Smectites on Early Mars

Workshop on Martian Phyllosilicates Wednesday, October 22, 2008

Smectites on Early Mars

• Gain a more quantitative description of conditions bracketing smectite formation in low-T martian geochemical systems– From the bottom, up

– Begin with experimental data on synthetic basalt weathering

• Experimental approach to smectite precipitation– Neoformation

– Transformation (?)

• Place results in context of weathering processes on early Mars– Describe smectite formation in relation to other phases (e.g.,

carbonates, oxides, amorphous silicates)


An Experimental Approach to Clay Formation

• Synthetic “basaltic weathering” fluids– Chemistry from Tosca et al., [2004]

– Addition of SO4/Cl salts for desired solution composition

• With constant stirring @ room temperature...1. Fluid synthesis by salt addition (no Fe, Si)

2. pH buffering (TRIS, KH2PO4, etc.)

3. SiO2(aq) addition with tetraethoxysilane

4. Fe2+-sulfate salt addition & equilibration

5. Age solution at 60oC (up to 6 weeks)

• “Anoxic” experiments– Deoxygenation with N2(g) saturation

– Hydrazine (N2H4) addition as O2(aq) scavenger


An Experimental Approach to Clay Formation

• Anoxic & oxic conditions

• pH 4-9 (buffered & unbuffered)

• Fe-free systems

• Effect of salinity (as Mg2+)

• Solid characterization– X-ray diffraction– Total X-ray scattering / PDF

analysis– XRF– SEM/EDS


Fe-Bearing Systems: Product Summary


Oxic Products: Vis/NIR


Fe3+-SiO2 phases: Unique Precipitates

• Fe3+-SiO2 precipitates occur from pH 5-8

• “Amorphous” to powder XRD, but exhibits local structure

– Ongoing work to model PDF

• Best described as SiO2-bearing (up to ~20 wt. %) Fe3+-(hydr)oxide

• Unique spectral features in Vis/NIR (2.21, 2.27 m)

– Similar to materials seen in CRISM (Valles Marineris)

• Represents “intermediate” between acid-sulfate and high-pH


• Strong hkl and 001 reflections observed after 21 days, pH = 7-9

• Peak position & XRF analysis consistent with:

– Trioctahedral Mg-smectite (Stevensite)

– Turbostratic ordering

Increased Mg2+ expands pH stability & increases crystallization rate


Mg-smectite precursor

Amorphous SiO2


pH Control on Epitaxial Smectite Formation


Application to Weathering: Topotaxy

• Inheritance of primary mineral crystal structure

– e.g., pyroxene to smectite

• Reaction stoichiometry poorly known

– Requires transport of some components & rearrangement

• Velbel & Barker [2008]– Opx to Fe/Mg-smectite– Fe transport required by

stoichiometry

• How does this process reflect aqueous chemistry?


Conclusions

• Fe(2+ or 3+)-smectites indicate a low-O2 environment– Nontronite formation through oxidation of Fe2+-phase, not from Fe3+ (aq)– May be able to target more “habitable”, reducing paleo-environments

• Increased [Mg] expands pH stability range & increases xtall. rates

• Amorphous Fe3+-SiO2 phases reflect transition between acid-sulfate and clay-bearing systems– Need to better understand structural & spectroscopic characteristics (vis/NIR &

X-ray scattering studies)

• Extend results to mineral weathering processes– Role of Fe in structural inheritance & topotactic crystallization?– Anoxic weathering studies


Ongoing Work

• Application of PDF to crystallization mechanism

• Integrate mafic minerals into clay formation experiments

• Quantify relationship between clays & carbonates

– Most experiments supersaturated wrt carbonates

– Reproducing experiments with controlled CO2

Smectites on Early Mars

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