Janice Bishop & Enver Murad

1. Spectral propertiesof akaganéite and schwertmannite and geochemical implications of their presence on Mars Janice Bishop & EnverMurad

2. Introduction – possible presence on mars • Geochemical analyses suggested presence of schwertmannite on Mars (Burns, 1994). • VNIR spectroscopy of schwertmannite (Bishop & Murad, 1996) consistent with CRISM spectra of hydrated materials. • CheMin identified nanophase material at Yellowknife Bay (Blake et al., 2013; Bish et al., 2013; Ming et al., 2014); akaganéite at Yellowknife Bay (Ming et al., 2014). • CRISM analyses found akaganéite in a few craters (Carter et al., 2014). • Work presented here from recent paper by Bishop, Murad & Dyar, submitted to American Mineralogist in April, 2014. • Akaganéite and schwertmannite: • Ferric oxyhydroxide minerals associated with acidic environments and iron alteration. • Tunnel structure with anions in tunnels. • Interactions of anions and OH in tunnels responsible for spectral features. Bishop & murad Goldschmidt ~ The mineralogy of Mars

3. akaganéite • Structural model of akaganéite • Created by D. Dyar. • Based on refinement from Post et al. (2003). • Fe oxide/hydroxide (FeOx) tunnels with H-bonded H2O molecules and Cl- ions filling 2/3 of tunnel sites (OH- in 1/3). • O and OH anions are shown in red, Fe cations in orange, H cations in blue, and Cl- ions in green. b-Fe3+O(OH)1-xClx•nH2O Bishop & murad Goldschmidt ~ The mineralogy of Mars

4. schwertmannite • Structural model of schwertmannite • Created by D. Dyar. • Based on refinement from Fernandez-Martinez et al. (2010). • FeOxtunnels with H-bonded H2O molecules and SO42- ions. • O and OH anions are shown in red, Fe cations in orange, H cations in blue, and SO42- ions in yellow. • H positions not yet refined, but H2O molecules located in the tunnels and adsorbed on the mineral surfaces. Fe3+8O8 (OH)8-2x(SO4)x•nH2O Bishop & murad Goldschmidt ~ The mineralogy of Mars

5. VNIR spectra • Fe electronic vibrations typical of FeOx. • NIR bands • very broad H2O bands consistent with hydrated material. • OH combination bands for akaganéite in unique position at ~2.46 µm. Bishop & murad Goldschmidt ~ The mineralogy of Mars

6. mid-IR spectra • Mid-IR spectra investigated • in order to understand NIR OH and H2O bands. • Both OH and H2O bands unusual due to constrained environment in tunnels. • OH vibrations for akaganéite • 800-850 cm-1 in-plane bending. • 623-653 cm-1 out-of-plane bending. • OH bending vibrations for schwertmannite • ~600-700 cm-1. SO42- Bishop & murad Goldschmidt ~ The mineralogy of Mars

7. mid-IR spectra Mid-IR spectra investigated in order to understand NIR OH and H2O bands. Both OH and H2O bands unusual due to constrained environment in tunnels. H2O bending vibrations for akaganéite • ~1430-1630 cm-1. H2O bending vibrations for schwertmannite • ~1430-1630 cm-1. • Additional mid-IR spectral analyses by Glotch and Kraft (2008), Song and Boily (2012, 2013). Bishop & murad Goldschmidt ~ The mineralogy of Mars

8. NIR spectra H2O and OH bands sensitive to H-bonding environment. Compared H2O stretching overtones with H2O stretching vibrations: (H2O 2n + 86 cm-1)/2 = H2O n Compared H2O stretching and bending vibrations with H2O combination bands: H2O n+ H2O d = H2O n+d • 3473 + 1523 = 4996 cm-1. Bishop & murad Goldschmidt ~ The mineralogy of Mars

9. NIR spectra ~ akaganéite H2O and OH bands sensitive to H-bonding environment. H2O 2n and H2O n+dbands depend on hydration level. OH n+dbands depend on coordination of OH with Cl- or H2O. Bishop & murad Goldschmidt ~ The mineralogy of Mars

10. NIR spectra H2O and OH bands sensitive to H-bonding environment. Compared H2O stretching overtones with H2O stretching vibrations: (H2O 2n + 86 cm-1)/2 = H2O n • NIR OH bands very weak near 4300-4500 cm-1 and difficult to characterize. Bishop & murad Goldschmidt ~ The mineralogy of Mars

11. Formation of akaganéite • Akaganéite is typically formed by hydrolysis of ferric chloride solution at low pH (e.g. Schwertmann and Cornell, 2000). • Akaganéite is an uncommon soil mineral on Earth. • it forms in Cl--rich environments including brines, marine rusts, and corrosion products (Johnston et al. 1978; Holm et al. 1983; Bibi et al. 2011). • Akaganéiteis the sole product of Fe2+ and ferrous chloride in anoxic environments (Rémazeilles and Refait 2007). Bishop & murad Goldschmidt ~ The mineralogy of Mars

12. Formation of schwertmannite • Schwertmanniteis generally formed at pH ~2.8-4.5 with sulfate concentrations on the order of 1000-3000 mg/L (Bigham et al. 1992; Bigham et al. 1996). • For higher sulfate concentrations, jarosite forms. • For higher pH levels goethite and ferrihydrite form. • Formation of schwertmannite is facilitated by Acidithiobacillusferrooxidans, which induces oxidation of Fe2+ to Fe3+ in solution and thrives in acidic environments (Kelly and Wood 2000). • Schwertmanniteis most commonly found as an alteration product from iron sulfides at mine drainage sites (Bigham et al. 1994; Bigham et al. 1996; Murad and Rojík 2003). • Schwertmanniteis also found in natural streams, e.g. draining from a pyritic schist in the Austrian alps (Schwertmann et al. 1995). Bishop & murad Goldschmidt ~ The mineralogy of Mars

13. Applications to mars - akaganéite • Akaganéite was identified on Mars by Ming et al. (2014) using XRD data of samples collected at the John Klein and Cumberland Hill drill holes at Yellowknife Bay in Gale Crater. • Clhas been found in Martian soil at all landing sites (e.g. Clark and Van Hart 1981; Gellert et al. 2006; Ming et al. 2014); some Cl could be present as akaganéite. • Akaganéitehas been identified using CRISM spectra of small outcrops at Robert Sharp, Gale and Antoniadi craters (Carter et al., 2014). • The presence of akaganéite on Mars likely indicates a hydrothermal environment with temperatures near 60 °C, low pH, excess Cl- and limited SO42- (Schwertmann and Cornell 2000). • Akaganéite converts to nanophase hematite at 300 °C (Glotch and Kraft 2008). • thus presence of akaganeiteindicates no elevated temperatures at these sites. • and akaganeite could be a source of the ubiquitous nanophase hematite found on Mars (e.g. Morris et al. 2006). Bishop & murad Goldschmidt ~ The mineralogy of Mars

14. Applications to mars - schwertmannite • Schwertmannite was proposed by Burns (1994) as a possible Fe sulfate-bearing mineral on Mars based on its’ formation and occurrences on Earth. • Schwertmanniteand goethite precipitated together with jarosite in Australian hypersaline sediments (Long et al., 1992; Burns, 1994; Henderson and Sullivan, 2010). • Analyses of CRISM spectra show numerous regions of hydrated material that could be consistent with schwertmannite or many other hydrous sulfates and other minerals (Murchie et al., 2009). • The presence of schwertmannite on Mars would indicate a low pHaqueous environment with moderate dissolved SO42-(Bigham et al., 1996). • Schwertmannite converts rapidly to goethite in solution (Cornell and Schwertmann2003) and to hematite at elevated temperatures (Henderson and Sullivan, 2010). • thus, the presence of schwertmannite on Mars would indicate that liquid water was not present at that location after formation of the schwertmannite. • and that surface temperatures did not raise above ~600 °C. Bishop & murad Goldschmidt ~ The mineralogy of Mars

15. summary • Spectral parameters used for detection of akaganéiteon Mars: • 2.46 µm OH combination band; may have shoulders at 2.23-2.42 µm. • 1.44-1.48 and 1.98-2.07 µm broad bands due to OH and H2O in constrained environments and H-bonding. • ~1430-1620 cm-1 (~6-7 µm) H2O bending vibrational band. • 800-850 cm-1 (~12 µm) in-plane bending band. • 623-653 cm-1(~15-16 µm) out-of-plane bending band. • Spectral parameters used for detection of schwertmanniteon Mars: • OH combination band too diffuse to characterize. • 1.44-1.48 and 1.95-2.00 µm broad bands due to OH and H2O in constrained environments and H-bonding. • ~1430-1620 cm-1 (~6-7 µm) H2O bending vibrational band. • 600-700 cm-1 (~14-16 µm) bending vibrational band. • Akaganéiteor schwertmanniteon Martian surface today implies little surface modification through aqueous or thermal alteration. • would be converted to nanophasegoethite or hematite. Bishop & murad Goldschmidt ~ The mineralogy of Mars