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Raman and XRD Investigation of Magnesium Perchlorate Hydration States, Fermi Resonance, and Thermal Behavior: Implications for Earth and Mars

Session: Advancing Mineralogy and Spectroscopy Across the Solar System in Honor of MSA Roebling Medalist M. Darby Dyar

Presenting Author:

Genesis Berlanga

Authors:

Berlanga, Genesis¹, Williams, Quentin², Bristow, Thomas³, Meusburger, Johannes⁴

(1) Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, USA; BMSIS, NASA Ames Research Center, Moffett Field, CA, USA, (2) Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, CA, USA, (3) Exobiology, NASA Ames Research Center, Moffett Field, CA, USA, (4) Exobiology, NASA Ames Research Center, Moffett Field, CA, USA,

Abstract:

Magnesium perchlorate (Mg(ClO₄)₂) is a highly deliquescent and oxidizing salt relevant to hyperarid Earth environments and Mars’ surface geochemistry. This study presents a comprehensive analysis of the hydration states, thermal stability, and vibrational behavior of Mg(ClO₄)₂ from –70 °C to 450 °C using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Raman measurements capture systematic evolution in perchlorate and bound water vibrational modes (v₁, v₂, v₃, v₄, OH) that correspond to structural phase transitions tracked via XRD. These hexahydrate-tetrahydrate-dihydrate-anhydrous phase transitions exhibit symmetry reduction and reorganization of polyhedral networks, consistent with previous diffraction-based studies.

Cl–O force constants were calculated using Wilson GF matrix methods, revealing increases in bond stiffness upon dehydration and values comparable to sulfate analogs. The hydrated hexahydrate phase exhibited distinct OH stretching bands that diminished sharply by ~150 °C, confirming loss of structural water. At low temperatures, broad OH and ClO₄⁻ bands indicated metastable brine phases under Mars-relevant humidity conditions. High-resolution Raman spectra showed Fermi resonance and overtone enhancement effects, particularly within the v₁-v₂ region, as a function of temperature. XRD provided insights into static time-averaged crystal structures and hydration-driven phase transitions, while Raman spectroscopy probed local bonding dynamics and hydrogen-related vibrations. The combined techniques offer a more complete picture of perchlorate behavior across conditions relevant to Earth and Mars.

We find that Mg(ClO₄)₂ exhibits vibrational and structural characteristics similar to tetrahedral sulfates (SO₄²⁻) but undergoes greater distortion across hydration states. This similarity implies that perchlorates may coexist with sulfates through cation exchange, solid solution formation, or coprecipitation in evaporite systems. However, due to the charge difference between ClO₄⁻ and SO₄²⁻, complete substitution requires charge-balancing mechanisms such as incorporation of Na⁺/K⁺, interlayer water, or facilitation through redox buffering with intermediate oxyanions (ClO₂⁻, ClO₃⁻). We propose that structural and vibrational similarities between ClO₄⁻ and SO₄²⁻ raise the potential for co-crystallization or metastable associations in brine-derived assemblages.

Our results suggest that Mg(ClO₄)₂ can retain water and deliquesce at Martian conditions, contributing to transient brine formation and possibly influencing recurring slope lineae. Structural changes across hydration states offer a model for interpreting spectroscopic data from Mars rovers and orbiters. This study provides key constraints on perchlorate mineralogy, lattice dynamics, and geochemical interactions in Earth analog and Martian settings.