IR Features of Hydrous Mg₂SiO₄-Ringwoodite, Unannealed and Annealed at 200–600 ^°C and 1 atm, with Implications to Hydrogen Defects and Water-Coupled Cation Disorder

• Main Authors: Xi Liu, Zhaoyang Sui, Hongzhan Fei, Wei Yan, Yunlu Ma, Yu Ye
• Format: Article
• Language: English
• Published: MDPI AG 2020-05-01
• Series: Minerals
• Subjects: annealing experiments; water-coupled cation disorder; hydrogen defects; hydrogen migration; hydrous ringwoodite; infrared spectrum
• Online Access: https://www.mdpi.com/2075-163X/10/6/499

Three batches of Mg₂SiO₄-ringwoodites (Mg-Rw) with different water contents (<i>C</i>_-H2O = ~1019(238), 5500(229) and 16,307(1219) ppm) were synthesized by using conventional high-<i>P</i> experimental techniques. Thirteen thin sections with different thicknesses (~14–113 μm) were prepared from them and examined for water-related IR peaks using unpolarized infrared spectra at ambient <i>P</i>-<i>T</i> conditions, leading to the observation of 15 IR peaks at ~3682, 3407, 3348, 3278, 3100, 2849, 2660, 2556, 2448, 1352, 1347, 1307, 1282, 1194 and 1186 cm⁻¹. These IR peaks suggest multiple types of hydrogen defects in hydrous Mg-Rw. We have attributed the IR peaks at ~3680, 3650–3000 and 3000–2000 cm⁻¹, respectively, to the hydrogen defects [<i>V</i>_Si(OH)₄], [<i>V</i>_Mg(OH)₂Mg_SiSi_Mg] and [<i>V</i>_Mg(OH)₂]. Combining these IR features with the chemical characteristics of hydrous Rw, we have revealed that the hydrogen defects [<i>V</i>_Mg(OH)₂Mg_SiSi_Mg] are dominant in hydrous Rw at high <i>P</i>-<i>T</i> conditions, and the defects [<i>V</i>_Si(OH)₄] and [<i>V</i>_Mg(OH)₂] play negligible roles. Extensive IR measurements were performed on seven thin sections annealed for several times at <i>T</i> of 200–600 °C and quickly quenched to room <i>T</i>. They display many significant variations, including an absorption enhancement of the peak at ~3680 cm⁻¹, two new peaks occurring at ~3510 and 3461 cm⁻¹, remarkable intensifications of the peaks at ~3405 and 3345 cm⁻¹ and significant absorption reductions of the peaks at ~2500 cm⁻¹. These phenomena imply significant hydrogen migration among different crystallographic sites and rearrangement of the O-H dipoles in hydrous Mg-Rw at high <i>T</i>. From the IR spectra obtained for hydrous Rw both unannealed and annealed at high <i>T</i>, we further infer that substantial amounts of cation disorder should be present in hydrous Rw at the <i>P</i>-<i>T</i> conditions of the mantle transition zone, as required by the formation of the hydrogen defects [<i>V</i>_Mg(OH)₂Mg_SiSi_Mg]. The Mg-Si disorder may have very large effects on the physical and chemical properties of Rw, as exampled by its disproportional effects on the unit-cell volume and thermal expansivity.