NDLI: Cordierite III: the site occupation and concentration of Fe3+

Content Provider	Springer Nature Link
Author	Geiger, Charles A. Rager, Helmut Czank, Michael
Copyright Year	2000
Abstract	Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18 .x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000 °C for 2 days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000 °C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000 cm−1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.
Starting Page	344
Ending Page	352
Page Count	9
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	140
Issue Number	3
e-ISSN	14320967
Language	English
Publisher	Springer Berlin Heidelberg
Publisher Date	2000-12-01
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Geology Mineral Resources Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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Cordierite III: the site occupation and concentration of Fe3+

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