SPECTR

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GEOCHEMICAL

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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.

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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.

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-b Fe3+O(OH)1-xClx•nH2O

SCHWERTMANNITE

Structural model of schwertmannite

•Created by D. Dyar.

•Based on refinement from Fernandez-Martinez et al. (2010).

FeOx tunnels with H-bonded H2O molecules and SO4

2- ions.

O and OH anions are shown in red, Fe cations in orange, H cations in blue, and SO4

2-

ions in yellow.

H positions not yet refined, but H2O molecules located in the tunnels and adsorbed on the mineral surfaces.

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Fe3+8O8 (OH)8-2x(SO4)x•nH2O

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.

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MID-IR SPECTRAMid-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.

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SO42-

MID-IR SPECTRA

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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).

NIR SPECTRA

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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.

Meas., cm-1 Calc., cm-1

H2O 2n H2O n

7038 35626930 35086860 34736740 3413

NIR SPECTRA ~ AKAGANÉITE

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H2O and OH bands sensitive to H-bonding environment.

H2O 2n and H2O + n d bands depend on hydration level.

OH + n d bands depend on coordination of OH with Cl- or H2O.

meas calc meas calc calc measH2O 2n H2O n H2O d H2O +n d H2O +n d H2O +n d

cm-1 cm-1 cm-1 cm-1 µm cm-1

7038 3562 1635 5197 1.92 52106930 3508 1617 5125 1.95 50406860 3473 1523 4996 2.00 49806740 3413 1430 4843 2.06 4830

meas meas meas calc calc measOH n OH d (ip) OH d (oop) OH +n d OH +n d OH +n dcm-1 cm-1 cm-1 cm-1 µm cm-1

OH…Cl- 3410 650 4060 2.46 4070OH isolated 3508 800 4308 2.32 4302OH isolated 3508 623 4131 2.42 4134OH…H2O 3642 850 4492 2.23 4492

NIR SPECTRA

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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.

meas calc calcH2O +n d H2O +n d H2O +n d

cm-1 cm-1 µm5190 5193 1.935120 5118 1.955005 5006 2.00

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 Fe2+ and ferrous chloride in anoxic environments (Rémazeilles and Refait 2007).

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FORMATION OF SCHWERTMANNITE Schwertmannite is 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 Acidithiobacillus ferrooxidans, which induces oxidation of Fe2+ to Fe3+ 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).

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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 SO4

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).

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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 SO4

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.

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SUMMARY Spectral parameters used for detection of akaganéite on 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 schwertmannite on 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.

Akaganéite or schwertmannite on Martian surface today implies little surface modification through aqueous or thermal alteration.• would be converted to nanophase goethite or hematite.

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