Details
Original language | English |
---|---|
Pages (from-to) | 55-67 |
Number of pages | 13 |
Journal | Schweizerische Mineralogische und Petrographische Mitteilungen |
Volume | 81 |
Issue number | 1 |
Publication status | Published - Jan 2001 |
Abstract
The OH ⇒ F substitution in trioctahedral ferrous micas has been investigated at 720 °C, 1 kbar P H2O, under fO 2 conditions set by the MW (Fe 2O 3-Fe 1-xO) buffer. The starting compositions belong to the annite-siderophyllite join: K(Fe 3-x)Al x)(Si 3-xAl 1+x)O 10(OH) 2 with x = 0 (annite), 0.5 (Fe-eastonite), and 0.75 (Es). In F-bearing system, the compositions investigated belong to (OH,F)-annite, (OH,F)-Fe-eastonite and (OH,F)-Es joins. A single mica phase was observed for (OH,F)-annite in 0 ≤ X F ≤ 0.5 compositional range, and in 0 ≤ X F ≤ 0.2 range for (OH,F)-Fe-eastonite and (OH,F)-Es. Beyond these values, toward to F-rich compositions, topaz, quartz, magnetite and glass coexist with mica. The correlation of the reticular distance d 060 as a function of X F shows that (OH,F)-annite and (OH,F)-Es behave differently as X F increases. Mössbauer measurements along the annite and Es (OH,F) joins demonstrate that the Fe 3+ content decreases from X Fe 3+ (Fe 3+/Fe ^t otal = 9.1% for X F-^a nn = 0 to 3.1% for X F-^a nn = 0.5 and it increases slightly from X Fe 3+ = 2.4% at X F-Es = 0 to 3% at X F-Es = 0.2. FTIR spectroscopy in the far-infrared region (200-50 cm -1) also shows an opposite trend wavenumber-shift between (OH,F)-annite and (OH,F)-Es with increasing X F. In the single phase range, the band observed at 67 cm -1 in the (OH)-annite, corresponding to torsional vibration motion of the tetrahedral layer (mode III), shifts to higher wavenumbers with X F. In contrast, this band (mode III) decreases from 91 cm -1 in the (OH)-Es end-member down to 88 cm -1 at X F = 0.2. These shifts are related to changes in the K-site configuration, particularly to the variations of the tetrahedral rotation angle (α). The angle a increases from ≈ 2° for X F-ann = 0 to 5.5° for X F+ann = 0.5 and it decreases from 8.5° at X F-Es = 0 to 5.3° at X F+Es = 0.2. The OH ⇒ substitution induces local cationic changes and consequently a dimensional adaptation of sheets (limited in such micas to α = 5.5°), which in turn controls the fluorine solubility in these studied micas. The results also show that the Fe 3+/Fe total ratio in F-bearing micas is not only controlled by fO 2 but also by structural constraints. The fluorine content of natural biotites has to be taken into account to estimate oxygen fugacities prevailing in the rocks.
Keywords
- FIR, Fluorine, Mössbauer, Tetrahedral rotation (α), Trioctahedral ferrous micas
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geology
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Schweizerische Mineralogische und Petrographische Mitteilungen, Vol. 81, No. 1, 01.2001, p. 55-67.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Structural effects of OH ⇒ F substitution in trioctahedral micas of the system
AU - Boukili, B.
AU - Robert, J. L.
AU - Beny, J. M.
AU - Holtz, Francois
N1 - Copyright: Copyright 2012 Elsevier B.V., All rights reserved.
PY - 2001/1
Y1 - 2001/1
N2 - The OH ⇒ F substitution in trioctahedral ferrous micas has been investigated at 720 °C, 1 kbar P H2O, under fO 2 conditions set by the MW (Fe 2O 3-Fe 1-xO) buffer. The starting compositions belong to the annite-siderophyllite join: K(Fe 3-x)Al x)(Si 3-xAl 1+x)O 10(OH) 2 with x = 0 (annite), 0.5 (Fe-eastonite), and 0.75 (Es). In F-bearing system, the compositions investigated belong to (OH,F)-annite, (OH,F)-Fe-eastonite and (OH,F)-Es joins. A single mica phase was observed for (OH,F)-annite in 0 ≤ X F ≤ 0.5 compositional range, and in 0 ≤ X F ≤ 0.2 range for (OH,F)-Fe-eastonite and (OH,F)-Es. Beyond these values, toward to F-rich compositions, topaz, quartz, magnetite and glass coexist with mica. The correlation of the reticular distance d 060 as a function of X F shows that (OH,F)-annite and (OH,F)-Es behave differently as X F increases. Mössbauer measurements along the annite and Es (OH,F) joins demonstrate that the Fe 3+ content decreases from X Fe 3+ (Fe 3+/Fe ^t otal = 9.1% for X F-^a nn = 0 to 3.1% for X F-^a nn = 0.5 and it increases slightly from X Fe 3+ = 2.4% at X F-Es = 0 to 3% at X F-Es = 0.2. FTIR spectroscopy in the far-infrared region (200-50 cm -1) also shows an opposite trend wavenumber-shift between (OH,F)-annite and (OH,F)-Es with increasing X F. In the single phase range, the band observed at 67 cm -1 in the (OH)-annite, corresponding to torsional vibration motion of the tetrahedral layer (mode III), shifts to higher wavenumbers with X F. In contrast, this band (mode III) decreases from 91 cm -1 in the (OH)-Es end-member down to 88 cm -1 at X F = 0.2. These shifts are related to changes in the K-site configuration, particularly to the variations of the tetrahedral rotation angle (α). The angle a increases from ≈ 2° for X F-ann = 0 to 5.5° for X F+ann = 0.5 and it decreases from 8.5° at X F-Es = 0 to 5.3° at X F+Es = 0.2. The OH ⇒ substitution induces local cationic changes and consequently a dimensional adaptation of sheets (limited in such micas to α = 5.5°), which in turn controls the fluorine solubility in these studied micas. The results also show that the Fe 3+/Fe total ratio in F-bearing micas is not only controlled by fO 2 but also by structural constraints. The fluorine content of natural biotites has to be taken into account to estimate oxygen fugacities prevailing in the rocks.
AB - The OH ⇒ F substitution in trioctahedral ferrous micas has been investigated at 720 °C, 1 kbar P H2O, under fO 2 conditions set by the MW (Fe 2O 3-Fe 1-xO) buffer. The starting compositions belong to the annite-siderophyllite join: K(Fe 3-x)Al x)(Si 3-xAl 1+x)O 10(OH) 2 with x = 0 (annite), 0.5 (Fe-eastonite), and 0.75 (Es). In F-bearing system, the compositions investigated belong to (OH,F)-annite, (OH,F)-Fe-eastonite and (OH,F)-Es joins. A single mica phase was observed for (OH,F)-annite in 0 ≤ X F ≤ 0.5 compositional range, and in 0 ≤ X F ≤ 0.2 range for (OH,F)-Fe-eastonite and (OH,F)-Es. Beyond these values, toward to F-rich compositions, topaz, quartz, magnetite and glass coexist with mica. The correlation of the reticular distance d 060 as a function of X F shows that (OH,F)-annite and (OH,F)-Es behave differently as X F increases. Mössbauer measurements along the annite and Es (OH,F) joins demonstrate that the Fe 3+ content decreases from X Fe 3+ (Fe 3+/Fe ^t otal = 9.1% for X F-^a nn = 0 to 3.1% for X F-^a nn = 0.5 and it increases slightly from X Fe 3+ = 2.4% at X F-Es = 0 to 3% at X F-Es = 0.2. FTIR spectroscopy in the far-infrared region (200-50 cm -1) also shows an opposite trend wavenumber-shift between (OH,F)-annite and (OH,F)-Es with increasing X F. In the single phase range, the band observed at 67 cm -1 in the (OH)-annite, corresponding to torsional vibration motion of the tetrahedral layer (mode III), shifts to higher wavenumbers with X F. In contrast, this band (mode III) decreases from 91 cm -1 in the (OH)-Es end-member down to 88 cm -1 at X F = 0.2. These shifts are related to changes in the K-site configuration, particularly to the variations of the tetrahedral rotation angle (α). The angle a increases from ≈ 2° for X F-ann = 0 to 5.5° for X F+ann = 0.5 and it decreases from 8.5° at X F-Es = 0 to 5.3° at X F+Es = 0.2. The OH ⇒ substitution induces local cationic changes and consequently a dimensional adaptation of sheets (limited in such micas to α = 5.5°), which in turn controls the fluorine solubility in these studied micas. The results also show that the Fe 3+/Fe total ratio in F-bearing micas is not only controlled by fO 2 but also by structural constraints. The fluorine content of natural biotites has to be taken into account to estimate oxygen fugacities prevailing in the rocks.
KW - FIR
KW - Fluorine
KW - Mössbauer
KW - Tetrahedral rotation (α)
KW - Trioctahedral ferrous micas
UR - http://www.scopus.com/inward/record.url?scp=0035021551&partnerID=8YFLogxK
M3 - Article
AN - SCOPUS:0035021551
VL - 81
SP - 55
EP - 67
JO - Schweizerische Mineralogische und Petrographische Mitteilungen
JF - Schweizerische Mineralogische und Petrographische Mitteilungen
SN - 0036-7699
IS - 1
ER -