Properties of Hydrated Minerals That May Be Present on Mars

The hydration properties of minerals play an important role in how water cycles between the surface and atmosphere on Mars as described in Chap. 8 by Tokano. This includes the ability of minerals to adsorb (attract and physically attach) water molecules and how fast they can take up this water. Besides adsorbed water, some minerals need bound (chemically attached) H2O as part of their structure. Other minerals require OH that is usually bound in an octahedral configuration. Most minerals that contain structural OH and H2O need aqueous conditions to form, but do not necessarily need water in their environment after formation. It is the ability of certain minerals to adsorb and desorb water that is important for understanding the hydrologic cycles on Mars. Thus, hydrated minerals on Mars provide information about the presence of water at the time of formation and about water in the current environment.

XRD (X-ray diffraction) is frequently used in the lab for mineral identification [e.g. 9, 1] and is arguably one of the most accurate techniques for identifying minerals in mixtures. However, XRD presents some challenges for remote investigations and has not yet been sent to Mars on a landed mission. Thermal analysis techniques are most useful for minerals with discrete thermal transitions and a variety of techniques are available in the lab for mineralogy studies [e.g. 90]. These include differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermal evolved gas analysis (TEGA) and all measure changes in a physical parameter with temperature. As described in Section 4.7, TEGA is planned for a future mission to Mars. VNIR spectroscopy is best for characterizing metal excitational transitions and water, and is also a good technique for identification of structural OH, CO3, NO3, SO4, PO4, or CH bonds [e.g. 71]. Mid-IR spectroscopy is most useful in characterizing Si-O and metal-O bonds and also has the advantage that information about sample texture (e.g. particle size) can be determined [e.g. 91]. Mid-IR spectroscopy is also sensitive to structural OH, CO3, etc. bonds when abundant. Raman spectroscopy is most effective for detecting polarized species such as CO3, OH and CH, and exhibits sharp bands for most highly crystalline minerals when they are abundant in mixtures [e.g. 92]. Mossbauer spectroscopy provides information about the electronic environment of the iron atom. This includes the oxidation state (e.g. Fe2+, Fe3+, etc.) and the bonding configuration. For geologic samples this is usually Fe-O bonds in either tetrahedral or octahedral coordination and is influenced by other cations bonded to the O. Mossbauer spectroscopy for geologic materials has been summarized recently by Murad and Cashion [93].

The emphasis in this section will be on the VNIR and mid-IR spectral properties of hydrated minerals because these techniques can be performed globally from orbit on Mars. Reflectance spectra are shown for several mineral groups in the following sections. The VNIR spectra span the range of upcoming VNIR instruments to Mars and the mid-IR spectra are shown as inverse reflectance for comparison with emittance spectra measured for Mars. VNIR spectra are commonly measured in wavelength units in remote sensing and emittance spectra in wavenumbers. This convention has been applied here. Selected wavelength units have been added to the upper axes of the mid-IR plots.

4.4.1 Iron Hydroxides/Oxyhydroxides

Iron oxide, hydroxide and oxyhydroxide minerals are common on Earth and are an important component of the surface of Mars. The properties of this mineral group have been described in detail by Cornell and Schwertmann [2]. The VNIR spectral region is dominated by Fe excitational and charge transfer bands from 0.4 to 1 pm, plus OH vibrations near 1.41, 2.3, and 2.7 pm, and H2O vibrations near 1.45, 1.95, and 3 pm. The mid-IR spectral region contains vibrational bands due to Fe-O, OH and H2O. Reflectance spectra of selected fine-grained iron oxide-bearing minerals are shown in Fig. 4.1. Hematite (a-Fe2O3), maghemite (y-Fe2O3), and magnetite (Fe3O4) are included as well because they may be associated with aqueous processes, although they do not contain structural OH or H2O. The spectral properties of hematite are highly sensitive to grain size and orientation [94, 60]; the coarse-grained hematite identified in the Sinus Meridiani region on Mars is much darker in the VNIR region and exhibits stronger features at longer wavelengths, than the fine-grained hematite spectrum shown here.

Many iron oxide-bearing minerals exhibit characteristic extended visible region spectra that can be used for identification of these minerals [e.g. 95, 96], although this becomes more difficult for mixtures of iron oxide-bearing minerals [97]. Many of the iron oxide-bearing species on Mars are thought to be fine-grained and even nanocrystalline. The spectral properties of nanophase hematite [94], ferrihydrite [98], and schwertmannite [99] have been studied as potential iron oxide phases on Mars. Nanophase iron oxides tend to have broader and weaker spectral features, thus increasing the challenge of uniquely identifying these minerals in mixtures. Temperature also has an effect on at least the Fe excitational bands in spectra of iron oxide-bearing minerals [100].

The presence of Al or Ti is frequently observed in many iron oxide-bearing minerals and can alter the structure and properties, while the presence of Si retards the alteration of hydrated ferric oxides to hematite [2]. Si is frequently present in natural ferrihydrites and appears to be influencing the spectral features [98]. Phosphorous also influences the formation and structure of iron oxide-bearing minerals and slows the alteration of hydrated ferric oxides to hematite [101, 102]. Torrent and Barron [103] found that lepidocrocite forms preferentially to goethite in the presence of P and they suggest that this P-lepidocrocite would produce maghemite that is more stable under Martian conditions and has magnetic properties consistent with those observed by Pathfinder. Si, Al, Ti and P are all present in the Martian rocks and soils [44] and may be contributing to the properties of the iron oxide-bearing minerals on Mars.

Goethite Ferrihydrite Spectrum
Fig. 4.1 Reflectance spectra of selected fine-grained iron oxides/ hydroxides/oxyhydrox-ides: magnetite (Mg), hematite (Hm), maghemite (Mh), akaganeite (Ak), goethite (Gt), feroxyhyte (Fx), lepidocrocite (Lp), natural ferrihydrite (Fh-n) and synthetic ferrihydrite (Fh-s).

4.4.2 Carbonates

Carbonates form in a variety of anhydrous and hydrated forms. Even anhydrous carbonates provide information about aqueous processes on Mars because they can form under aqueous conditions [e.g. 5]. The spectral properties of anhydrous carbonates have been studied by Gaffey [104] and Lane and Christensen [67]. The spectral properties of hydrous carbonates have been studied by Calvin et al. [66]. Spectra of calcite (CaCO3) and siderite (FeCO3) are shown in Fig. 4.2. In addition to the CO3 bands observed near 2.30-2.34, 2.50-2.55, 3.5, 4.0, and 4.7 pm in the NIR region and near 1550, 880 and 730 cm-1 in the mid-IR region [8, 104, 66, 67], siderite contains Fe excitational bands. The siderite spectrum shown here also contains bands due to water near 2, 3 and 6 pm that indicate that this sample has become hydrated. Some examples of hydrated carbonate minerals are listed in Table 4.1. Grain size plays an important role in the mid-IR spectral character of carbonates because of their unique index of refraction. Depending on the grain size, the spectral features will occur either as peaks or as troughs [105].

Much debate has occurred about the carbonates associated with Martian meteorites. Isotopic measurements on Fe-Mg carbonates in ALH 84001 [106, 107] and theories involving precipitation of carbonates [108, 109] suggest a low-temperature origin [106, 107], while grain morphology and fluid reactions suggest high-temperature origins [83, 110, 111]. Despite the evidence for carbonates in Martian meteorites, carbonates on Mars remain elusive.

1 2 3 4 2000 1600 1200 800 400 Wavelength (¡im) Wavenumber (cm-1)

Fig. 4.2 Reflectance spectra of two carbonates. The calcite is a < 63 |m separate from [67]. The siderite sample is a < 125 |m powder of a sample acquired from Wards.

4.4.3 Sulfates

Sulfate minerals exhibit bands due to SO4 in the NIR region near 1.7-1.85, 4.4-5, and in the mid-IR region near 1160, 680, 600, and 490 cm-1 [70-73]. Many also contain OH bands near 2.3 pm and water bands near 1.45, 1.95, 3 and 6 pm. As seen in Table 4.1, jarosite and alunite contain OH in their structures, while gypsum and schwertmannite contain H2O. Reflectance spectra of several sulfates are shown in Fig. 4.3. The VNIR sulfate bands near 1.7-1.85 and 4.5-4.9 pm fall where few other minerals exhibit spectra features, and thus may be useful in upcoming missions for detection of sulfates on Mars.

Wavelength (jim)

Wavelength (jim)

1 2 3 4 2000 1600 1200 800 400 Wavelength (/im) Wavenumber (cm1)

Fig. 4.3 Reflectance spectra of selected sulfates. The alunites and jarosites were prepared for a study by Kodama [112]. The gypsum is a < 63 pm separate from [73]. The schwertmannite spectra are from [99].

1 2 3 4 2000 1600 1200 800 400 Wavelength (/im) Wavenumber (cm1)

Fig. 4.3 Reflectance spectra of selected sulfates. The alunites and jarosites were prepared for a study by Kodama [112]. The gypsum is a < 63 pm separate from [73]. The schwertmannite spectra are from [99].

4.4.4 Phyllosilicates

Phyllosilicates or layer silicates, or more commonly clay minerals, include micas, talc, chlorites, smectites, the kaolin-serpentine group, the chain-structure group (e.g. prehnite, sepiolite, attapulgite), and vermiculites [9, 5]. Smecites, kaolin-serpentines, and chain-structure clays (although they are more rare) are alteration products of volcanic material on Earth and have therefore been thought to be present in the surface dust and soils on Mars [e.g. 12, 5].

Reflectance spectra of selected clay minerals are shown in Fig. 4.4. These spectra include bands due to OH near 1.4, 2.2-2.3, 2.7, and 12 pm, due to water near 1.45, 1.95, 3, and 6 pm, due to SiO4 near 1000-1100 cm-1 (9-10 pm) and 450-550 cm-1 (18-22 pm). The nontronites and serpentines also exhibit Fe electronic transitions from 0.6 to 1 pm.

2 3 4 2000 1600 1200 800 Wavelength (//m) Wavenumber (crrr1)

Fig. 4.4 Reflectance spectra of selected phyllosilicates. The montmorillonite and nontronite spectra are from [113] and the kaolinite and serpentine spectra are from [114].

A number of smectites has been collected and characterized by the Clay Minerals Society Source Clays Repository and are available in bulk for study; these include montmorillonites SWy and SAz from Wyoming and Arizona, respectively [14]. Terrestrial smectites are most commonly found with Na and/or Ca interlayer cations [e.g. 14], however, Mg and Fe are also observed and could be present as interlayer cations in smectites on Mars. The polarizing power of these cations increases from Na to Ca to Mg to Fe3+ and should be related to how strongly they attract water [10]. A study of the adsorption kinetics for water on Na-smectite indicated that this is too slow to act as a significant diurnal water reservoir on Mars [115]. However, Na-smectite is one of the least hygroscopic forms of this mineral as shown by IR spectra [10], calculations of partial molar enthalpy [116], and calculations of the chemical potential of H2O in smectite [13]. The effect of the interlayer cation on hydration of smectites is illustrated in Figs. 4.5 and 4.6.

1« 100 % Relative Humidity

Fig. 4.5 Water content vs relative humidity for montmorillonites with different interlayer cations.

1« 100 % Relative Humidity

Fig. 4.5 Water content vs relative humidity for montmorillonites with different interlayer cations.

Water abundance in SWy-1 montmorillonite with Na, Ca and Fe3+ interlayer cations was determined at several relative humidity (RH) levels by measuring weight loss after heating. Data were averaged for four replicates for the full RH range and six replicates for the low RH range and are shown in Fig. 4.5. The amount of water present, as determined by H2O lost on heating to 105 and 350°C, increased from Na to Fe3+ to Ca. The results of a TGA study with these samples, plus Mg-SWy, are displayed in Fig. 4.6 and show increasing water contents for the trend Na, Fe3+, Ca, or Mg interlayer cation [10]. The amount of water present as measured by spectral band strength [10], weight loss on heating (Fig. 4.5) and TGA (Fig. 4.6) all follow the polarizing power trends for Na, Ca, and Mg, but not for Fe3+. This implies that Fe3+ binds differently than typical interlayer cations in smectites, perhaps because iron is more reactive than the other cations.

Chipera et al. [117] performed XRD measurements on SAz montmorillonite as a function of RH. They found variations in the d-spacing, and hence water content, depending on the interlayer cation. The amount of adsorbed water for a given RH increased from K to Na to Ca. The polarizing power of K is less than that of Na, so this compares well with the trends shown in Figs. 4.5 and 4.6.

Fig. 4.6 TGA of montmorillonites with different interlayer cations (modified from Bishop et al. [10]).

4.4.5 Other Hydrated Minerals

Other hydrated minerals that could be present on Mars include zeolites, opal, phosphates, and nitrates. Spectra of two apatites, a nitrate and two zeolites are shown in Fig. 4.7. Zeolite spectra contain bands due to bound and adsorbed H2O that are influenced by the extra-framework cations as well as bands due to SiO4 and AlO4 groups near 1000, 800, and 500 cm-1 that vary depending on the zeolite structure and cations present [118]. Opal and amorphous silica are frequently hydrated and exhibit OH and H2O bands as well. The phosphate mineral apatite contains OH bands when in the hydroxyapatite form, but not F/Cl-apatite. NIR bands are primarily due to OH and H2O, but the 4.1 pm band is due to phosphate. Bands due to PO4 include a doublet near 1100 and another band near 600 cm-1, as well as transparency features near 800 cm-1 for fine-grained samples [119, 73]. The nitrate minerals niter (KNO3) and nitratine (NaNO3) follow the aragonite and calcite structures, respectively, and hence exhibit related spectral features [119]. The nitrate spectrum shown here is for nitratine and exhibits NO3 bands near 2.4, 2.6, 3.6, 4.1, and 4.8 pm in the NIR region as well as NO3 bands near 1790, 1550, 835, and 724 cm-1 in the mid-IR region.

A review of the properties of water in several zeolites including chabazite and clinoptilolite [15], coupled with recent calculations of water on clinoptilolite by Bish et al. [13], indicate that if zeolites are present on the surface of Mars they will be partially hydrated. This has implications for the surface-atmosphere water cycling as discussed in Chap. 8.

Wavelength i//m)

Wavelength i//m)

O.Iii iiliiiiliiiiliiiilii.il -.-1-1-1-1-1-1-0.8

1 2 3 4 2000 1600 1200 800 400 Wavelength (//m) Wavenumber (cm1)

Fig. 4.7 Reflectance spectra of selected apatites, zeolites and a nitrate. The dark green spectrum is of a white hydroxyapatite powder, the light green spectrum is the < 90 |m reddish-brown apatite from [73], the sodium nitrate sample is from a museum in Chile (contributed by C.P. McKay), and the zeolites are from Wards.

O.Iii iiliiiiliiiiliiiilii.il -.-1-1-1-1-1-1-0.8

1 2 3 4 2000 1600 1200 800 400 Wavelength (//m) Wavenumber (cm1)

Fig. 4.7 Reflectance spectra of selected apatites, zeolites and a nitrate. The dark green spectrum is of a white hydroxyapatite powder, the light green spectrum is the < 90 |m reddish-brown apatite from [73], the sodium nitrate sample is from a museum in Chile (contributed by C.P. McKay), and the zeolites are from Wards.

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