acidic weathering of phyllosilicates workshop on martian phyllosilicates cnes paris, france oct. 21...
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Acidic Weathering of Phyllosilicates
Workshop on Martian Phyllosilicates
CNES Paris, France
Oct. 21 – 23, 2008
Travis Altheide*, Vincent Chevrier, and Patricia Gavin
psp_001784, HiRISE Mawrth Vallis, OMEGA
Mineralogical Context • Layered deposits in outflow channels (Wray, et al.
2008; Bishop, et al. 2008)• Fe,Mg-smectite: orange/red• Fe2+ bearing phases: yellow/green• Al-phyllosilicate and hydrated silica: blue
• Small deposits with phyllosilicates and sulfates mixed: Aram Chaos, Terra Meridiani, and Becquerel crater (Poulet et al., 2005)
• Silica deposits idendified in Gusev Crater (Squyres, et al., 2008)
Bibring, et al. 2005
Complex Aqueous History: phyllosilicates vs. sulfates
• Different geochemical environments between phyllosilicates and sulfate deposits (Bibring et al., 2006; Chevrier et al., 2007)
• But small deposits of phyllosilicates mixed with sulfates
• Gusev Crater: no phyllosilicates, mostly sulfates and low pH minerals
• Questions: what, if any, is the effect of acidic weathering on phyllosilicates? Is there a mineralogical relationship between the two?
Materials and Protocol
Mineral Formula pH before
pH after
Kaolinite Al2Si2O5(OH)4
024
0.22.44.0
Montmorillonite(Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·n
H2O
024
0.94.5n.a.
Nontronite Na0.3Fe2(Si,Al)4O10(OH)2·nH2O024
1.23.06.6
Chlorite (Mg,Fe)3(Si,Al)4O10(OH)2 0
2
4
0.8
2.9
5.2
Analytical Methods
Infrared Spectroscopy– 1.0 – 2.4 μm
• X-ray Diffraction– Method described in abstract
• ESEM – EDAX– Relative elemental analysis
• Atomic Absorption– Quantify extent of Fe and Mg leaching
FT-IR: nontronite
FT-IR: montmorillonite
FT-IR: kaolinite
FT-IR: chlorite
XRD: nontronite, pH 0
Silica
ControlSample
RhoRho
Rho = rhomboclase
ESEM/EDAX: nontronite, pH 0
20 μm
O = 45.86Si = 40.31Al = 2.52S = 9.40Ti = 1.90
O = 45.91Si = 41.99Al = 2.05S = 8.08Ti = 1.98
XRD: montmorillonite, pH 0ControlSample
Mus/Kao
Al-Si Mus/Kao
Kao = kaoliniteMus = muscovite
Al-Si = hydrated aluminum silicate
ESEM/EDAX: montmorillonite, pH 0
O = 45.59Si = 41.22Al = 4.10S = 7.71Mg = 1.38
O = 48.63Si = 40.19Al = 4.38S = 6.80Mg = 0.00
XRD: kaolinite, pH 0ControlSampleKao
KaoKao
Si02
Kao = kaolinite
ESEM/EDAX: kaolinite, pH 0
O = 47.03Si = 34.36Al = 15.05S = 3.56
O = 44.86Si = 34.92Al = 15.35S = 4.87
Summary of Results: Solid Phase
Nontronite Montmorillonite Kaolinite Chlorite
pH 4 Leaching of Fe n.a. No leachingMinimal leaching
of Fe,Mg
pH 2 Leaching of FeLeaching of
Mg:Al,Mg-silicate
Minimal leachingLeaching of
Fe,Mg
pH 0Leaching of Fe,Al: Silica,
Fe-sulfate
Leaching of Mg:
Al,hydrated silicate
Minimal leaching:
Al,hydrated silicate
Leaching of Fe,Mg
Stability of Mg-rich Phyllosilicates: pH and Si02
Implications for Mars
• Transition from nontronite to kaolinite indicates a “global” decrease in pH
• Precipitation of phyllosilicates in increasing acidic conditions
• Progressive leaching of Fe, Mg, Ca, Na ions from phyllosilicate due to acidic waters (pH < 6)– More compatible with formation of amorphous
silica (hydrothermalism can also explain)– Association of sulfates plus hydrated silica
resulting from acidic weathering of phyllosilicates?
Continuing Work
• Investigation of Fe2+ phases– Chlorite– Serpentine
• Liquid phase characterization– Atomic absorption: Quantify Fe and Mg
leaching
• Thermodynamic and kinetic modeling (Chevrier, 2008: this session)
Acidic Weathering of Phyllosilicates
Workshop on Martian Phyllosilcates
CNES Paris, France
Oct. 21 – 23, 2008
Travis Altheide*, Vincent Chevrier, and Patricia Gavin
Psp_001784, HiRISE Mawrth Vallis, OMEGA