The high-pressure C2/c–P2₁/c phase transition along the LiAlSi₂O₆–LiGaSi₂O₆ solid solution

Abstract

Two synthetic single-crystals with composition Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ and space group C2/c at room conditions have been studied under pressure by means of X-ray diffraction using a diamond anvil cell. The unit-cell parameters were determined at 12 and 10 different pressures up to P = 8.849 and P = 7.320 GPa for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆, respectively. The sample with mixed composition shows a C2/c to P2₁/c phase transformation between 1.814 and 2.156 GPa, first-order in character. The transition is characterised by a large and discontinuous decrease in the unit-cell volume and by the appearance of the b-type reflections (h + k = odd) typical of the primitive symmetry. The Ga end-member shows the same C2/c to P2₁/c transformation at a pressure between 0.0001 and 0.39 GPa. The low-pressure value at which the transition occurred did not allow collecting any data in the C2/c pressure stability field except that on room pressure. Our results compared with those relative to spodumene (LiAlSi₂O₆, Arlt and Angel 2000a) indicate that the substitution of Al for Ga at the M1 site of Li-clinopyroxenes strongly affects the transition pressure causing a decrease from 3.17 GPa (spodumene) to less than 0.39 GPa (LiGaSi₂O₆) and decreases the volume discontinuity at the transition. As already found for other compounds, the C2/c low-pressure phases are more rigid than the P2₁ /c high-pressure ones. Moreover, the increase of the M1 cation radius causes a decrease in the bulk modulus K _T0. The axial compressibility among the Li-bearing clinopyroxenes indicates that the c axis is the most rigid for the C2/c phases while it becomes the most compressible for the P2₁ /c phases.

Introduction

The P2₁/c–C2/c phase transformation in natural and synthetic pyroxenes occurs both at high temperature and high pressure for several compositions (e.g. Angel et al. 1992; Arlt and Angel 2000a; Tribaudino et al. 2001, 2002; Nestola et al. 2004; Pommier et al. 2005). The high temperature and high pressure C2/c phases, however, are characterized by a different structural asset of the tetrahedral chains running along c. Based upon this difference, we call HTC2/c those clinopyroxenes showing extended tetrahedral chains (e.g. spodumene LiAlSi₂O₆, O3–O3–O3 = 170°, Arlt and Angel 2000a) and HPC2/c those having kinked tetrahedral chains (e.g. iron-free pigeonite Ca_0.15Mg_1.85Si₂O₆, O3–O3–O3 = 137°, Nestola et al. 2004). The complete sequence of phase transformations from HTC2/c to P2₁/c to HPC2/c has been found for ZnSiO₃ clinopyroxene (Arlt and Angel 2000a) at room temperature through a pressure range included between room pressure and 7.4 GPa. For Li-bearing pyroxenes instead only the HTC2/c to P2₁/c phase transition has been observed so far (Arlt and Angel 2000a; Pommier et al. 2005), whereas P2₁/c Li-bearing pyroxenes appear to be very stable with increasing pressure (Gatta et al. 2005). The pyroxene composition plays a substantial role in determining the transition pressure or temperature as well as the compressibility; high pressure and high temperature studies of pyroxene solid solution are, therefore, crucial to better constrain the structure—physical properties’ relationships in these minerals.

Here, we present high-pressure data obtained for two synthetic samples belonging to the LiAlSi₂O₆–LiGaSi₂O₆ solid solution using single-crystal X-ray diffraction, in order to define a possible trend between the size of the M1 crystallographic site and the HTC2/c–P2₁/c transition pressure, as well as to improve the systematic compressibility of Li-bearing pyroxenes. The samples studied can be defined as HTC2/c as they show at room conditions a strongly extended tetrahedral chain (e.g. Ohashi et al. 2003).

Experimental

Two synthetic single-crystals with compositions Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ (both showing space group HTC2/c at room pressure) and crystal sizes 175 × 120 × 50 and 100 × 70 × 55 μm³, respectively, were selected for the high-pressure X-ray diffraction on the basis of their sharp optical extinction, absence of twinning and sharp diffraction profiles (about 0.06° in ω). The samples are selected from the same synthesis run of Ohashi (2003) done in a belt-type apparatus using mixtures of crystalline Li₂Si₂O₅, Al₂O₃, Ga₂O₃ and SiO₂ sealed in platinum capsules and maintained at 1,770 K and 6 GPa for 20 h. The crystals were loaded in a BGI-type and an ETH diamond anvil cells using T301 steel gaskets preindented to 80 and 85 microns for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆, respectively, and with holes of 250 μm in diameter. A methanol:ethanol = 4:1 mixture was used as hydrostatic medium and two single-crystals of quartz (Angel et al. 1997) were used as internal pressure standards. Unit-cell parameters were determined using a HUBER four-circle diffractometer. Full details of the instrument and the peak-centring algorithms are provided by Angel et al. (2000). During the centring procedure, the effects of crystal offsets and diffractometer aberrations were eliminated from refined peak positions by the eight-position centring method of King and Finger (1979). Unconstrained unit-cell parameters were found to be within one estimated standard deviation of the symmetry constrained ones, calculated using SINGLE04 (Angel 2004) and reported in Table 1. In order to monitor the possible space group change of the two samples investigated, scans of three intense b-type reflections, i.e. (102), (031), and ${\left( {\ifmmode\expandafter\bar\else\expandafter\=\fi{2}31} \right)}$ (selected among those relative to spodumene, Arlt and Angel 2000a), typical of the primitive P2₁/c lattice and forbidden in the C2/c space group, were performed at all measured pressures.

Table 1 Unit-cell parameters at different pressures for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ samples

Results

Phase transitions

The unit-cell parameters for the samples studied in this work were determined at 12 and 10 different pressures and room temperature reaching 8.85 and 7.32 GPa for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆, respectively. Their evolution with pressure together with the evolution of the unit-cell volumes are reported in Figs. 1 and 2. For the Li(Al_0.53Ga_0.47)Si₂O₆ sample the unit-cell parameters decrease up to about 1.710 GPa, and between this pressure and 2.732 GPa they show a strong discontinuity (1.7% of volume change), which indicates a first order phase transition from C2/c to P2₁/c as indicated by the appearance of the b-type reflections, forbidden in C2/c. Then, to better bracket the transition, we decreased the pressure and at 2.156 GPa the sample symmetry was still P2₁/c as confirmed by the b-type reflections. At a polarizing microscope, during a further decrease of pressure from 2.156 to 1.814 GPa we observed the coexistence of the P2₁/c and C2/c domains in the pyroxene crystal separated by a sharp Becke line (Fig. 3). This phenomenon was already reported in a previous work on spodumene LiAlSi₂O₆ by Arlt and Angel (2000b), and was related to the occurrence of pressure buffering in the DAC during a first-order phase-transition. In that work, the authors proposed that the phase transition boundary is parallel to the (100) plane. Since we used the same crystal orientation in the DAC used by Arlt and Angel (2000b), it is likely that also in our experiment the (100) plane represents the transition boundary, with the crystal oriented with the (001) plane parallel to the diamond culet. Once the pressure reached 1.814 GPa the P2₁/c domain disappeared as also confirmed by the zero intensity of the (102) reflection and the evident change in volume (Table 1; Fig. 2). During the second compression we observed that the P2₁/c symmetry is stable to 8.85 GPa (the maximum pressure reached in this experiment).

Fig. 1

Evolution of the unit-cell parameters as a function of pressure for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ samples

Fig. 2

Evolution of the unit-cell volumes with pressure for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ samples

Fig. 3

Coexistence of the two C2/c and P2₁/c phases in Li(Al_0.53Ga_0.47)Si₂O₆ sample (larger crystal in the picture) viewed under a polarizing microscope decreasing the pressure from 2.156 GPa to slightly higher pressure than 1.814 GPa: from the top left the single domain is the P2₁/c transforming to C2/c starting to grow in the top middle image and evolving through the other images. The smaller crystal is the quartz used as an internal standard pressure. The phenomenon is perfectly reversible from one phase to the other across this two-phase coexistence field. The scale of the images is given by the size of the crystal

At the transition the a and c axes as well as the β angle decrease discontinuously, whereas the b axis slightly increases. This behaviour was already observed for spodumene, Sc-spodumene (Arlt and Angel 2000a), clinoenstatite and pigeonite (Angel and Hugh-Jones 1994; Nestola et al. 2004).

The high-pressure behaviour of LiGaSi₂O₆ is different with respect to that observed for Li(Al_0.53Ga_0.47)Si₂O₆. Going from room pressure (C2/c symmetry) to 0.39 GPa (Table 1) the sample already showed a phase transformation to the P2₁/c symmetry as confirmed by the appearance of the b-type reflections, obviously absent at room pressure. For this composition, we were not able to observe optically the coexistence of the P and C domains as described above for Li(Al_0.53Ga_0.47)Si₂O₆. By extrapolating the high-pressure P2₁/c data down to room pressure we can estimate that the change in volume due to the transition is smaller than 0.2%, much smaller than that showed by Li(Al_0.53Ga_0.47)Si₂O₆ and LiAlSi₂O₆; however, since we do not have any data between room pressure and 0.39 GPa we cannot speculate on the thermodynamic character of the transition.

Equations of state

In this study we could determine the equation of sate for the HTC2/c and P2₁/c phases of Li(Al_0.53Ga_0.47)Si₂O₆ and for the P2₁/c phase of LiGaSi₂O₆. The limited pressure range in which the HTC2/c of LiGaSi₂O₆ is stable did not allow evaluating any compressibility data for such symmetry. For the P2₁/c symmetry of LiGaSi₂O_6, we fitted the pressure–volume data (including those measured during decompression) using a third-order Birch–Murnagahan equation of state (Birch 1947, BM3-EoS) with the EoS-FIT5.2 program (Angel 2002) refining simultaneously the unit-cell volume, V ₀, the bulk modulus, K _T0 and its first pressure derivative, K′. The following EoS parameter values were obtained: V ₀ = 404.32(15) ų, K _T0 = 86(2) GPa, K′ = 10.4(9) (Table 2). These values are in excellent agreement with those obtained from the “normalized stress–finite strain” (F–f) plot calculated using as V ₀ the BM3-EoS value (Fig. 4). The values obtained with F _E–f _E plot gave K _T0 = 86 GPa and K′ = 10.4. A second-order BM-EoS (BM2-EoS) was used to fit the pressure–volume data up to 1.814 GPa for the C2/c symmetry of Li(Al_0.53Ga_0.47)Si₂O₆ giving the following coefficients: V ₀ = 396.89(2) ų, K _T0 = 135.2(9) GPa, K′ was fixed to 4 during the refinement. Also for the C2/c phase the F _E–f _E plot gave a very good agreement with the refined K _T0, being the obtained value of 137 GPa. Moreover, the plot (Fig. 4) clearly indicates that the BM2-EoS well describes the PV trend for C2/c symmetry of Li(Al_0.53Ga_0.47)Si₂O₆, as the slope of the linear regression is practically zero within one standard deviation. For the high pressure P2₁/c symmetry of Li(Al_0.53Ga_0.47)Si₂O₆ the P–V data were fitted using a BM3-EoS from 2.156 GPa up to the maximum pressure reached (8.849 GPa). In a first cycle of refinement V ₀, K _T0 and K′ were refined simultaneously and gave the following coefficients: V ₀ = 396.3(3) ų, K _T0 = 81(4) GPa, K′ = 12(1). The relatively large error in K _T0 is due to the limited number of measured points and to the fact that an experimental V ₀ is not available. Since the “normalized stress–finite strain” (F–f) plot calculated using as V ₀ the volume obtained from the BM3-EoS fit, indicates a K′ = 12 (Fig. 4), we decided to perform a further refinement cycle with fixed K′ = 12 obtaining the following results: V ₀ = 396.63(7) ų, K _T0 = 80.5(4) GPa (Table 2). Note that large values of K′ are quite common for Li-clinopyroxenes (see for example Arlt and Angel 2000a for spodumene: K′ = 8.9(6) and Gatta et al. 2005 for Fe–Mg spodumene: K′ = 9.6).

Table 2 Equation of state coefficients (see the text) for Li(Al_0.53Ga_0.47)Si₂O₆ and LiGaSi₂O₆ samples (this study) and LiAlSi₂O₆ (Arlt and Angel 2000a)

Fig. 4

F _E–f _F plot (see the text) for LiGaSi₂O₆ (P2₁/c symmetry) and LiAl_0.53Ga_0.47Si₂O₆ (P2₁/c and C2/c symmetry) studied in this work

The volume compressibilities, calculated as β _V  = −1/K _T0, are the following: β _V  = 0.00740(2) GPa⁻¹ and β _V  = 0.01242(3) GPa⁻¹ for the C2/c and P2₁/c Li(Al_0.53Ga_0.47)Si₂O₆, respectively, and β _V  = 0.01163(3) GPa⁻¹ for the P2₁/c LiGaSi₂O₆. For comparison the volume compressibilities of the C2/c and P2₁/c phases of spodumene LiAlSi₂O₆ (Arlt and Angel 2000a) are β _V  = 0.00694(3) GPa⁻¹ and β _V  = 0.01099(3) GPa⁻¹, respectively.

Axial compressibility

In order to determine the axial moduli for a, b, and c for the C2/c and P2₁/c phases of the Li(Al_0.53Ga_0.47)Si₂O₆ and for the P2₁/c symmetry of the LiGaSi₂O₆ we used a parameterised form of the BM-EoS in which the individual axes are cubed, following the procedure implemented in the EoS-FIT5.2 program (Angel 2002). The results obtained are reported in Table 2. For Li(Al_0.53Ga_0.47)Si₂O₆ the axial bulk modulus ratios are 1.00:1.05:1.40 and 2.60:2.67:1.00 for the C2/c and the P2₁/c phase, respectively, indicating both a change in the compressibility scheme and a more pronounced anisotropy associated with the transformation. For the P2₁/c LiGaSi₂O₆ end-member the axial bulk modulus ratios is 1.23:2.23:1.00, which is similar to that of the P2₁/c Li(Al_0.53Ga_0.47)Si₂O₆ sample, the major difference being the compressibility along the a axis which is softer for LiGaSi₂O₆ than for Li(Al_0.53Ga_0.47)Si₂O₆. Concerning the LiAlSi₂O₆ (Arlt and Angel 2000a) the axial bulk modulus ratios are 1.00:1.05:1.29 and 1.54:2.22:1.00 for the C2/c and the P2₁/c phases, respectively, with the Ga causing an increase in anisotropy for both the C and P symmetries.

Discussion and conclusions

Phase transition and elasticity: comparison with LiAlSi₂O₆, LiFeSi₂O₆ and LiScSi₂O₆

Beside LiAlSi₂O₆ (Arlt and Angel 2000a) and the samples studied in this work, high-pressure data are also available for LiFeSi₂O₆ and LiScSi₂O₆ (Pommier et al. 2005; Arlt and Angel 2000a). These two end-members, HTC2/c at ambient conditions, show a phase transition to P2₁/c between 0.7 and 1 GPa and between 0.3 and 0.66 GPa, respectively. For purpose of comparison, we can assume as their transition pressure the average values of 0.85 and 0.48 GPa for LiFeSi₂O₆ and LiScSi₂O₆, respectively. For these Li-clinopyroxenes, changes in composition are limited to cation substitution at the M1 site; therefore we can expect some relationship between the M1 cation size and the transition pressure. In Fig. 5, we report the M1 aggregate cation radius (Shannon 1976) versus the transition pressure for all samples. It appears that for the LiAlSi₂O₆–LiGaSi₂O₆ join and LiFeSi₂O₆ the pressure decreases almost linearly with decreasing the cation radius, whereas the LiScSi₂O₆ datum lies out of the trend. This behaviour is similar to that observed as a function of temperature for the HTC2/c–P2₁/c transition in LiM³⁺Si₂O₆ clinopyroxenes (Redhammer et al. 2001; Redhammer and Roth 2004). The following transition temperatures, taken from Redhammer and Roth (2004) and Redhammer et al. (2001): T _c  = 330(1) K for LiCrSi₂O₆; T _c  = 286(1) K for LiGaSi₂O₆; T _c  = 205(1) K for LiVSi₂O₆; T _c  = 234(1) K for LiScSi₂O₆; T _c  = 90(1) K for LiSc_0.72In_0.28Si₂O₆; and T _c  = 229(1) K for LiFeSi₂O₆, are plotted versus the M1 cation radius for the LiM³⁺Si₂O₆ in Fig. 6. Also in this case the transition temperature decreases linearly with decreasing the cation radius with exception of the Sc-bearing pyroxenes. A major difference between the high-pressure and the high-temperature behaviour is given by spodumene LiAlSi₂O₆ (Arlt and Angel 2000a; Redhammer and Roth 2004), since this C2/c pyroxene transforms to the P2₁/c symmetry with increasing pressure, but it is C centred at all temperatures. It appears, therefore, that the variation of the M1 cation size is not the only factor to determine the transition pressure or temperature, but that also the different electronic configuration of the M1 cations (as found for the P2₁/c to HPC2/c phase transition in pyroxenes Arlt et al. 1998), as well as differences in covalent bonds between the M1 cations and the oxygens should be taken into account.

Fig. 5

Cation radius at M1 site versus the pressure of C2/c–P2₁/c transition for different Li bearing clinopyroxenes. The solid line is a weighted linear fit

Fig. 6

Cation radius at M1 site versus the temperature of C2/c–P2₁/c transition for different Li bearing clinopyroxenes. The solid line is a weighted linear fit

The substitution of Ga for Al (i.e. increase of the cation radius at the M1 site) gives rise to a decrease of the bulk modulus for both C2/c and P2₁/c phases. In particular, for the P2₁/c phases, for which we have the bulk moduli of both end-members, it appears that the K _T0 variation across the solid solution is not linear (Fig. 7), having the sample with mixed composition a bulk modulus smaller than that expected from an ideal behaviour of the solid solution.

Fig. 7

Bulk modulus K _T0 versus the Ga content at M1 site for the P2₁/c phases of LiGaSi₂O₆, LiAl_0.53Ga_0.47Si₂O₆ (this study) and spodumene LiAlSi₂O₆ (Arlt and Angel 2000a)

The P2₁/c phases of spodumene and Li(Al_0.53Ga_0.47)Si₂O₆ are more compressible than the corresponding C2/c phases, this is due mostly to the softening of the c direction (which is the most rigid in the C2/c phases) as a result of the phase transformation.