Post on 12-Jan-2016
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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)
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Fe-Bearing Systems: Product Summary
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Oxic Products: Vis/NIR
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
• 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
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Mg-smectite precursor
Amorphous SiO2
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
pH Control on Epitaxial Smectite Formation
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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?
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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
Workshop on Martian Phyllosilicates Wednesday, October 22, 2008
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