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