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Article

High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds

by
Masaki Akaogi
*,
Taisuke Tajima
,
Masaki Okano
and
Hiroshi Kojitani
Department of Chemistry, Gakushuin University, Mejiro, Toshima-ku, Tokyo 171-8588, Japan
*
Author to whom correspondence should be addressed.
Minerals 2019, 9(10), 614; https://doi.org/10.3390/min9100614
Submission received: 8 September 2019 / Revised: 3 October 2019 / Accepted: 3 October 2019 / Published: 6 October 2019
(This article belongs to the Special Issue Mineral Physics—In Memory of Orson Anderson)
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Abstract

Phase transitions of Mg2TiO4 and Fe2TiO4 were examined up to 28 GPa and 1600 °C using a multianvil apparatus. The quenched samples were examined by powder X-ray diffraction. With increasing pressure at high temperature, spinel-type Mg2TiO4 decomposes into MgO and ilmenite-type MgTiO3 which further transforms to perovskite-type MgTiO3. At ~21 GPa, the assemblage of MgTiO3 perovskite + MgO changes to 2MgO + TiO2 with baddeleyite (or orthorhombic I)-type structure. Fe2TiO4 undergoes transitions similar to Mg2TiO4 with pressure: spinel-type Fe2TiO4 dissociates into FeO and ilmenite-type FeTiO3 which transforms to perovskite-type FeTiO3. Both of MgTiO3 and FeTiO3 perovskites change to LiNbO3-type phases on release of pressure. In Fe2TiO4, however, perovskite-type FeTiO3 and FeO combine into calcium titanate-type Fe2TiO4 at ~15 GPa. The formation of calcium titanate-type Fe2TiO4 at high pressure may be explained by effects of crystal field stabilization and high spin–low spin transition in Fe2+ in the octahedral sites of calcium titanate-type Fe2TiO4. It is inferred from the determined phase relations that some of Fe2TiO4-rich titanomagnetite inclusions in diamonds recently found in São Luiz, Juina, Brazil, may be originally calcium titanate-type Fe2TiO4 at pressure above ~15 GPa in the transition zone or lower mantle and transformed to spinel-type in the upper mantle conditions.

1. Introduction

Titanomagnetite (Fe3–xTixO4), one of the important magnetic minerals in igneous and metamorphic rocks, is a solid solution between Fe3O4 magnetite (x = 0) and Fe2TiO4 ulvöspinel (x = 1), formed by replacement of Fe3+ by Ti4+. Fe2TiO4 ulvöspinel has an inverse spinel structure where tetrahedral sites are occupied by Fe2+ and octahedral sites are occupied randomly by Fe2+ and Ti4+ [1,2]. Mg2TiO4 qandilite is one of endmembers of spinel solid solution in the system Fe2TiO4–Mg2TiO4–Fe3O4–MgFe2O4, and is important in industrial ceramics. Mg2TiO4 qandilite also has the inverse spinel structure [3]. High-pressure phase transitions of A2BO4 and AB2O4 spinels have received much attention in geophysics and mineral physics, because they provide valuable information on post-spinel transitions of spinel-structured Mg2SiO4 ringwoodite which occurs at 660 km depth in the mantle and on stability of aluminous phases with the AB2O4 stoichiometry in the deep mantle.
Phase transitions of Fe2TiO4 spinel at room temperature and high pressure have been studied in detail [2,4,5,6]. The studies indicated that at room temperature Fe2TiO4 cubic spinel (space group Fd-3m) first changes into tetragonal spinel (I41/adm) at 9 GPa due to a Jahn-Teller effect of Fe2+ in the tetragonal site. At 12 GPa, the tetragonal spinel transforms to CaTi2O4-type structure (Cmcm), in which octahedral sites are occupied by disordered Fe2+ and Ti4+ and eight-fold sites by Fe2+. The CaTi2O4-type Fe2TiO4 further transforms at 53 GPa to another high-pressure polymorph (Pmma) with ordered Fe2+ and Ti4+ in the octahedral sites. Yamanaka et al. [4] reported that the high spin-low spin transition in Fe2+ in the octahedral sites of the CaTi2O4-type Fe2TiO4 started to occur above about 15 GPa at room temperature and that all of Fe2+ in the octahedral sites were in the low spin state in the Pmma phase, while Wu et al. [6] reported that the spin transition in Fe2+ initiated at around 40 GPa.
In contrast, studies on high-pressure and high-temperature transitions of Fe2TiO4 and Mg2TiO4 spinels have been limited. Phase transitions of the spinels were examined only up to 5 GPa at 800–1600 °C by Akimoto and Syono [7]. They reported that both Fe2TiO4 and Mg2TiO4 spinels decompose into ATiO3 ilmenite and AO with rock-salt structure (A = Mg, Fe). The phase transitions at high temperature and high pressure above ~5 GPa, however, have not been well examined so far. Changes in elastic properties associated with the phase transitions in Fe2TiO4 and Mg2TiO4 shown above were investigated by Liebermann et al. [8]. The physical basis of elastic properties at high pressure and high temperature is described in detail by Anderson [9].
In this study, we have investigated phase transitions in Fe2TiO4 and Mg2TiO4 up to 28 GPa and 1600 °C, using a Kawai-type multianvil apparatus. We have found that the transition sequences in Fe2TiO4 at high pressure and high temperature are substantially different from those at high pressure and room temperature. Based on the results on Fe2TiO4 and Mg2TiO4, we discuss the difference in the transition behaviors between Fe2TiO4 and Mg2TiO4 from a crystal–chemical point of view. In addition, we discuss stability and possible origin of Fe2TiO4-rich titanomagnetites which were found as inclusions in superdeep diamonds from Juina area, Brazil [10,11].

2. Experimental Methods

Starting materials for high-pressure and high-temperature experiments of Mg2TiO4 and Fe2TiO4 were synthesized as follows. Spinel(Sp)-type Mg2TiO4 (qandilite) was synthesized from a 2:1 mixture (molar ratio) of MgO (>99.9% purity, Wako Co., Osaka, Japan) and TiO2 (>99.9%, Wako Co.) by heating at 1300 °C for 20 h. Sp-structured Fe2TiO4 (ulvöspinel) was synthesized from a 1:1 mixture of Fe2O3 (>99.9%, Wako Co.) and TiO2 by heating at 1270 °C for 27 h in controlled oxygen fugacity with mixed gasses of CO2, H2 and Ar (3:2:5 volume ratios). Powder X-ray diffraction measurements and composition analyses using a scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS) indicated that the synthesized samples were single-phase Sp-type Mg2TiO4 and Fe2TiO4 with the stoichiometric compositions. The lattice parameters determined by powder X-ray diffraction were a = 8.4398(1) Å for Mg2TiO4 Sp and a = 8.5375(3) Å for Fe2TiO4 Sp, which were in good agreement with those by Wechsler et al. [3] and Wechsler et al. [1], respectively. For high-pressure and high-temperature experiments of Fe2TiO4, 5 wt % metallic iron (>99.9 %, Johnson Matthey Co., London, UK) was added to the above synthesized Fe2TiO4 Sp to keep iron in the Fe2TiO4 samples in ferrous state.
High-pressure and high-temperature experiments were performed with the quench method using a Kawai-type 6-8 multianvil apparatus [12] at Gakushuin University. Tungsten carbide anvils with 2.5 mm truncated edge-length were used as the second-stage anvils. A 5 wt % Cr2O3-doped MgO octahedron of 7 mm edge length was used as the pressure medium. A cylindrical Re heater was placed in the central part of the octahedron. For the experiments of Mg2TiO4, the powdered starting material was directly put into the Re heater. For thermal insulation, a LaCrO3 sleeve was placed between the heater and the MgO octahedron along with two LaCrO3 plugs in both ends of the furnace. Two Pt discs were placed between the sample and the plugs to avoid any reaction between them. For the experiments of Fe2TiO4, the mixture of 95 wt % Fe2TiO4 and 5 wt % Fe was put into an Fe capsule which was placed in the central part of the Re heater. A BN sleeve was inserted between the Fe capsule and Re heater for electrical insulation. Temperature was measured at the central part of the outer surface of the furnace with a Pt/Pt-13%Rh thermocouple. Effect of pressure on emf of the thermocouple was ignored.
Pressure was calibrated at room temperature using pressure-fixed points: Bi I-II (2.55 GPa), Bi III-V (7.7 GPa), ZnS (15.6 GPa), and GaAs (18.3 GPa) in Ito [12] and GaP (23 GPa) by Dunn and Bundy [13]. The pressure was further corrected at 1200 and 1600 °C, using transition pressures of Mg2SiO4 olivine–wadsleyite–ringwoodite [14,15] and MgSiO3 akimotoite–bridgmanite [16] and decomposition pressure of Mg3Al2Si3O12 pyrope to MgSiO3-rich bridgmanite + Al2O3-rich corundum [16]. Uncertainties of pressure and temperature were estimated to be ±0.3 GPa and ±20 °C, respectively. In each run, pressure was raised to a targeted pressure of 11–28 GPa at a constant rate during about 2–4 h, and then temperature was raised to a targeted temperature of 1000–1600 °C at a rate of about 100 °C/min. The samples were kept for 60–120 min at the pressure-temperature conditions, and then quenched under pressure, slowly decompressed, and recovered to ambient conditions.
The recovered samples were pulverized and examined by a powder X-ray diffractometer (Rigaku, RINT2500, Tokyo, Japan) operated at 45 kV and 250 mA with a rotating Cr anode. Monochromatized CrKα radiation was used for identification of phases in the run products and determination of lattice parameters. High-purity Si powder was used to calibrate 2θ angle of the powder X-ray diffractometer. The lattice parameters were refined by the least squares method. By using some parts of run products which were mounted on slide-glass plates with epoxy resin and polished flat, phase identification and chemical analysis were made using a SEM (JEOL, JMS-6360, Tokyo, Japan) in combination with an EDS (SGX Sensortech Sirius SD-10133, High Wycombe, UK). The SEM was operated with acceleration voltage of 15 kV and probe current of 0.55 nA. Natural forsterite and synthetic fayalite and MnTiO3 provided by JEOL were used as standard materials for Mg, Fe and Ti, respectively.

3. Results

3.1. Phase Transitions in Mg2TiO4

Table 1 summarizes results of phase transition experiments in Mg2TiO4 at 21–28 GPa and 1000–1600 °C. The previous study showed that at pressure below 1–2 GPa spinel (Sp)-type Mg2TiO4 first dissociates into ilmenite (Ilm)-type MgTiO3 + MgO periclase (Pc) [7]. Figure 1a shows a powder X-ray diffraction pattern of the run product (No. O161021) quenched from 21 GPa and 1600 °C. The diffraction pattern exhibits MgO Pe and a phase with very similar diffraction peaks to MgTiO3 Ilm (space group R-3). However, some diffraction peaks such as 003 and 101 of MgTiO3 Ilm in 2θ range of 20–30° are absent. Almost all the peaks were indexed with hexagonal symmetry, and refined cell parameters were a = 5.0774(5) Å, c = 13.716(2) Å, giving c/a = 2.701. This c/a value is considerably smaller than 2.750 of MgTiO3 Ilm [3]. The diffraction data in Figure 1a and the lattice parameters are consistent with those of LiNbO3 (Ln)-type MgTiO3 (R3c) [17]. Linton et al. [17] reported that Ilm-type MgTiO3 transforms at about 15–20 GPa to the perovskite (Pv) phase, which is converted to the Ln-type phase on decompression. Therefore, our results in Table 1 and Figure 1a reveal that MgTiO3 Pv + MgO Pe are stable at 21 GPa and 1600 °C and that MgTiO3 Pv transforms to the Ln-type on release of pressure.
Figure 1b shows a powder X-ray diffraction pattern of the run product (No. O160924) quenched from 28 GPa and 1300 °C, in which diffraction peaks of MgO Pe and αPbO2 type TiO2 are observed. Broad diffraction peaks of αPbO2 type TiO2 in Figure 1b suggest that the phase was a retrograde transformation product from a high-pressure phase. Previous studies on TiO2 indicated that at about 800–1000 °C TiO2 rutile transforms to αPbO2 type at about 6–8 GPa, which further transforms to baddeleyite (Bd)-type TiO2 (akaogiite) [18,19] at about 15–20 GPa [20,21,22]. The Bd-type TiO2 transforms to orthorhombic-I (OI) type TiO2 at about 25–30 GPa at 1200–1500 °C [23,24]. These studies also showed that both of Bd- and OI-type TiO2 phases back-transform to αPbO2 type TiO2 on release of pressure. Therefore, based on the experimental pressure range and the broad X-ray diffraction peaks, we conclude that the αPbO2 type TiO2 found in the run products of 28 GPa and 1300 °C was the retrograde transition product from Bd- or OI-type TiO2 synthesized at the P, T conditions. This leads to the conclusion that MgTiO3 Pv stable at 21 GPa decomposes to the constituent oxides at higher pressure.
Figure 2 shows the phase diagram of Mg2TiO4. Our experimental results at 21–28 GPa and 1000–1600 °C indicate that the transition boundary of MgTiO3 Pv + MgO Pe to 2MgO Pe + Bd-type TiO2 which is the decomposition boundary of MgTiO3 Pv is located at 21–23 GPa and 1000–1600 °C with a small positive dP/dT slope. Figure 2 also includes the dissociation boundary of Mg2TiO4 Sp to MgTiO3 Ilm + MgO Pe below ~1–2 GPa [7], as well as the Ilm-Pv transition boundary of MgTiO3 at ~ 15–20 GPa [25]. The assemblage of MgO and TiO2 is stable to at least 35 GPa at 1400 °C in our preliminary study on phase transitions in MgTiO3 [25].
Molar volumes of high-pressure phases in the systems MgO-TiO2 and FeO-TiO2 are listed in Table 2. Using the data in Table 2, for the following transitions in Mg2TiO4:
Mg2TiO4(Sp) → MgTiO3(Ilm) + MgO(Pc) → MgTiO3(Pv) + MgO(Pc) → 2MgO(Pc) + TiO2(Bd) → 2MgO(Pc) + TiO2(OI),
we obtain molar volume changes at ambient conditions of −3.15, −1.55, −1.17 and −0.07 cm3/mol, respectively, which are −7.0, −3.7, −2.9 and −0.2%, respectively.

3.2. Phase Transitions in Fe2TiO4

Results of high-pressure transition experiments in Fe2TiO4 are summarized in Table 3. Phase identification was made mostly by powder X-ray diffraction method. Metallic iron in the samples was identified by the SEM-EDS analysis and/or powder X-ray diffraction. Fe2TiO4 ulvöspinel first dissociates into Ilm-type FeTiO3 and FeO wustite (Wu) at 4–5 GPa and 1000–1300 °C [7]. Figure 3a shows a powder X-ray diffraction pattern of the run product (No. M150115) synthesized at 14 GPa and 1100 °C. The diffraction pattern of Figure 3a is composed of FeO Wu, metallic iron, and the other phase whose diffraction peaks agree with those of Ln-type FeTiO3 by Akaogi et al. [26]. The Ln-type FeTiO3 phase is interpreted to be the retrograde transition product from Pv-type FeTiO3 [27]. Natural occurrences of Ln-type FeTiO3 were reported in shocked meteorites, and the new mineral was recently named wangdaodeite [28,29].
Figure 3b shows a powder X-ray diffraction pattern of the run product (No. M140710) synthesized at 18 GPa and 1300 °C. The diffraction pattern is similar to calcium titanate (CaTi2O4), and the peaks were indexed with orthorhombic symmetry (Figure 3b). The Miller indices were consistent with extinction rules of CaTi2O4(CT)-structured phase (Cmcm). The lattice parameters refined using twenty nine diffraction peaks of the Fe2TiO4 phase were a = 2.9473(5) Å, b = 9.6448(2) Å, c = 9.9085(2) Å, V = 281.663(7) Å3. The cell parameters agree well with those extrapolated to ambient pressure of CT-type Fe2TiO4 at 38–50 GPa and room temperature by Yamanaka et al. [4]. These results indicate that the Fe2TiO4 phase in the run product synthesized at 18 GPa and 1300 °C is the CT-type phase. This is consistent with the results in the previous studies that Fe2TiO4 CT is quenchable at ambient conditions [5,30]. Compositions of coexisting Fe2TiO4 CT and FeTiO3 Ln with very small grains of FeO Wu in the run product (No. M140626) at 15 GPa and 1300 °C were analyzed using the SEM-EDS: Fe1.97(1)Ti1.01(1)O4 for CT-type phase and Fe0.95(2)Ti1.02(1)O3 for Ln-type phase, indicating almost stoichiometric compositions within the analytical errors.
Figure 4 shows the phase diagram of Fe2TiO4. Our experimental results indicate that at 1000–1300 °C the assemblage of FeO Wu + FeTiO3 Ln which was Pv at high P, T was observed in the run products synthesized at 12.5–15.5 GPa, while FeO Wu + FeTiO3 Ilm was observed at 11–12.5 GPa. The Ilm-Pv transition boundary has a small negative slope. The Ilm-Pv transition pressure in this study is generally consistent with that obtained by extrapolation of the boundary determined at 500–900 °C by Ming et al. [35]. We found that the assemblage of FeTiO3 Ln (Pv at high P, T) and FeO Wu changes to Fe2TiO4 CT in the run products at 15–16 GPa at 1100–1300 °C with a small negative slope boundary. Therefore, the stability field of FeTiO3 Pv and FeO Wu is limited in a narrow pressure interval of about 2 GPa. This study indicates that Fe2TiO4 CT is stable up to at least 22–24 GPa at 1100–1300 °C. In our previous study in FeTiO3 composition, we found that Fe2TiO4 CT coexists with OI-type TiO2 up to about 33 GPa, above which it decomposes into FeO Wu + TiO2 OI [26]. Therefore, the upper stability pressure of Fe2TiO4 CT is about 33 GPa.
The results in Figure 4 combined with those by Akaogi et al. [26] indicate that the following transitions occur in Fe2TiO4 with increasing pressure:
Fe2TiO4(Sp) → FeTiO3(Ilm) + FeO(Wu) → FeTiO3(Pv) + FeO(Wu) → Fe2TiO4(CT) → 2FeO(Wu) + TiO2(OI),
where molar volume changes at ambient conditions are −2.95, −1.38, −0.10 and −1.24 cm3/mol, which are −6.3%, −3.1%, −0.2% and −2.9%, respectively, using the data in Table 2.

4. Discussion

Our results indicate that Fe2TiO4 Sp undergoes phase transitions to FeTiO3 Ilm + FeO Wu and subsequently to FeTiO3 Pv + FeO Wu at high pressure and high temperature (Figure 4). The decomposition of Fe2TiO4 Sp to FeTiO3 Ilm + FeO Wu is consistent with the result by Akimoto and Syono [7]. However, at room temperature and high pressure, the transition behavior is different: Fe2TiO4 cubic Sp is distorted to tetragonal Sp at ~9 GPa due to the Jahn-Teller effect of Fe2+ at the tetrahedral site [2]. The difference may arise from kinetic hindrance to decomposition into FeTiO3 Ilm + FeO Wu at room temperature. In the experiments at room temperature and high pressure, tetragonal Sp transforms to Fe2TiO4 CT at ~15 GPa [4,6]. The pressure is generally consistent with transition pressure to Fe2TiO4 CT at room temperature extrapolated from the results at 1000–1300 °C, considering the uncertainty of the boundary slope (Figure 4).
Here, we compare the transitions in Mg2TiO4 and Fe2TiO4. As shown in Figure 2 and Figure 4, both of Sp-type Mg2TiO4 and Fe2TiO4 dissociate into ATiO3 Ilm and AO (A = Mg, Fe) at pressures below ~5 GPa, and both of the ATiO3 Ilm transform to ATiO3 Pv at ~13–15 GPa. At higher pressure, however, the transition behaviors are different between Mg2TiO4 and Fe2TiO4. In Mg2TiO4, the assemblage of MgTiO3 Pv + MgO Pe changes into 2MgO Pe + TiO2 Bd due to decomposition of MgTiO3 Pv. However, in Fe2TiO4 the assemblage of FeTiO3 Pv + FeO Wu changes at ~15 GPa to CT-type Fe2TiO4 which is stable up to ~33 GPa.
Figure 5 illustrates the CT-type structure (Cmcm). The structure consists of double chains of edge-sharing octahedra running parallel to the a-axis, and tunnel spaces are formed by four corner-sharing double chains [37,38]. In the structure of CT-type Fe2TiO4, Fe2+ and Ti4+ are randomly distributed in the octahedral sites and only Fe2+ in the eight-fold sites in the tunnel spaces. Compared with Mg2TiO4, crystal-field effect of Fe2+ in the octahedral sites may stabilize the CT-type phase of Fe2TiO4 composition. Using effective ionic radii by Shannon [39], the difference of ionic radii between Fe2+ (0.78 Å in high-spin state) and Ti4+ (0.605 Å) in octahedral site is larger than that between Mg2+ (0.72 Å) and Ti4+. However, the ionic radius of Fe2+ in low-spin state in the octahedral site is 0.61 Å, which is much closer to that of Ti4+ compared with Mg2+. When we adopt that the high-spin to low-spin transition of Fe2+ in the octahedral site of Fe2TiO4 CT starts to occur at pressure of ~15–20 GPa at room temperature and proportion of low-spin Fe2+ increases with pressure [4], it is suggested that the very similar ionic radii of low-spin Fe2+ and Ti4+ is more favorable for the CT-type structure. Although the high-spin state and low-spin state would be mixed at high temperature such as 1000–1300 °C [40], it is likely that both of the crystal-field effect and the high-spin to low-spin transition of Fe2+ may facilitate the stability of Fe2TiO4 CT above ~15 GPa.
Sp-type titanomagnetites were found as inclusions in diamonds from some localities in Juina, Brazil, and the diamonds were interpreted to be derived from the deep mantle [10,11]. Two different occurrences of titanomagnetites as the diamond inclusions have been reported: one was a titanomagnetite-bearing mineral composite, and the other was separate single-phase inclusions of titanomagnetite. As the former-type inclusion, Walter et al. [10] discovered a ~30 μm-sized composite consisting mostly of orthopyroxene together with olivine and titanomagnetite (called “ulvöspinel” in their study) in a diamond from the Juina-5 kimberlite, Brazil. The analyzed composition of the titanomagnetite was approximately 36 mol % Fe2TiO4⋅36 mol % Fe3O4⋅28 mol % Mg2TiO4. Walter et al. [10] interpreted that the three-phase composite was originally a homogeneous Mg-rich bridgmanite in the lower mantle, and that the diamond was transported to the upper mantle at a depth range of ~150–200 km where immiscibility into the three phases occurred before eruption of the kimberlite. Based on the experimental data on NaAlSiO4-MgAl2O4 [42,43], Walter et al. [10] estimated the depth range of ~150–200 km using nepheline–spinel composite inclusions which were interpreted to be originally calcium ferrite (CF)-type phase and hexagonal aluminous (NAL) phase in the lower mantle and the unmixing occurred at the depth range. Figure 1 and Figure 3 and the results of Woodland et al. [44] indicate that the upper bound of stability field of Sp-type Mg2TiO4, Fe2TiO4 and Fe3O4 increases in the order of ~1, ~4 and ~10 GPa, respectively. Using the pressures, we roughly estimate the upper limit of stability field of the titanomagentite inclusion of the above composition to be ~5–6 GPa. The pressure is compatible with the estimated depth range of ~150–200 km from the CF and NAL inclusions.
The other kind of titanomagnetites was separate single-phase inclusions in superdeep diamonds. By a new combined method of synchrotron microtomography and single-crystal X-ray diffraction with fast, non-destructive methodology, Wenz et al. [11] discovered ~10–20 μm-sized inclusions of separate titanomagnetite crystals and powders in diamonds from São Luiz, Juina, Brazil. They observed that titanomagnetite inclusions were relatively abundant next to (Mg,Fe)O magnesio-wustite and Fe2−xTixO3 titanohematite, and determined lattice parameters of the titanomagnetites embedded in the diamonds to be 8.511–8.405 Å by in situ X-ray diffraction method. Wenz et al. [11] assumed that compositions of the titanomagnetites were on the join Fe2TiO4-Fe3O4, because chemical analysis of the titanomagnetites was not made in the study. However, it is likely that the titanomagnetites contained minor amounts of Mg2TiO4 and MgFe2O4 components, as that found by Walter et al. [10]. At ambient conditions, cell parameters of Fe2TiO4 Sp and Fe3O4 Sp are 8.5375(3) Å (this study) and 8.3984(8) Å [4], respectively, while Mg2TiO4 Sp and MgFe2O4 Sp are 8.4398(1) Å (this study) and 8.391 Å [45], respectively. Therefore, there is large uncertainty in estimating compositions of the titanomagnetites in the diamonds only from the lattice parameters determined by the in situ measurements. It should also be considered that remnant pressure probably remained in the titanomagnetites embedded in the diamonds. When we use measured bulk modulus of Fe2TiO4 Sp of 121 GPa [8,46] or 147 GPa [47], the pressure dependence of cell parameter is calculated to be −0.023 or −0.019 Å/GPa, respectively.
We suggest, however, that the remnant pressure and possible presence of Mg2TiO4 and MgFe2O4 components, if any, decrease the lattice parameter of titanomagnetite from that in the Fe2TiO4–Fe3O4 system at 1 atm. When we assume that the titanomagnetites in the diamonds are Mg-free solid solutions in the system Fe2TiO4–Fe3O4 and the remnant pressure is negligibly small, we can estimate 81 mol % of Fe2TiO4 component for the titanomagnetite with the largest cell parameter (a = 8.511 Å). Because this is the lower bound for the Fe2TiO4 component, as discussed above, it is probable that the titanomagnetite composition was more Fe2TiO4-rich, possibly almost pure Fe2TiO4. Such titanomagnetites of very high Fe2TiO4 content probably could not be directly incorporated at a depth where the diamonds were formed, because the graphite–diamond transition boundary [36] is placed by ~1 GPa at higher pressure than the upper bound of the stability field of Fe2TiO4 Sp (Figure 4). Therefore, the above estimate would suggest that the Fe2TiO4-rich titanomagnetites were originally CT-type phases which are stable above ~15 GPa in the transition zone and the lower mantle conditions, and that they back-transformed to the Fe2TiO4-rich titanomagnetites in the diamonds after being transported to the upper mantle. The decomposition into FeTiO3 Ilm (or Pv) + FeO Wu might have been kinetically hindered. Further experimental studies, particularly compositional analysis of the titanomagnetites, would be required to better clarify the origin of the titanomagnetites in the superdeep diamonds.

Author Contributions

T.T., M.O. and M.A. conducted the experiments; T.T., M.O., H.K. and M.A. analyzed the obtained data; M.A. and H.K. wrote the manuscript.

Funding

This work was supported in part by JSPS KAKENHI (grant nos. 25287145 and 17H02986 to M.A.) and by the MEXT-supported program of Gakushuin University for the Strategic Research Foundation at Private Universities.

Acknowledgments

I (M.A.) thank Robert C. Liebermann (Bob-san) for his suggestion to submit the paper to the Special Issue, Mineral Physics−In Memory of Orson Anderson, and for his encouragements on various occasions to me. We are grateful to T. Ishii, H. Yusa, D. Mori and Y. Inaguma for useful suggestions and discussion.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Powder X-ray diffraction patterns (Cr Kα) of run products of Mg2TiO4, (a) No. O161021 (21 GPa, 1600 °C) and (b) No. O160924 (28 GPa, 1300 °C). Miller indices in (a) are for Ln-type MgTiO3, and those in (b) are for αPbO2-type TiO2. Ln: LiNbO3-type, Pe: periclase, α: αPbO2-type.
Figure 1. Powder X-ray diffraction patterns (Cr Kα) of run products of Mg2TiO4, (a) No. O161021 (21 GPa, 1600 °C) and (b) No. O160924 (28 GPa, 1300 °C). Miller indices in (a) are for Ln-type MgTiO3, and those in (b) are for αPbO2-type TiO2. Ln: LiNbO3-type, Pe: periclase, α: αPbO2-type.
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Figure 2. Phase diagram of Mg2TiO4. An open circle: Mg2TiO4(Sp), solid circles: MgTiO3(Pv) + MgO(Pe), open squares: 2MgO(Pe) + TiO2 (Bd, OI). A thick solid line: this study, a thin line with A-S67: Sp dissociation boundary to Ilm + Pe [7], a thin line with A: Ilm-Pv boundary [25]. Sp: spinel-type, Ilm: ilmenite-type, Pe: periclase, Pv: perovskite-type, Bd: baddeleyite-type, OI: orthorhombic I-type.
Figure 2. Phase diagram of Mg2TiO4. An open circle: Mg2TiO4(Sp), solid circles: MgTiO3(Pv) + MgO(Pe), open squares: 2MgO(Pe) + TiO2 (Bd, OI). A thick solid line: this study, a thin line with A-S67: Sp dissociation boundary to Ilm + Pe [7], a thin line with A: Ilm-Pv boundary [25]. Sp: spinel-type, Ilm: ilmenite-type, Pe: periclase, Pv: perovskite-type, Bd: baddeleyite-type, OI: orthorhombic I-type.
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Figure 3. Powder X-ray diffraction patterns (Cr Kα) of run products of Fe2TiO4, (a) No. M150115 (14 GPa, 1100 °C) and (b) No. M140710 (18 GPa, 1300 °C). Miller indices in (a) are for Ln-type FeTiO3, and those in (b) are for CT-type Fe2TiO4. Ln: LiNbO3-type, Wu: wustite, Fe: metallic iron, CT: calcium titanate-type.
Figure 3. Powder X-ray diffraction patterns (Cr Kα) of run products of Fe2TiO4, (a) No. M150115 (14 GPa, 1100 °C) and (b) No. M140710 (18 GPa, 1300 °C). Miller indices in (a) are for Ln-type FeTiO3, and those in (b) are for CT-type Fe2TiO4. Ln: LiNbO3-type, Wu: wustite, Fe: metallic iron, CT: calcium titanate-type.
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Figure 4. Phase diagram of Fe2TiO4. An open triangle: Fe2TiO4(Sp), open circles: FeTiO3(Ilm) + FeO(Wu), solid circles: FeTiO3(Pv) + FeO(Wu), open squares: Fe2TiO4(CT). Thick solid lines: this study, a thin line with A-S67: Sp dissociation boundary to Ilm + Wu [7], a thin dashed line with Gr-Dia: the graphite–diamond boundary [36]. Sp: spinel-type, Ilm: ilmenite-type, Wu: wustite, Pv: perovskite-type, CT: calcium titanate-type.
Figure 4. Phase diagram of Fe2TiO4. An open triangle: Fe2TiO4(Sp), open circles: FeTiO3(Ilm) + FeO(Wu), solid circles: FeTiO3(Pv) + FeO(Wu), open squares: Fe2TiO4(CT). Thick solid lines: this study, a thin line with A-S67: Sp dissociation boundary to Ilm + Wu [7], a thin dashed line with Gr-Dia: the graphite–diamond boundary [36]. Sp: spinel-type, Ilm: ilmenite-type, Wu: wustite, Pv: perovskite-type, CT: calcium titanate-type.
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Figure 5. Crystal structure of CaTi2O4-type AB2O4. Small red spheres, middle blue ones and large green ones express oxygen, B and A cations, respectively. The structure was drawn by VESTA [41].
Figure 5. Crystal structure of CaTi2O4-type AB2O4. Small red spheres, middle blue ones and large green ones express oxygen, B and A cations, respectively. The structure was drawn by VESTA [41].
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Table 1. Results of high-pressure and high-temperature experiments in Mg2TiO4.
Table 1. Results of high-pressure and high-temperature experiments in Mg2TiO4.
Run No.Pressure (GPa)Temperature (°C)(min)Run Product
O160915211000120Pe, Ln
O16102523100090Pe, α
O16101721130090Pe, Ln
O17011622.5130050Ln, Pe, α
O16112823.5130060Pe, α, Ln(tr)
O16090924.5130060Pe, α
O16092428130090Pe, α
O16110628140090Pe, α, Ln(tr)
O17021922150090Ln, Pe, α
O17020225150060Pe, α
O17012628150060Pe, α
O16102121160090Pe, Ln
O16112422160060Pe, Ln
O16120923.5160060Ln, Pe, α
O16090324.5160060Pe, α, Ln(tr)
Abbreviations are as follows: Ln, LiNbO3-type MgTiO3; Pe, MgO periclase; α, αPbO2 type TiO2; tr, trace.
Table 2. Molar volumes of phases in the systems MgO-TiO2 and FeO-TiO2.
Table 2. Molar volumes of phases in the systems MgO-TiO2 and FeO-TiO2.
Comp.StructureV0 (cm3/mol)Ref.Comp.StructureV0 (cm3/mol)Ref.
MgOrock-salt11.24aFeOrock-salt12.17i
MgTiO3ilmenite30.86bFeTiO3ilmenite31.72j
MgTiO3perovskite29.31cFeTiO3perovskite30.34k
MgTiO3LiNbO330.71dFeTiO3LiNbO331.34j
Mg2TiO4spinel45.25eFe2TiO4spinel46.84e
TiO2αPbO218.41fFe2TiO4CaTi2O442.41e
TiO2baddeleyite16.90g
TiO2OI16.83h
V0: molar volume at ambient conditions. Ref. a: Hazen [31], b: Wechsler and Von Dreele [3], c: Linton et al. [17], d: Linton et al. [32], e: This study, f: Kojitani et al. [22], g: Al-Khatatbeh et al. [24], h: Nishio-Hamane et al. [33], i: McCammon [34], j: Akaogi et al. [26], k: Leinenweber et al. [27].
Table 3. Results of high-pressure and high-temperature experiments in Fe2TiO4.
Table 3. Results of high-pressure and high-temperature experiments in Fe2TiO4.
Run no.Pressure (GPa)Temperature (°C)Time (min)Run Product *
M15010912.5100060Ilm, Wu
M15010712.5110060Ilm, Wu
M14080613110060Ilm, Ln, Wu
M15011514110060Ln, Wu
M14112815.5110060Ln, Wu
M14070716110060CT
M15021817110060CT
M15060320110060CT
M15022623110060CT
M15051624110060CT
M15012712.5120060Ilm, Ln, Wu
M15041618120060CT
M15070711130060Ilm, Wu
M15072512130060Ilm, Wu
M14072813130060Ln, Wu
M15062514130060Ln, Wu
M15020215130060CT
M14062615130060CT, Ln, Wu
M14070118130060CT
M14071018130060CT
M15022720130060CT
M15060622130060CT
* Metallic iron coexisted in the run products. Ilm: ilmenite-type FeTiO3, Wu: FeO wustite, Ln: LiNbO3-type FeTiO3, CT: CT-type Fe2TiO4.

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Akaogi, M.; Tajima, T.; Okano, M.; Kojitani, H. High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds. Minerals 2019, 9, 614. https://doi.org/10.3390/min9100614

AMA Style

Akaogi M, Tajima T, Okano M, Kojitani H. High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds. Minerals. 2019; 9(10):614. https://doi.org/10.3390/min9100614

Chicago/Turabian Style

Akaogi, Masaki, Taisuke Tajima, Masaki Okano, and Hiroshi Kojitani. 2019. "High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds" Minerals 9, no. 10: 614. https://doi.org/10.3390/min9100614

APA Style

Akaogi, M., Tajima, T., Okano, M., & Kojitani, H. (2019). High-Pressure and High-Temperature Phase Transitions in Fe2TiO4 and Mg2TiO4 with Implications for Titanomagnetite Inclusions in Superdeep Diamonds. Minerals, 9(10), 614. https://doi.org/10.3390/min9100614

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