SPECTRAL PROPERTIES OF AKAGANÉITE AND SCHWERTMANNITE AND GEOCHEMICAL IMPLICATIONS OF THEIR PRESENCE ON MARS JANICE BISHOP & ENVER MURAD

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,  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. June 9, 2014 BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS 2

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 H 2 O 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. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS  Fe 3+ O(OH) 1-x Cl x nH 2 O

SCHWERTMANNITE Structural model of schwertmannite Created by D. Dyar. Based on refinement from Fernandez-Martinez et al. (2010).  FeOx tunnels with H-bonded H 2 O molecules and SO 4 2- ions.  O and OH anions are shown in red, Fe cations in orange, H cations in blue, and SO 4 2- ions in yellow.  H positions not yet refined, but H 2 O molecules located in the tunnels and adsorbed on the mineral surfaces. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS Fe 3+ 8 O 8 (OH) 8-2x (SO 4 ) x nH 2 O

VNIR SPECTRA Fe electronic vibrations typical of FeOx. NIR bands  very broad H 2 O bands consistent with hydrated material.  OH combination bands for akaganéite in unique position at ~2.46 µm. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS

MID-IR SPECTRA Mid-IR spectra investigated  in order to understand NIR OH and H 2 O bands.  Both OH and H 2 O bands unusual due to constrained environment in tunnels.  OH vibrations for akaganéite cm -1 in-plane bending cm -1 out-of-plane bending.  OH bending vibrations for schwertmannite ~ cm -1. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS SO 4 2-

MID-IR SPECTRA June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS Mid-IR spectra investigated  in order to understand NIR OH and H 2 O bands.  Both OH and H 2 O bands unusual due to constrained environment in tunnels.  H 2 O bending vibrations for akaganéite ~ cm -1.  H 2 O bending vibrations for schwertmannite ~ cm -1.  Additional mid-IR spectral analyses by Glotch and Kraft (2008), Song and Boily (2012, 2013).

NIR SPECTRA June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS H 2 O and OH bands sensitive to H- bonding environment.  Compared H 2 O stretching overtones with H 2 O stretching vibrations: (H 2 O cm -1 )/2 = H 2 O  Compared H 2 O stretching and bending vibrations with H 2 O combination bands: H 2 O + H 2 O  = H 2 O  = 4996 cm -1. Meas., cm -1 Calc., cm -1 H 2 O 2 H 2 O

NIR SPECTRA ~ AKAGANÉITE June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS H 2 O and OH bands sensitive to H-bonding environment.  H 2 O 2 and H 2 O  bands depend on hydration level.  OH  bands depend on coordination of OH with Cl - or H 2 O. meascalcmeascalc meas H 2 O 2 H 2 O H 2 O  H 2 O  cm -1 µmcm meas calc meas OH OH  (ip)OH  (oop)OH  cm -1 µmcm -1 OH … Cl OH isolated OH isolated OH … H2O

NIR SPECTRA June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS H 2 O and OH bands sensitive to H-bonding environment.  Compared H 2 O stretching overtones with H 2 O stretching vibrations: (H 2 O cm -1 )/2 = H 2 O  NIR OH bands very weak near cm -1 and difficult to characterize. meascalc H 2 O  cm-1 µm

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).  Akaganéite is the sole product of Fe 2+ and ferrous chloride in anoxic environments (Rémazeilles and Refait 2007). June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS

FORMATION OF SCHWERTMANNITE  Schwertmannite is generally formed at pH ~ with sulfate concentrations on the order of 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 Acidithiobacillus ferrooxidans, which induces oxidation of Fe 2+ to Fe 3+ in solution and thrives in acidic environments (Kelly and Wood 2000).  Schwertmannite is 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).  Schwertmannite is also found in natural streams, e.g. draining from a pyritic schist in the Austrian alps (Schwertmann et al. 1995). June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS

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.  Cl has 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.  Akaganéite has 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 SO 4 2- (Schwertmann and Cornell 2000).  Akaganéite converts to nanophase hematite at 300 °C (Glotch and Kraft 2008). thus presence of akaganeite indicates 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). June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS

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.  Schwertmannite and 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 pH aqueous environment with moderate dissolved SO 4 2- (Bigham et al., 1996).  Schwertmannite converts rapidly to goethite in solution (Cornell and Schwertmann 2003) 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. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS

SUMMARY  Spectral parameters used for detection of akaganéite on Mars: 2.46 µm OH combination band; may have shoulders at µm and µm broad bands due to OH and H 2 O in constrained environments and H-bonding. ~ cm -1 (~6-7 µm) H 2 O bending vibrational band cm -1 (~12 µm) in-plane bending band cm -1 (~15-16 µm) out-of-plane bending band.  Spectral parameters used for detection of schwertmannite on Mars: OH combination band too diffuse to characterize and µm broad bands due to OH and H 2 O in constrained environments and H-bonding. ~ cm -1 (~6-7 µm) H 2 O bending vibrational band cm -1 (~14-16 µm) bending vibrational band.  Akaganéite or schwertmannite on Martian surface today implies little surface modification through aqueous or thermal alteration. would be converted to nanophase goethite or hematite. June 9, BISHOP & MURADGOLDSCHMIDT ~ THE MINERALOGY OF MARS