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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

by Xi Liu 1,2,*, Zhaoyang Sui 1,2, Hongzhan Fei 3, Wei Yan 1,2, Yunlu Ma 1,2 and Yu Ye 4
1
School of Earth and Space Sciences, Peking University, Beijing 100871, China
2
Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, Beijing 100871, China
3
Bayerisches Geoinstitut, University of Bayreuth, D95440 Bayreuth, Germany
4
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(6), 499; https://doi.org/10.3390/min10060499
Received: 9 May 2020 / Revised: 26 May 2020 / Accepted: 28 May 2020 / Published: 30 May 2020
Three batches of Mg2SiO4-ringwoodites (Mg-Rw) with different water contents (CH2O = ~1019(238), 5500(229) and 16,307(1219) ppm) were synthesized by using conventional high-P 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 P-T 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 [VSi(OH)4], [VMg(OH)2MgSiSiMg] and [VMg(OH)2]. Combining these IR features with the chemical characteristics of hydrous Rw, we have revealed that the hydrogen defects [VMg(OH)2MgSiSiMg] are dominant in hydrous Rw at high P-T conditions, and the defects [VSi(OH)4] and [VMg(OH)2] play negligible roles. Extensive IR measurements were performed on seven thin sections annealed for several times at T of 200–600 °C and quickly quenched to room T. 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 T. From the IR spectra obtained for hydrous Rw both unannealed and annealed at high T, we further infer that substantial amounts of cation disorder should be present in hydrous Rw at the P-T conditions of the mantle transition zone, as required by the formation of the hydrogen defects [VMg(OH)2MgSiSiMg]. 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. View Full-Text
Keywords: annealing experiments; water-coupled cation disorder; hydrogen defects; hydrogen migration; hydrous ringwoodite; infrared spectrum annealing experiments; water-coupled cation disorder; hydrogen defects; hydrogen migration; hydrous ringwoodite; infrared spectrum
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Liu, X.; Sui, Z.; Fei, H.; Yan, W.; Ma, Y.; Ye, Y. 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. Minerals 2020, 10, 499.

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