on the formation of twin boundaries, microstructure development and electrical properties. Nb2O5 addition shifts densification to higher temperatures (up to 1430 °C), producing microstructures composed of twinned SnO2 grains. Already 0.1 mol% Nb2O5 triggers a three-fold increase in growth rate via the diffusion induced grain boundary mobility (DIGM). At 0.5 mol % of Nb2O5 chemical equilibrium is achieved and SnO2 grains undergo normal grain growth. Electron back-scatter diffraction (EBSD) has shown that the prevailing type of twins is {101}. Cyclic twins are common. High-angle annular dark-filed scanning transmission electron microscopy (HAADF-STEM) image analysis revealed non-uniform segregation of Nb along the twin boundaries, indicating that they are not directly triggered by Nb2O5, but are a result of yet unexplained sequence of topotaxial replacement reactions. Energy dispersive spectroscopy (EDS) has shown that by dual doping of SnO2 with CoO and Nb2O5 the amount of Co dissolved in SnO2 grains is always ~4x lower compared to the amount of incorporated Nb and propose the following mechanism of tin out-diffusion: 6 Sn(IV)˟Sn(IV) ⇋ Sn(II)''Sn(IV) + Co(II)''Sn(IV) + 4 Nb(V)˙Sn(IV). Optimal electrical properties were achieved at 1 mol% Nb2O5 addition displaying high nonlinearity (α=50), high break-down voltage (571±12 V/mm) and low leakage current (IL = 4.2 µA). The addition of 2 mol% of Nb2O5 has an inhibiting effect on densification and SnO2 grain growth, resulting in a collapse of nonlinearity and increase of leakage current.
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Electron backscatter diffraction (EBSD) was used for the analysis of multiple cyclic twins in cassiterite (SnO2), which form during sintering of SnO2 with small additions of CoO and Nb2O5. Grain misorientation analysis has shown that about one third of all grains contain {101} twin boundaries (TBs). The majority of these grains are contact twins, whereas a small fraction of grains are multiple, mainly cyclic twins. A procedure was developed in MTEX [Bachmann, Hielscher & Schaeben (2010). Solid State Phenom. 160, 63-88] for automated identification of crystallographically different types of cyclic twins and found two main types: coplanar twins composed of three or four domains with a common [010] axis and alternating twins composed of three to seven domains oriented along the [111] axis. Both types of cyclic twins have a characteristic common origin (nucleus) of all TBs, which is positioned eccentric relative to the grain section and the cycle is closed with a shorter non-crystallographic contact between the first and the last twin domain. The morphology of cyclic twins suggests that they form by nucleation in the initial stages of grain growth. The average size of twinned grains increases with the number of twin domains indicating the influence of TBs formation on the growth of composite grains.
Full-text available
Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the <010>R{101}R||<001>H{110}H orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact <010>R{401}R||<001>H{170}H. The intergrowth is dictated by the formation of {170}H|{401}R equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian-ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form {301}R|{030}H equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian-ilmenite crystals was close to Ilm67–Hem33, typical for Fe–Ti rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian-ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.
Full-text available
We studied the atomic structure and the chemical composition of (101)-type rutile (TiO2) twins from Diamantina in Brazil by electron microscopy methods to resolve the mechanism of their formation. The twin boundaries were studied in two perpendicular orientations to reveal their 3D structure. The pres-ence of a precursor phase, such as Al-rich hydroxylian pseudorutile (HPR; kleberite), during the initial stages of the crystallization appears to be the necessary condition for the formation of (101) twins of rutile at this locality. The precursor with a tivanite-type structure serves as a substrate for the topotaxial crystallization of rutile. Depending on the initial crystallization pattern the rutile can grow either as a single crystal or as a twin. During the progressive crystallization of the rutile Al-rich oxyhydroxide (diaspore, α-AlOOH) clusters are concentrated at the center of the precursor where they are pinned to the twin boundary as the precursor is fully recrystallized into rutile. At the increased temperatures the remaining diaspore precipitates are converted to corundum (α-Al2O3), while the two crystal domains continue to grow in the (101) twin orientation. In addition to the primary (101) twin, series of secondary {101} twins are formed to accommodate the residual tensile stress caused by the diaspore-to-corundum transformation. Based on the observed corundum-rutile [0001]C (1120)C || [010]R (101)R and ilmenite-rutile [0001]I (1100)I || [010]R (301)R crystallographic relations a unified mechanism of the genesis of the {101} and {301} reticulated sagenite twin clusters is proposed.
Orientation relations between rutile, hematite, and spinel structure-types are based on parallel oxygen layers between host and guest crystals. Oxygen chains in rutile exist along <101> and <001> and cross under 57o15'. In spinel (111) or hematite (0001) those chains cross under 60o. Alignment of rutile <101> or <001> with corresponding directions in close-packed structures leads to two different orientation relations which can be observed in nature and laboratory experiments. (Author's abstract) -W.T.
Some aggregates of rutile, classically considered to be "elbow" twinned, instead are topotactic replacements of ilmenite or other hexagonal titaniferous precursors. Twinned rutile can be differentiated from the reticulated rutile of topotactic replacements by the angle of prism intersections, junction morphology, and the overall form of the aggregate. In a special case of topotactic replacement of ilmenite, rutile forms pseudomorphs of "trellis"-textured ilmenite lamellae in {111} of precursor magnetite. We trace the progress of rutile formation through the alteration of fine-grained magnetite-bearing host rocks. The sequential two-step topotaxy from magnetite through ilmenite to rutile requires rutile prisms to parallel the intersections of {111} planes in precursor magnetite. Some coarse reticulated rutile may result from the same paragenetic sequence.
Removal of gangue minerals from ilmenite concentrates can be accomplished by relatively low temperature roasting in controlled gas atmospheres to selectively enhance the magnetic separation characteristics of the ilmenite. At roasting temperatures below 800°C the magnetising reactions are often not consistent with phase equilibria considerations. Metastable phases can nucleate and control the roasting products. In weathered ilmenites the phase assemblages and microstructures in the altered ilmenite grains can influence the roasting reactions. In this paper we give an overview of the phase chemistry of chemically weathered ilmenites and of metastable phases formed by low temperature roasting. We also report the results of roast experiments conducted in a neutral gas atmosphere. Major increases in magnetic properties after short roasting times at low temperatures were obtained for a range of ilmenite concentrates. For example heating a secondary ilmenite concentrate from Western Australia for 15 minutes in nitrogen at 600°C gave an increase in the three kilogauss magnetic fraction from three to 92 wt per cent. The solid state phase changes accompanying the inert atmosphere roasts were characterised by combining elemental analyses with quantitative phase analyses of the ilmenite concentrates and their roast products. It was found that the phase reaction that is principally responsible for the increase in the magnetic properties of weathered ilmenites involves a transformation of disordered ferrous-containing pseudorutile to an ordered ilmenite-hematite solid solution containing typically ten to 35 mole per cent of the hematite component. Examples are given of the application of the inert atmosphere roast to effect separations of ilmenite from gangue minerals.
The structures of two single-crystals of synthetic ilmenite were refined using X-ray intensity data collected at 24, 400, 600, 800 and 1050oC (1 atm) and 0.001, 25.4, 34.6 and 46.1 kbar (room T). The responses of cell parameters, bond lengths, coordination, and ordering to T and P were analysed and found, generally, not to be inverse in character. -J.A.Z.
Phase transitions, morphology changes, and oxidation mechanism of the ilmenite oxidation process were investigated. FeTiO3 transforms to hematite and rutile when oxidation at 700-800 C, and pseudobrookite is formed when the oxidation temperature reaches 900 C. The initial ilmenite powder exhibits paramagnetism; however, after being oxidized at the intermediate temperature (800-850 C), the oxidation product exhibits weak ferromagnetism. The oxidation mechanism was discussed. The microstructure observations show that a lot of micro-pores emerge on the surfaces of ilmenite particles at the intermediate temperature, which is deemed to be capable of enhancing the mass transfer of oxygen during oxidation.
Chromium iron titanates, with general formula (Cr,Fe) 2Ti n-2 O 2 n-1 , n = 3, 4, and 5 have been prepared by reacting the component oxides in air at 700-1650°C. The compounds have been characterized by X-ray crystallographic techniques and, where necessary, lattice parameters have been confirmed by selected area electron diffraction. The structures of the compounds are closely related to that of α-PbO 2 and may be derived from it by crystallographic shear parallel to (110) α-PbO 2. Preliminary results are reported for two series of ordered intergrowth phases, (M 3O 5) n(M 4O 7) m and (M 4O 7) n(M 5O 9) m.
Morphological changes with the phase transitions were investigated in detail for the oxidation roasting of Panzhihua ilmenite in air from 873 K to 1173 K (600 °C to 900 °C). It was found that a thin hematite layer of 1 to 2 μm formed rapidly at the initial stage of the oxidation process, independent of the oxidation temperature, on the surface of ilmenite particles, and the thickness of the hematite layer kept nearly constant with increasing time of oxidation. The morphology inside an ilmenite particle, however, changed with the oxidation temperature. Needlelike rutile network enwrapped by hematite grains was observed after oxidation below 1073 K (800 °C), and the structure can be well preserved during the oxidation process, although grain growth of rutile and hematite did occur with the extension of the oxidation time. The morphological changes at 1073 K (800 °C) and higher showed two distinct stages, where in the first stage ilmenite was oxidized to form rutile and hematite with morphology similar to that of oxidation below 1073 K (800 °C). The as-formed rutile and hematite were not stable and recombined to form pseudobrookite in the second stage, which caused significant morphology change; i.e., the needlelike rutile network gradually “dissolved” to form the final morphology of irregular rutile grains dispersed in the pseudobrookite matrix with some isolated hematite grains. Moreover, the oxidation temperature also has a great effect on the rate of morphological changes, i.e., the increased evolution rate with increasing the oxidation temperature.