Skip to Main Content

Dehydration-hydration mechanisms in synthetic Fe-poor diopside

Rickard Sundvall, Henrik Skogby and Roland Stalder
Dehydration-hydration mechanisms in synthetic Fe-poor diopside (in Sixth European conference on Mineralogy and spectroscopy, Eugen Libowitzky (editor) and Manfred Wildner (editor))
European Journal of Mineralogy (January 2009) 21 (1): 17-26


Small amounts (ppm) of OH in nominally anhydrous minerals (NAMs) can have a dramatic effect on the physical properties of the upper mantle. The pyroxenes of the upper mantle have been shown to incorporate substantial numbers of protons forming hydroxyl ions. Enstatite and diopside are the most important endmembers of the pyroxenes in terms of bulk volume in the upper mantle. To further constrain the behavior of hydroxyl ions in clinopyroxene, the dehydration-hydration mechanisms of synthetic (super 57) Fe-doped diopside were investigated. Dehydration was carried out by stepwise heating in air of crystals synthesized at high pressure under water-saturated conditions. FTIR spectra were obtained after each step. Mossbauer spectra were recorded for three of the crystals when there had been a significant decrease in FTIR absorbance intensity. From the Mossbauer spectra we see an increase in the Fe (super 3+) doublet with successive dehydration, although this increase is less than the decrease in OH in terms of atoms per formula unit. This means that the dehydration only partly follows the redox reaction OH (super -) +Fe (super 2+) = O (super 2-) +Fe (super 3+) +1/2 H (sub 2) , and that additional reactions occur. Hydration experiments were conducted on one crystal in the same manner as the dehydrations, with the exception that hydrogen gas was used during heating. Hydration experiments resulted in re-hydration of the sample to 73% of the original amount of OH. The calculated Arrhenius equation derived from the diffusion rates during dehydration along [010] yields an activation energy (E (sub a) ) of -292+ or -50 kJ mol (super -1) , and D (sub 0) = 10 (super -1.9+ or -2.3) m (super 2) s (super -1) . The result of the rehydration experiment agrees well with the established diffusion law. Diffusion rates determined for synthetic diopside are almost two orders of magnitude slower than for synthetic enstatite with comparable Fe contents. Compared to natural diopside, diffusion rates in these synthetic samples are slower, probably because of the low iron content. E (sub a) is similar to that of dehydration of pure and low-Fe enstatite.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 21
Serial Issue: 1
Title: Dehydration-hydration mechanisms in synthetic Fe-poor diopside
Title: Sixth European conference on Mineralogy and spectroscopy
Author(s): Sundvall, RickardSkogby, HenrikStalder, Roland
Author(s): Libowitzky, Eugeneditor
Author(s): Wildner, Manfrededitor
Affiliation: Naturhistoriska Riksmuseet, Sektionen foer Mineralogi, Stockholm, Sweden
Pages: 17-26
Published: 200901
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Meeting name: Sixth European conference on Mineralogy and spectroscopy
Meeting location: Stockholm, SWE, Sweden
Meeting date: 20070908Sept. 8-11, 2007
References: 12
Accession Number: 2009-032661
Categories: Mineralogy of silicates
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: Institutionen foer Geologi och Geokemi, SWE, SwedenUniversitaet Innsbruck, AUT, Austria
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200918
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal