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The fate of ammonium in phengite at high temperature

Yang Yan, Vincent Busigny, Wang Zhongping and Xia Qunke
The fate of ammonium in phengite at high temperature
American Mineralogist (November 2017) 102 (11): 2244-2253


Nitrogen (N) is the main component of the atmosphere and is largely considered as a volatile element. However, most researchers now agree that a significant amount of N, in the form of ammonium (NH (sub 4) (super +) ) substituting for K (super +) in some K-bearing minerals such as clays, micas, and feldspars, can be transferred to the deep Earth through subduction. The fate of ammonium in those minerals during subduction is still poorly known but is likely controlled by temperature and pressure pathways. In an attempt to contribute to understanding the fate of N during high-temperature processes, we carried out in situ high-temperature IR and Raman spectra measurements to investigate the rate and mechanism of NH (sub 4) (super +) loss in phengite. We observed that a new OH band at 3425 cm (super -1) became prominent above 400 degrees C, and did not change with times during isothermal annealing at 500 and 700 degrees C. The N-H stretching band shifted to higher wavenumbers in the temperature interval from -150 to 20 degrees C, while linearly shifted to lower wavenumbers in the temperature interval from 20 to 500 degrees C and remained stable above 500 degrees C. The N-H bending band linearly shifted to lower wavenumbers in the temperature interval from -150 to 400 degrees C and remained stable. The K-O stretching frequency decreased with increasing temperature to 600 degrees C, and then remained stable. These processes were reversible until dehydration and ammonium loss from phengite starting at 800 degrees C. The results suggest that (1) at low temperatures, ammonium is ordered and hydrogen bonding between ammonium and the framework evolves during cooling; (2) at high temperatures, the N-H interatomic distance of NH (sub 4) (super +) lengthens with increasing temperature until 500 degrees C. N-H bond subsequently no longer lengthens, accompanied by H transferring from N to neighboring O and forming a new OH band at 3425 cm (super -1) At 800 degrees C, H (super +) starts breaking from N and leaving others to form NH (sub 3) and OH (super -) This study has implications for evaluating the extent to which these minerals can preserve information regarding nitrogen behavior during high-temperature processes.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 102
Serial Issue: 11
Title: The fate of ammonium in phengite at high temperature
Affiliation: Zhejiang University, Institute of Geology and Geophysics, Hangzhou, China
Pages: 2244-2253
Published: 201711
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 75
Accession Number: 2018-021718
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Institut de Physique du Globe de Paris, FRA, FranceUniversity of Science and Technology of China, CHN, China
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201806
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