Acidic Weathering of Phyllosilicates Workshop on Martian Phyllosilicates CNES Paris, France Oct. 21 – 23, 2008 Travis Altheide *, Vincent Chevrier, and Patricia Gavin psp_001784, HiRISEMawrth Vallis, OMEGA

Mineralogical Context Layered deposits in outflow channels (Wray, et al. 2008; Bishop, et al. 2008) Fe,Mg-smectite: orange/red Fe 2+ 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 MineralFormula pH before pH after KaoliniteAl 2 Si 2 O 5 (OH) Montmorillonite (Na,Ca) 0.33 (Al,Mg) 2 Si 4 O 10 (OH) 2 ·n H 2 O n.a. NontroniteNa 0.3 Fe 2 (Si,Al) 4 O 10 (OH) 2 ·nH 2 O Chlorite (Mg,Fe) 3 (Si,Al) 4 O 10 (OH)

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 Control Sample Rho Rho = rhomboclase

ESEM/EDAX: nontronite, pH 0 20 μm O = Si = Al = 2.52 S = 9.40 Ti = 1.90 O = Si = Al = 2.05 S = 8.08 Ti = 1.98

XRD: montmorillonite, pH 0 Control Sample Mus/ Kao Al-Si Mus/ Kao Kao = kaolinite Mus = muscovite Al-Si = hydrated aluminum silicate

ESEM/EDAX: montmorillonite, pH 0 O = Si = Al = 4.10 S = 7.71 Mg = 1.38 O = Si = Al = 4.38 S = 6.80 Mg = 0.00

XRD: kaolinite, pH 0 Control Sample Kao Si0 2 Kao = kaolinite

ESEM/EDAX: kaolinite, pH 0 O = Si = Al = S = 3.56 O = Si = Al = S = 4.87

Summary of Results: Solid Phase NontroniteMontmorilloniteKaoliniteChlorite pH 4Leaching of Fen.a.No leaching Minimal leaching of Fe,Mg pH 2Leaching of Fe Leaching of Mg: Al,Mg-silicate Minimal leaching Leaching of Fe,Mg pH 0 Leaching 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 Si0 2

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 Fe 2+ 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, HiRISEMawrth Vallis, OMEGA