Workshop on Martian PhyllosilicatesWednesday, 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 PhyllosilicatesWednesday, October 22, 2008 An Experimental Approach to Clay Formation Synthetic “basaltic weathering” fluids –Chemistry from Tosca et al., [2004] –Addition of SO 4 /Cl salts for desired solution composition With constant room temperature... 1.Fluid synthesis by salt addition (no Fe, Si) 2.pH buffering (TRIS, KH 2 PO 4, etc.) 3.SiO 2 (aq) addition with tetraethoxysilane 4.Fe 2+ -sulfate salt addition & equilibration 5.Age solution at 60 o C (up to 6 weeks) “Anoxic” experiments –Deoxygenation with N 2 (g) saturation –Hydrazine (N 2 H 4 ) addition as O 2 (aq) scavenger

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 An Experimental Approach to Clay Formation Anoxic & oxic conditions pH 4-9 (buffered & unbuffered) Fe-free systems Effect of salinity (as Mg 2+ ) Solid characterization –X-ray diffraction –Total X-ray scattering / PDF analysis –XRF –SEM/EDS

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Fe-Bearing Systems: Product Summary

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Oxic Products: Vis/NIR

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Fe 3+ -SiO 2 phases: Unique Precipitates Fe 3+ -SiO 2 precipitates occur from pH 5-8 “Amorphous” to powder XRD, but exhibits local structure –Ongoing work to model PDF Best described as SiO 2 -bearing (up to ~20 wt. %) Fe 3+ -(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 PhyllosilicatesWednesday, October 22, 2008

Workshop on Martian PhyllosilicatesWednesday, 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 Mg 2+ expands pH stability & increases crystallization rate

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 Mg-smectite precursor Amorphous SiO 2

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008

Workshop on Martian PhyllosilicatesWednesday, October 22, 2008 pH Control on Epitaxial Smectite Formation

Workshop on Martian PhyllosilicatesWednesday, 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 PhyllosilicatesWednesday, October 22, 2008 Conclusions Fe(2+ or 3+)-smectites indicate a low-O 2 environment –Nontronite formation through oxidation of Fe 2+ -phase, not from Fe 3+ (aq) –May be able to target more “habitable”, reducing paleo-environments Increased [Mg] expands pH stability range & increases xtall. rates Amorphous Fe 3+ -SiO 2 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 PhyllosilicatesWednesday, 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 CO 2