Thermal Infrared Emission Spectroscopy of Salt Minerals Predicted for Mars

ICARUS

135, 528–536 (1998) IS985998

ARTICLE NO.

Thermal Infrared Emission Spectroscopy of Salt Minerals Predicted for Mars Melissa D. Lane and Philip R. Christensen Department of Geology, Box 871404, Arizona State University, Tempe, Arizona 85287–1404 E-mail: [email protected] Received September 12, 1997; revised May 11, 1998

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Thermal emissivity spectra (2000–400 cm ) of select carbonate, sulfate, phosphate, and chloride minerals are presented in this study. The suite of samples was chosen to most closely represent the chemistry of the salts predicted for Mars on the basis of the Viking lander data, theoretical and experimental studies, and analyses of the Shergottite–Nakhlite–Chassignite and ALH84001 meteorites. Midinfrared spectra are presented to demonstrate the variation in emissivity between evaporite mineral classes (e.g., carbonates versus sulfates) and within a mineral class (e.g., calcite versus dolomite). Additional emissivity spectra are presented of particle-size fractions for each of the four mineral classes to show the effects of increased energy scattering with decreased particle size. Understanding particlesize effects may be critical for interpreting the emission of Mars where particulate material is common. The unique appearance of the emissivity spectra will aid the identification of the salt minerals on Mars from the midinfrared vibrational data to be acquired by the Thermal Emission Spectrometer aboard the Mars Global Surveyor spacecraft.  1998 Academic Press Key Words: spectroscopy; mineralogy; Mars, surface; infrared observations.

INTRODUCTION

Carbonates, sulfates, phosphates, and chlorides have been proposed to occur on Mars on the basis of theoretical and experimental calculations, Viking lander data of sediment crusts, and meteorite data of the Shergottite– Nakhlite–Chassignite (SNC) suite and ALH84001 thought to be derived from Mars. These minerals have suggested origins that include various combinations of water, rock, and atmosphere interactions. The studies that have focused on the interaction between water and mafic igneous rocks include Gooding 1978, Clark and Van Hart 1981, Schaefer 1990, 1993, DeBraal et al. 1992, and Plumlee et al. 1992. The interaction of the martian atmosphere with both water and rocks has also received attention by various authors (e.g., Gooding 1978, Fanale et al. 1982, 1992, Kahn 1985, 528 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.

Postawko and Kuhn 1986, Siderov and Zolotov 1986, Pollack et al. 1987). The experimental sampling of Mars both directly (at the Viking and Pathfinder lander sites) and indirectly (studying martian meteorites) also has led to the predictions of carbonates, sulfates, phosphates, and chlorides on Mars (Baird et al. 1976, Clark et al. 1976, Toulmin et al. 1977, Gooding 1978, Clark and Van Hart 1981, Gooding and Muenow 1986, Gooding et al. 1988, 1991, Chatzitheodoridis and Turner 1990, Treiman and Gooding 1991, Wentworth and Gooding 1991a,b, 1994, Gooding 1992a,b, Mittlefehldt 1994, McKay et al. 1996, Rieder et al. 1997). Detection and identification of these minerals is an objective of the Thermal Emission Spectrometer (TES) science team. The TES instrument (Christensen et al. 1992) was launched aboard the Mars Global Surveyor (MGS) spacecraft in November 1996. MGS-TES is slated to begin global data acquisition in April 1999 following circularization of the spacecraft orbit. The objective of this study is to present thermal (midinfrared) emission spectra of carbonates, sulfates, phosphates, and chlorides in order to illustrate the spectral differences between the mineral classes that arise due to mineral structure as well as those that arise with variation in particle size.

VIBRATIONAL SPECTROSCOPY The study of minerals and rocks using vibrational spectroscopy is based on the principle that molecules vibrate as they interact with propagating electromagnetic (EM) energy. The fundamental (hence, strongest) frequencies of internal molecular vibration for most geologic materials occur in the midinfrared range of the EM spectrum and are related to the physical properties of the bonds. Vibrations of the molecular groups offset the end atoms and their associated electrons, thus inducing an oscillating electronic dipole. When the energy absorbed for this vibrational process is emitted, the frequency of vibration is seen as an

THERMAL INFRARED EMISSION SPECTROSCOPY OF SALT MINERALS

emission maximum if the emitting medium is optically thin (i.e., when the radiance is dominated by volume scattering). However, if the sample is optically thick, the emitted phonon will be reabsorbed by another molecule, causing the frequency of vibration to be seen as an emission minimum (i.e., when the radiance is dominated by surface scattering). As a result of the preferred vibrations and the optical properties, the radiation emitted from a geologic material is not blackbody in character, but rather exhibits wavelength-dependent reduced radiance that appears as absorption features in the emissivity spectra. For a recent review of vibration theory as it relates to the optical properties of a mineral and thermal emission see the work by Wenrich and Christensen (1996). EXPERIMENTAL METHOD

The emitted radiation from various mineral and rock samples was measured using a modified Mattson Cygnus 100 interferometric spectrometer over the EM range of 2000 to 400 cm21 with 2 cm21 spectral sampling. The laboratory spectrometer was equipped with a KBr beam splitter and an uncooled, deuterated triglycine sulfate (DTGS) detector. The modifications of the spectrometer included removal of the internal ‘‘globar’’ radiation source because the samples, prior to analysis, were heated to p808C (p508C for the gypsum samples) and provided passive radiation for the experiments. Each sample was placed into a sample chamber within a glove box external to the spectrometer. To direct the radiation from the heated sample into the ray path of the spectrometer, an external parabolic mirror was added atop the sample chamber. The sample chamber is maintained at a constant temperature (24.0 6 0.18C) by a circulating water bath enclosed in the copper chamber wall. The spectrometer and the external glove box are continuously purged with air scrubbed of water vapor, dust particulates, and carbon dioxide. During sample analysis, the external sample chamber and glove box were purged with additional nitrogen gas to further reduce the humidity and CO2 vapor introduced during sample placement. Two blackbody targets (at p708 and 1008C) were measured to determine the instrument response function and instrument temperature used for data calibration. The emissivity spectra of the minerals were obtained by reducing the raw wavelength- and temperature-dependent data according to the one-temperature procedure of Ruff et al. (1997), a refinement of the twotemperature method of Christensen and Harrison (1993). This one-temperature calibration method assumes that the sample emissivity («) equals unity at some point within the data range. The position where « 5 1 is commonly referred to as the Christiansen frequency. The samples used in this analysis range from uncrushed, consolidated hand samples to particles #1000 em, divided

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into separate size fractions. The hand samples, heated overnight in a gravity convection oven, were placed into an insulated, foam-filled holder within the sample chamber just before analysis. Experiments showed that hand-sample cooling during the scan duration affected the absorption band emissivity («) of the majority of the samples by less than 1%. Particulates, however, can cool rapidly and therefore were actively heated during the spectral analysis. The particulate samples were poured into small copper sample cups coated with thermally black paint and heated overnight in the convection oven to remove adsorbed water. During data collection, the copper sample cups were placed onto a small heater unit regulated by a thermocouple buried within the particulate sample. Because of the ability to prevent sample cooling, the particulate samples were analyzed for a longer scan duration than the hand samples (i.e., 5.5 versus 3.5 min per sample). ABSOLUTE CALIBRATION OF SAMPLES

The calibration technique used in this study has been validated by a rigorous analysis of the potential errors associated with the measurement and calibration of radiant energy (Ruff et al. 1997). Emissivity errors were calculated for each ‘‘single error’’ case such as the measurement of instrument temperature, sample temperature, or environmental (sample chamber) temperature. ‘‘Multiple error’’ analyses were also conducted to assess the errors associated with two blackbody reference temperature measurements, whose values propagate through the calibration procedure. These two reference temperatures are used to derive the instrument response function that is used in the determination of both sample and instrument temperatures. The detailed error analysis suggests that the derived emissivity, «, for the worst case, is correct to p4%, and in most cases the emissivity is accurate to within 2% uncertainty. The largest error associated with the calibration technique relates to the derivation of sample temperature. Sample temperature is derived by fitting a Planck blackbody function (whose « 5 1) to the calibrated sample radiance, with the assumption that at some wavenumber, the sample also has an emissivity of unity. Commonly, however, geologic materials have a maximum emissivity that ranges between 0.98 and 1.0 (Salisbury et al. 1991 as discussed in Ruff et al. 1997). The final sample emissivity error was determined to correlate one-to-one with the departure from the assumption of unity of the true sample emissivity. For example, spectral analysis of a sample with a true maximum emissivity of 0.98 would produce a spectrum with a 2% emissivity error. SAMPLE DESCRIPTION

The carbonate, sulfate, phosphate, and chloride samples presented in this study represent the chemistry of the min-

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TABLE I Carbonate Samples Analyzed Using Vibrational Emission Spectroscopy Sample

ID

Mineral hand samples Calcite C2 Calcite C7 Calcite C8 Calcite C9 Calite

C10

Calcite Calcite Magnesite Magnesite Magnesite

C27 C40 C55 C59 C60

Siderite Siderite Siderite Rhodochrosite Rhodochrosite Dolomite

C48 C50 C62 C3 C29 C17

Dolomite

C18

Dolomite

C19

Dolomite Dolomite

C20 C28

Rock hand samples Limestone GW-R3 Dolomite GW-M2

Locale

Rodeo, Durango, Mexico (Burminco 1100-H) Diamond Bar, Los Angeles Co., California (Burminco 3030) Near Joplin, Jasper Co., Missouri (Burminco 1100) Near Yates Mine, near Otter Lake, Quebec, Canada (Burminco 1100-A) Crestmore Quarry, Riverside Co., California (Burminco 1100-F) Unknown Unknown Chewalah, Washington (Ward’s 46E4829) Bruck/Mur, Austria (Smithsonian 137985) Well’s Island, St. Lawrence River, New York (Smithsonian 70678) Roxbury Iron Mine, Connecticut Sovacon Infiernillos, Colavi, Bolivia Mt. St. Hilarie, Quebec, Canada (Smithsonian 162054) Butte, Montana (Burminco 4160) unknown (Spair or Portugal) Crystal Pass area, near Goodsprings, Nevada (Burminco 1840-B) San Antonio Canyon, Los Angeles Co., California (Burminco 1840) Sultan Mine, near Goodsprings, Clark Co., Nevada (Burminco 1840-C) Palas Verdes, Los Angeles Co., California (Burminco 1840-D) Corydon Crushed Rock Quarry, Corydon, Indiana

Redwall Formation, Granite Wash, Arizona Martin Formation, Granite Wash, Arizona

These carbonate lithologies have been predicted for Mars, so samples of these rocks are also included in this study (limestone GW-R3, dolomite GW-M2). Sulfates. The sulfate samples consist of five gypsums (CaSO4 ? 2 H2O) and two anhydrites (CaSO4). X-ray diffraction analyses confirmed their composition. The sulfate that was crushed and sieved for the particle-size analysis was a gypsum sample (S6). The grain size fractions were 710 to 1000 em, 500 to 710 em, 355 to 500 em, 250 to 355 em, 180 to 250 em, 125 to 180 em, 90 to 125 em, 63 to 90 em, and ,63 em. Phosphates. Two apatite [Ca5(PO4)3(F,Cl,OH)] samples are included. X-ray diffraction analyses confirmed their composition. Apatite P1 was crushed and sieved for particle-size analysis. The grain size fractions were 710 to 1000 em, 500 to 710 em, 355 to 500 em, 250 to 355 em, 180 to 250 em, 125 to 180 em, 90 to 125 em, and ,90 em. Chlorides. Two samples of halite (NaCl) and one of sylvite (KCl) are included. X-ray diffraction analyses confirmed their composition. A halite sample (HAL2) was crushed and sieved for the particle-size analyses of chlorides. The grain size fractions were 710 to 1000 em, 500 to 710 em, 355 to 500 em, 250 to 355 em, 180 to 250 em, 125 to 180 em, 90 to 125 em, and ,90 em. RESULTS AND DISCUSSION

erals either identified in the SNC and ALH84001 meteorites or predicted to occur on Mars. The analyzed evaporites occur either as coarsely crystalline mineral hand samples, as crystalline particulates, or as consolidated grains comprising a rock. This paper will present the spectra of evaporite minerals and rocks with a variety of compositions and textures. The samples included in this study are described in Tables I and II and discussed below. Carbonates. The carbonate minerals included in this study are calcite (CaCO3), dolomite [CaMg(CO3)2], magnesite (MgCO3), siderite (FeCO3), and rhodochrosite (MnCO3). Prior work by the authors (Lane and Christensen 1997) exclusively concerned the emission spectroscopy of carbonates and discussed the entire subset of carbonate mineral hand samples used in this study. This shared suite of carbonate mineral samples includes seven calcites, five dolomites, three magnesites, three siderites, and two rhodochrosites. Electron microprobe analyses confirmed the carbonate compositions (see Lane and Christensen 1997). One calcite sample (C2) was crushed and sieved for the particle-size analysis. The particle-size fractions were 710 to 1000 em, 500 to 710 em, 355 to 500 em, 250 to 355 em, 180 to 250 em, 125 to 180 em, 90 to 125 em, 63 to 90 em, and ,63 em. On Earth carbonates commonly comprise monomineralic rocks such as limestone (pure CaCO3) and dolomite [pure CaMg(CO3)2].

Mineral Hand Samples The emission spectra of the coarsely crystalline mineral hand samples are shown in Figs. 1 (carbonates) and 2 (sulfates, phosphate, and chlorides). Each spectrum shown in Figs. 1 and 2 represents the average of all the sample spectra of a specific mineral composition. These spectra

TABLE II Sulfate, Phosphate, and Chloride Samples Analyzed Using Vibrational Emission Spectroscopy Sample Sulfates Gypsum Gypsum

ID

S5 S6

Locale

Gypsum Gypsum Gypsum

S8 S17 S18

Anhydrite Anhydrite

S9 S16

Near St. George, Washington Co., Utah (Burminco 2480-A) Mule Canyon, near Calico, San Bernardino Co., California (Burminco 2480-C) Near White City, Eddy Co., New Mexico (Burminco 2480) Morocco (Burminco 2480-C) Near White City, Eddy Co., New Mexico (Burminco 2480); acquired 5 years after S8 Near Carson City, Nevada (Burminco 300) Near Carson City, Nevada (Burminco 300); acquired 5 years after S9

Phosphates Apatite Apatite

P1 P2

Yates Mine, near Otter Lake, Quebec, Canada (Burminco 440-A) Dacey Mine, near Cantley, Quebec, Canada (Burminco 440)

Chlorides Halite Halite Sylvite

HAL1 HAL2 SYL1

Detroit, Michigan (Burminco 2520) Detroit, Michigan (Burminco 2520) Near Carlsbad, Eddy Co., New Mexico (Burminco 4620)

THERMAL INFRARED EMISSION SPECTROSCOPY OF SALT MINERALS

FIG. 1. Emissivity spectra of carbonates. Vertical lines are for reference of band position relative to calcite. The 1800 to 2000 cm21 data are not shown because the spectra show no absorption features in that range.

exemplify the variety of spectral shapes that arise between and within the mineral classes. The carbonate emissivity spectra all share a similar shape; however, the exact positions of the absorption band minima vary between carbonate minerals. This variation in position is sufficient to distinguish the carbonates from each other (Lane and Christensen 1997). The spectral absorption features arise from the vibrations of the carbonate anion, specifically of the C–O bonds. The carbonate features at approximately 1540, 890, and 730 cm21 result from the n3 (asymmetric stretch), n2 (out-of-plane bend), and n4 (in-plane bend) normal modes of vibration, respectively. The n1 mode (symmetric stretch) is infrared inactive because no dipole moment results from that vibration. The spectral features seen in the sulfate spectra (Fig. 2) arise from the S–O bond oscillations of the sulfate anion. The gypsum and anhydrite spectra each exhibit three absorption bands having comparable shapes and positions. The sulfate feature at approximately 1160 cm21 results from the n3 mode (asymmetric stretch); the 680 and 600 cm21 features are both components of the n4 (in-plane bend) normal mode of vibration (Kohlrausch 1943, Vassallo and Finnie 1992). The n1 symmetric stretch is infrared inactive. Gypsum also displays a fourth band at p490 cm21 that is shallow in the anhydrite spectrum. This band is related to the n2 vibrational mode (Griffith 1970). Phosphate absorption features shown in Fig. 2 (apatite)

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arise from the oscillations of the P–O bonds within the phosphate anion. The n3 and n4 fundamental vibrations occur at p1140 to 1010 cm21 and p620 to 550 cm21, respectively. The n1 symmetric stretch is infrared inactive and the n2 symmetric bend occurs at wavelengths longer than those of this study. The n3 feature is a distinctive doublet whose band septum occurs at 1085 cm21 and is as diagnostic for phosphates as are the positions of the emissivity minima. The chlorides, halite and sylvite, are both isometric crystals with three equal-length axes that are orthogonal to each other. Thus the energy emitted from any direction is identical. Halite and sylvite are also both highly ionic, thus having uniform electrostatic charges across the crystal. This strong ionic bonding prevents the chemical bond between individual diatomic pairs (e.g., Na and Cl) from vibrating independently and causes the entire crystal lattice to vibrate as a whole (Simon 1966). These organized crystal lattice vibrations produce broad, strong absorption features in the emissivity spectra—absorption features much broader than the discrete bands of anion group oscillations seen in the spectra of carbonates, sulfates, and phosphates. The spectrum of halite shown in Fig. 2 is the emissivity of a cleavage face. The broad absorption band of halite (the low emissivity region at frequencies higher than p600 cm21) is apparent in the midinfrared; however, the absorp-

FIG. 2. Emissivity spectra of sulfates (gypsum and anhydrite), phosphate (apatite), and chlorides (halite and sylvite). The 1800 to 2000 cm21 data are not shown because the spectra show no absorption features in that range.

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THERMAL INFRARED EMISSION SPECTROSCOPY OF SALT MINERALS

tion band for sylvite falls out of range of this midinfrared study and thus exhibits graybody radiation (« 5 p0.96) in the 2000 to 400 cm21 region (Fig. 2). Although the sylvite emissivity spectrum was obtained by dividing the calibrated sample radiance by a best-fit blackbody function, the lack of an obvious emission maximum (i.e., a Christiansen frequency) in the data range may have provided an incorrect sample temperature, resulting in potentially incorrect graybody emissivity values. Mineral Particulate Samples As the grain size of any particulate sample is decreased so that it approaches the wavelength of the emitted energy, the emissivity spectra change shape. Previous studies (e.g., Lyon 1964, Aronson et al. 1966, Hunt and Vincent 1968, Vincent and Hunt 1968, Conel 1969, Hunt and Logan 1972, Aronson and Emslie 1973, Salisbury and Eastes 1985, Salisbury and Wald 1992, Wald 1994, Moersch and Christensen 1995, Wald and Salisbury 1995, Mustard and Hays 1997) have addressed the issue of energy scattering and the effect of particle size on midinfrared spectra. The differences between the particulate emissivity spectra and the consolidated hand sample spectra are due to both increased surface scattering associated with the increased number of grain/air interfaces per unit volume as grain size decreases and increased volume scattering associated with the decreased optical thickness of finer particles. The readers are encouraged to refer to those studies for discussion of the ‘‘type’’ of band behaviors associated with energy scattering (Hunt and Vincent 1968) which Moersch and Christensen (1995) explain in terms of band-behavior ‘‘classes’’ related to the mineral’s optical properties, n and k. The unique appearance of each particulate spectrum can be exploited for determining composition as well as effective particle size of the target. The salts predicted to occur on Mars may be unconsolidated. For this reason, a particle-size suite of one mineral from each mineral class was prepared and the emissivity spectra are presented. The minerals chosen to represent the carbonate, sulfate, chloride, and phosphate classes are calcite, gypsum, halite, and apatite, respectively. The particle-size fractions for each class are listed under Sample Description, above. The particle-size-dependent spectra discussed below will serve to present spectral emissivities that represent the mineral classes at narrower particle-size fractions than are common in the literature (e.g., Hunt and Vincent 1968, Vincent and Hunt 1968, Hunt and Logan 1972, Salisbury et al. 1991, Salisbury and Wald 1992). Figure 3a presents the calcite particle-size spectra. The n3

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(p1523 cm21) band clearly exhibits type 1/class 1 behavior. That is, the emissivity feature remains as the particle size decreases, but the absolute depth of the feature decreases. The first noticeable change in the spectrum with decreasing particle size, however, occurs at the n4 (p712 cm21) feature and, to a lesser initial extent, at the n2 (p883 cm21) feature. As particle size decreases, these two bands become essentially obliterated by the features developing and deepening (type 3/class 3 behavior) in the neighboring interband (transparency) areas. As a result the n2 and n4 features may become inverted and appear as emissivity maxima (type 2/class 2 behavior). Between p1600 and 2000 cm21 the emissivity decreases with decreasing particle size due to interband transparency. Within this area of transparency are distinct high-emissivity features at p1800 and 1960 cm21 that result from combination or over tones of the fundamental vibrations. Observation of the fundamental/ combination/overtone bands and the transparency features, as well as their relative depths can provide insight into the effective particle size of the sample in addition to composition. The sulfate grain spectra (Fig. 3b) show the same relative absorption band depths as the hand sample spectrum, until fine particle sizes are reached. This particle-size threshhold occurs approximately at the 90- to 125-em grain size, at which point significant interband (transparency) features arise at frequencies higher than p1300 cm21. Similar to the n3 carbonate band, the absorption bands of gypsum exhibit type 1 behavior, generally shallowing with decreasing grain size. However, this trend occurs only after an initial anomalous band deepening of the n3 (p1160 cm21) feature. This initial apparent band deepening likely is a result of the particle shape and placement in the sample cup rather than a true emissivity behavior of the mineral. That is, the coarsest grains are most affected by sample porosity due to the fibrous nature of the grains. Separate experiments showed that the porosity of the sample in the sample cup depended upon how the sample was prepared (e.g., poured into the cup only versus poured and tapped on the counter to settle the particles). The nonequant particle shape prevents the porosity of all of the samples from being identical. The spectra suggest that for the 250- to 355-em and finer particle size fractions the influence of particle shape is not dominant and the n3 feature shallows with decreasing particle size as expected. The spectra in Fig. 3b each represent the average of two to five separate data sets. Precision analysis of the n3 band showed that standard deviation of the n3 band emissivity was s 5 0.0045. This value correlates with a precision of p1% «.

FIG. 3. Emissivity spectra of particulate (a) calcite, (b) gypsum, (c) apatite, and (d) halite at various grain size fractions.

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The spectra of the particle-size suite of apatite are shown in Fig. 3c. The spectral behavior of phosphate is similar to that of gypsum, in that the particulate spectra resemble the hand sample spectrum, until small particle sizes are reached. For apatite, this threshhold also occurs at approximately the 90- to 125-em grain size, at which point the fundamental absorption features shallow (type 1/class 1 behavior). Significant interband (transparency) features arise even in the coarsest samples at frequencies higher than p1400 cm21 and deepen with a continued decrease in particle size. Within the transparency bands, apatite particulate samples exhibit two sharp emissivity maxima at p1760 and 1785 cm21 that result from combination/over tones. Other interband features (e.g., at p1400 to 1200 cm21 and p950 to 650 cm21) dominate only in the ,90em size fraction. The spectrum of halite presented in Fig. 2 is that of a single-crystal cleavage face. In Fig. 3d, however, the spectra of halite grain separates are presented. These particle-size samples consist of grains with no preferred axis orientation. Because halite is isometric, the energy emitted in any direction should be identical to that measured from the cleavage face. The halite particulate spectra all display pronounced spectral features (increased emissivity features) that have arisen due to extremely minor contamination of the sample. Spectral features due to minor impurities appear quite pronounced when the host medium is a spectrally featureless, nonabsorbing material such as halite (Eastes 1989). These features, attributable to the contaminant, typically resemble transmission spectra of that mineral impurity (Eastes 1989). This is to be expected because transmission measurements of minerals commonly use alkali halides such as KBr or KCl as the substrate into which the mineral of study is mixed for spectral analysis. The superimposed transmission features appear inverted (i.e., appear as peaks rather than valleys) from conventional transmission spectra. Optical analysis of the halite (HAL2) sample shows that the halite sample is contaminated with less than 1 wt% of quartz and calcite. This amount of contamination is far below the detectability of the XRD technique (p5%); no peaks arose in the X-ray diffraction (XRD) spectrum that were not due to halite. With the assumption that the spectral bands arising from the contaminants are akin to those seen in transmission analyses, the features seen superimposed on the halite emissivity spectra appear to be due to the presence of quartz (for the feature at p1000 cm21 and the small doublet at p800 cm21), carbonate (for the features at p1400 to 1500 cm21), and perhaps clay (for the feature at p1050 cm21). Although the specific carbonate was not identified, the presence of two carbonate features suggests that the mineral may be pirssonite (Na2Ca (CO3)2 ? 2H2O) (Adler and Kerr 1963), a mineral known to precipitate in saline environments. The emissivity maximum at p1620 cm21 is due likely to water inclusions (Urban

FIG. 4. Radiance curves of a 710- to 1000-em-fraction halite sample and a Planck blackbody curve at the same temperature as the sample (74.498C).

1993, Farmer 1974, J. Salisbury, pers. commun. 1997). Attempts to concentrate the halite contaminants for further analysis were unsuccessful. Inspection of the calibrated sample radiance curves suggested that the maximum sample emissivity of halite occurs at p 410 cm21. Figure 4 shows the calibrated sample radiance curve for the 710- to 1000-em fraction in addition to the corresponding Planck function at the derived sample temperature of 74.498C. The noise below p400 cm21 results from the physical throughput limitation of the KBr beam splitter and does not represent sample information. The maximum emissivity value of 410 cm21 was used during calibration of each of the halite separates to derive the sample emissivity spectra. Given the uncertainty of the assumption that halite has an « 5 1.0 at 410 cm21 and its high reflectivity (low emissivity), the derived emissivity spectra of the halite samples (including the coarsely crystalline sample) should not be considered to represent ‘‘absolute’’ emissivity. Hence the trends of band depths will not be addressed. Previous experimental midinfrared mixture studies (Eastes 1989) of quartz, calcite, gypsum, or montmorillonite with halite showed that the mixture spectra are a unique combination of reflectance and transmittance spectra. The common trend of these mixture spectra is toward increased reflectivity (decreased emissivity) with increasing abundance of halite. Thus we argue that the broad, low-emissivity character of the halite spectra, with or without superposed ‘‘transmission’’ features from contaminants, is unique and can be used to identify the presence of abundant chloride on Mars. Rock Hand Samples Some minerals, particularly the carbonates, occur in monomineralic rocks. Two examples are CaCO3 in limestone (sample GW-R3) and CaMg(CO3)2 in dolomite

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REFERENCES Adler, H. H., and P. F. Kerr 1963. Infrared spectra, symmetry and structure relations of some carbonate minerals. Am. Mineral. 48, 839–853. Aronson, J. R., and A. G. Emslie 1973. Spectral reflectance and emittance of particulate materials. 2. Application and results. Appl. Opt. 12, 2573– 2584. Aronson, J. R., A. G. Emslie, and H. G. McLinden 1966. Infrared spectra from particulate surfaces. Science 152, 345–346. Baird, A. K., P. Toulmin III, B. C. Clark, H. J. Rose, Jr., K. Keil, R. P. Christian, and J. L. Gooding 1976. Mineralogic and petrologic implications of Viking geochemical results from Mars: Interim report. Science 194, 1288–1293. Chatzitheodoridis, E., and G. Turner 1990. Secondary minerals in the Nakhla meteorite. Meteoritics 25, 354. Christensen, P. R., and S. T. Harrison 1993. Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks. J. Geophys. Res. 98, 19819–19834.

FIG. 5. Emissivity spectra of monomineralic carbonate rocks [(a) limestone and (b) dolomite] compared to the appropriate pure mineral spectrum. The 1800 to 2000 cm21 data are not shown because the spectra show no absorption features in that range.

(sample GW-M2). Although the individual grains in the rocks are small, the emissivity spectra of the rocks (Figs. 5a and 5b) appear more similar to the coarsely crystalline mineral hand sample spectra than the fine-grained particulate sample spectra with regard to the number of absorption bands, band position, and shape. This occurs because the consolidated, massive character of the rocks minimizes the number of grain/air interfaces and porosity and hence the scattering at the grain boundaries. The limestone spectrum does include minor features that suggest a spectral component with an effective grain size between the 250to 355-em and 355- to 500-em size fractions (refer to Fig. 3a). The dolomite rock spectrum is almost identical to the dolomite mineral spectrum. The similarity of the carbonate rock spectra to their respective constituent mineral spectra allows for the chemical compositions of the rocks to be easily identified.

ACKNOWLEDGMENTS The authors thank Doug Howard at Arizona State University for his assistance with the preparation of the calcite and sulfate particle-size fractions. Thanks are given to Dr. John Holloway for his assistance with the optical analysis of the halite sample. The authors thank Drs. Fraser Fanale and Jack Salisbury for their helpful reviews of the manuscript. This study was supported by NASA’s Planetary Geology and Geophysics Program and the Mars Global Surveyor Science Office.

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