In Honor of Nobel Laureate Dr. Aaron Ciechanover
| SESSION: GeochemistryTuePM3-R5 |
Dutrow International Symposium (4th Intl. Symp. on Geochemistry for Sustainable Development) |
| Tue. 18 Nov. 2025 / Room: Orchid | |
| Session Chairs: Sabine Dietrich; Darrell Henry; Student Monitors: TBA | |
Mg-Al spinel (MgAl2O4, MAS) is a common rock-forming mineral widely distributed in the upper mantle, crust, and extraterrestrial bodies [1]. The order-disorder transition (ODT) in spinel is a critical physical characteristic that significantly affects its properties and applications as a geothermometer for probing the thermal history of Earth and extraterrestrial bodies [2]. Although activation energy—a key thermodynamic parameter of ODT—is reported to be influenced by Cr content [3,4], the relationship between these two factors remains unclear.
In this work, six natural Mg-Al spinel samples with varying Cr contents (419 – 6918 ppma) were studied. Each sample was cut into eight slices and heated to seven different temperatures (550–850 ◦C) by a muffle furnace (model YFX9/16Q-YC), creating a stepwise increase in the degree of disorder. Photoluminescence (PL) spectra were subsequently collected at liquid nitrogen temperature (JASCO NRS7500 Raman spectrometer).
The PL spectrum emitted by Cr in the samples consists of zero-phonon lines (ZPL) and phonon sidebands, with a spectral range between 13800 and 14640 cm-1. The ZPL, located between 14500 and 14640 cm-1, comprises R peaks (R1 and R2) and N peaks (n5, N2, n3, and N1). R1 and R2 are emitted by the Cr3+ ions around which the cation arrangement is ordered, while the disordered arrangement emits the N peaks. The intensity of the N peaks in the zero-phonon line range and the N4 peak increased with increasing Cr content.
By integrating parameters extracted from these spectra into a thermodynamic model, which we established in previous work based on the Arrhenius relationship [5], we calculated the ODT activation energies, which decreased from 335 to 163 kJ/mol as the Cr content increased. A quantitative relationship between activation energy and Cr content was established: Ea = − 0.03[Cr] + 356.28. As a result, we clarify the negative correlation between ODT activation energy and Cr content in Mg-Al spinel, which may be attributed to local structural distortion caused by the presence of Cr-pairs.
This study provides a method to correct the influence of Cr content on ODT thermodynamic models, thereby enhancing the viability of spinel as geothermometers and deepening the scientific understanding of how elemental composition affects the thermal stability of spinel [6].
