A series of Si-bearing MgAl₂O₄-spinels were synthesized at 1500–1650 °C and 3–6 GPa. These spinels had SiO₂ contents of up to ~1.03 wt % and showed a substitution mechanism of Si⁴⁺ + Mg²⁺ = 2Al³⁺. Unpolarized Raman spectra were collected from polished single grains, and displayed a set of well-defined Raman peaks at ~610, 823, 856 and 968 cm⁻¹ that had not been observed before. Aided by the Raman features of natural Si-free MgAl₂O₄-spinel, synthetic Si-free MgAl₂O₄-spinel, natural low quartz, synthetic coesite, synthetic stishovite and synthetic forsterite, we infer that these Raman peaks should belong to the SiO₄ groups. The relations between the Raman intensities and SiO₂ contents of the Si-bearing MgAl₂O₄-spinels suggest that under some P-T conditions, some Si must adopt the M-site. Unlike the SiO₄ groups with very intense Raman signals, the SiO₆ groups are largely Raman-inactive. We further found that the Si cations primarily appear on the T-site at P-T conditions ≤~3–4 GPa and 1500 °C, but attain a random distribution between the T-site and M-site at P-T conditions ≥~5–6 GPa and 1630–1650 °C. This Si-disordering process observed for the Si-bearing MgAl₂O₄-spinels suggests that similar Si-disordering might happen to the (Mg,Fe)₂SiO₄-spinels (ringwoodite), the major phase in the lower part of the mantle transition zone of the Earth and the benchmark mineral for the very strong shock stage experienced by extraterrestrial materials. The likely consequences have been explored.

The effects of Al³⁺–Fe³⁺ substitution on 10 synthesized garnet samples along the grossular–andradite binary solid solution were investigated using both powder and single-crystal X-ray diffraction. Results showed that cell volume increased with andradite content. Small negative excess volume was observed in the Al-rich samples. By measuring the bond length, polyhedral volume, octahedral distortion parameter (σ), and tetrahedral rotation angle (α), we determined that the distortion occurred on the dodecahedral site. The width of the diffraction peaks was obviously related to the composition of the solid solution. Full width at half maximum of diffraction peaks was used to calculate the microstrain, which may have a relationship with enthalpy of mixing.

MgAl₂O₄-spinel has wide industrial and geological applications due to its special structural and physical–chemical features. It is presumably the most important endmember of complex natural spinel solid solutions, and therefore provides a structural model for a large group of minerals with the spinel structure. There exists a well known but still inadequately understood phenomenon in the structure of MgAl₂O₄-spinel, the Mg–Al cations readily exchanging their positions in response to variations of temperature, pressure, and composition. A large number of experiments were performed to investigate the Mg–Al cation order-disorder process usually quantified by the inversion parameter x (representing either the molar fraction of Al on the tetrahedral T-sites or the molar fraction of Mg on the octahedral M-sites in the spinel structure), and some thermodynamic models were thereby constructed to describe the x-T relation. However, experimental data at some key T were absent, so that the different performance of these thermodynamic models could not be carefully evaluated. This limited the interpolation and extrapolation of the thermodynamic models. By performing some prolonged annealing experiments with some almost pure natural MgAl₂O₄-spinel plates and quantifying the x values with single-crystal X-ray diffraction technique, we obtained some critical equilibrium x values at T down to 773 K. These new x-T data, along with those relatively reliable x values at relatively high T from early studies, clearly indicate that the CS94 Model (a model constructed by Carpenter and Salje in 1994) better describes the Mg–Al cation order-disorder reaction in MgAl₂O₄-spinel for a wide range of T. On the basis of the CS94 Model, a geothermometer was established, and its form is T_-closure = 21362 × x³ − 12143 × x² + 6401 × x − 10 (T_-closure standing for the closure temperature of the Mg–Al cation exchange reaction). This geothermometer can be used to constrain the thermal history of the geological bodies containing MgAl₂O₄-spinel.

Bredigite with the composition Ca₇MgSi₄O₁₆ (Ca₇MgSi₄O₁₆-Bre) has been synthesized by a solid-state reaction method at 1.2 GPa and 1373 K for 7 days, and its structure has been determined by single-crystal X-ray diffraction data. Following a relevant genealogy analysis in the literature, we have refined the structure into two space groups, Pnnm and Pnn2, and found that Ca₇MgSi₄O₁₆-Bre belongs to the space group Pnnm, which can be essentially derived from the space group Pnn2 via an atomic coordinate transformation (with an average deviation of 0.039 Å only). Furthermore, some thermodynamic properties of the Ca₇MgSi₄O₁₆-Bre have been obtained in this study. Using first-principles simulations based on density functional theory, the isothermal bulk modulus has been determined as 90.6(4) GPa with a pressure derivative of 5.7(1). Using density functional perturbation technique, the phonon dispersions and vibrational density of the states (VDoS) have been calculated. The VDoS has been combined with a quasi-harmonic approximation to compute the isobaric heat capacity (C_p) and standard vibrational entropy ($S_{298}^0$), yielding C_p = 8.22(2) × 10² − 3.76(6) × 10³T^−0.5 − 1.384(4) × 10⁷T⁻² + 1.61(8) × 10⁹T⁻³ J mol⁻¹ K⁻¹ for the T range of 298-1000 K and $S_{298}^0$ = 534.1 (22) J mol⁻¹ K⁻¹.

The in-situ X-ray diffraction (XRD) and Raman spectra of synthetic (Mg, Fe, Mn)CO₃ single crystals for (Mg_0.37Fe_0.31Mn_0.32)CO₃, (Mg_0.49Fe_0.51)CO₃, (Fe_0.47Mn_0.53)CO₃, and (Mg_0.50Mn_0.50)CO₃ were investigated up to 871 K. Lattice parameters and the volumetric thermal expansion coefficients were achieved, demonstrating an inherent anisotropy of c/a near to 3.0. The Raman spectra of our intermediate components all exhibit a negative temperature dependence similar to that of the end members. The temperature dependence of the external modes T, L, anti-symmetric stretching mode ν₃, and out-of-plane bending mode 2ν₂ is generally more prominent than the in-plane bending mode ν₄ and symmetric stretching mode ν₁ except for siderite FeCO₃. Combining thermal expansion coefficients α_V with the relative changes in frequency δνi/δT of Raman spectra, the average thermodynamic Grüneisen parameters (γ_th) for (Mg_0.37Fe_0.31Mn_0.32)CO₃, (Fe_0.47Mn_0.53)CO₃, (Mg_0.50Mn_0.50)CO₃, MgCO₃, and MnCO₃ were obtained as 1.18, 1.36, 1.34, 1.13, and 1.53, respectively. The relationship between Fe²⁺ and Mn²⁺ contents and γ_th illustrates that the concentration of Fe²⁺ causes a negative effect while the concentration of Mn²⁺ causes a positive effect on γ_th. These results could provide a reference for the contribution of Fe²⁺ and Mn²⁺ on the structural and thermodynamic properties of (Mg, Fe, Mn)CO₃ carbonates.