Thermal behavior of polyhalite: a high-temperature synchrotron XRD study

Xu, Hongwu; Guo, Xiaofeng; Bai, Jianming

doi:10.1007/s00269-016-0842-5

Thermal behavior of polyhalite: a high-temperature synchrotron XRD study

Journal Article · Sat Sep 17 00:00:00 EDT 2016 · Physics and Chemistry of Minerals

DOI:https://doi.org/10.1007/s00269-016-0842-5· OSTI ID:1439795

Xu, Hongwu ^[1]; Guo, Xiaofeng ^[1]; Bai, Jianming ^[2]

Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source II

As an accessory mineral in marine evaporites, polyhalite, K₂MgCa₂(SO₄)₄·2H₂O, coexists with halite (NaCl) in salt formations, which have been considered as potential repositories for permanent storage of high-level nuclear wastes. However, because of the heat generated by radioactive decays in the wastes, polyhalite may dehydrate, and the released water will dissolve its neighboring salt, potentially affecting the repository integrity. Thus, studying the thermal behavior of polyhalite is important. In this paper, a polyhalite sample containing a small amount of halite was collected from the Salado formation at the WIPP site in Carlsbad, New Mexico. To determine its thermal behavior, in situ high-temperature synchrotron X-ray diffraction was conducted from room temperature to 1066 K with the sample powders sealed in a silica-glass capillary. At about 506 K, polyhalite started to decompose into water vapor, anhydrite (CaSO₄) and two langbeinite-type phases, K₂Ca _x Mg_2-x (SO₄)₃, with different Ca/Mg ratios. XRD peaks of the minor halite disappeared, presumably due to its dissolution by water vapor. With further increasing temperature, the two langbeinite solid solution phases displayed complex variations in crystallinity, composition and their molar ratio and then were combined into the single-phase triple salt, K₂CaMg(SO₄)₃, at ~919 K. Rietveld analyses of the XRD data allowed determination of structural parameters of polyhalite and its decomposed anhydrite and langbeinite phases as a function of temperature. Finally, from the results, the thermal expansion coefficients of these phases have been derived, and the structural mechanisms of their thermal behavior been discussed.

View Accepted Manuscript (DOE)

Research Organization:: Brookhaven National Lab. (BNL), Upton, NY (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)

Grant/Contract Number:: SC0012704; AC52-06NA25396

OSTI ID:: 1439795

Alternate ID(s):: OSTI ID: 1458950

Report Number(s):: BNL--114219-2017-JAAM

Journal Information:: Physics and Chemistry of Minerals, Journal Name: Physics and Chemistry of Minerals Journal Issue: 2 Vol. 44; ISSN 0342-1791

Publisher:: SpringerCopyright Statement

Country of Publication:: United States

Language:: English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
anhydrite
crystal structure
langbeinite
polyhalite
salt repository
synchrotron X-ray diffraction
thermal decomposition
thermal expansion

Thermal behavior of polyhalite: a high-temperature synchrotron XRD study

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