The results of spectral measurements for mafic silicates are given. The study provided valuable spectral reflectance information about mafic silicates and phyllosilicates in the 2.5 to 4.6 micron wavelength region. It was shown that the reflectance of these materials is strongly affected by the presence of H2O and OH. Therefore, the identification of these absorbing species is greatly enhanced

Mccord, Thomas B.

Roush, Ted L.

Singer, Robert B.

English

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GTMCE W E  SiISCATES FBM)H .(6 TO 4Xipnra; 
Ted L. Roush, Robert B. Singer, and Thomas B. McCord, Univ. Hawaii, Planetary Geosciences Div., 
Hawaii Inst. Geophysics, 2525 Correa Rd., Honolulu, HI 96822 
DUmOM Spectral reflectance, obtained by spacecraft and earth-based telescopic 
observations, is the dominant method of collecting mineralogical information concerning the 
surface properties of solar system objects. O u r  ability to interpret this data, in terms of the 
identity and abundance of minerals present on a surface, is important for addressing more gen- 
eral questions concerning the origin and evolution of that body as well as the solar system as a 
whole. The reflectance of geologically important materials in the 0.3 to 2 . 5 ~  wavelength region 
has been the subject of intense laboratory research for many years(1,2,3,4,5). However, there 
has been a paucity of research concerning the reflectance behavior of geologic materials in the 
2.5 to 4 . 6 ~  wavelength region. 
The mapping spectrometers on future spacecraft missions, such as Galileo and the Mars Orbiter, 
are designed to return spectral reflectance data from 0.7 to 5.2pm. Thus, it is important for 
interpretation of such data to expand laboratory reflectance measurements to include the 
longer wavelength region. The study presented here provides empirical laboratory data concern- 
ing the reflectance behavior of geologically important materials in the 2.5 to 4.6pm wavelength 
region. 
MPEizwENTAL OD For this study two infrared, cooled, circular variable filter (CVF) spec- 
trometers with InSb detectors were used and the combined wavelength coverage spanned the .65 
to 4.5pm wavelength region. In both instruments the detector, filter, and other internal com- 
ponents are operated a t  liquid nitrogen temperature (77 K). The spectral resolution (Ah/h) of 
both CVF's is about 1.5%. 
The mafic samples were chosen to represent minerals which result in mafic to ultramafic assem- 
blages. All the mafic samples were dry sieved to the < 3 8 ~ m  grain size. The phyllosilicates were 
chosen to represent a variety of structural types and amounts of natural hydration states. All 
phyllosilicate samples were dry sieved to the 38-45pm grain size. Additionally, a palagonite was 
included in the sample suite since this material has been suggested as a Mars soil analog 
material(6,7). The 2-3pm grain size of the palagonite was separated by liquid suspension in 
methanol. 
All samples were placed in a furnace prior to being situated in an environment chamber. The 
environment chamber was flushed with inert nitrogen gas during the entire data collection and 
heating sequence. The samples were allowed to equilibrate in the nitrogen environment for 
thirty minutes before room temperature spectral data were collected. The samples were then 
heated a t  200°C for thirty minutes and subsequently allowed to cool to room temperature before 
additional data were collected. 
BACKGROUND Electronic tranyt+ions of he f i s t  transition element series, especially the petro- 
logically significant cations Fe and T$+, result in absorptions in the visible and near-infrared 
(0.3 to 2.5pm) region of the electromagnetic spectrum(8), and are due to ions located within 
specific crystallographic sites. Hence, from the spectra direct information is obtained concern- 
ing the chemistry and crystal structure of the material. 
For the silicates and of this study, the most important species which have vibrationally induced 
absorptions are the H 0 molecule and the OH- ion. The H20 molecule has strong absorption 
features located near 89pm (0-H asymmetric stretch). 3 . 0 ~  (H-0-H bend overtone), and 3. lpm 
(0-H symmetric stretch)(9). Overtones and/or combinations of these bands can occur near 0.94, 
1.135, 1.38, 1.45, and 1.88pm(9). There is a fundamental of the OH- group located near 2.75ym 
and the f i s t  overtone is located near 1.4pm(9). Additionally, bands in the 2.2 to 2.4pm region 
involve a combination of the OH- fundamental with a X-OH bending mode (where X is usually Al or 
Mg) (9). 
DISCUSSION The results of the spectral measurements for the mafic silicates are presented in 
Figures 1 to 3. All spectra, excep the plagioclase, exhibit electronic absorptions located near 1 
, andlor 2pm which are due to Feh+ located in octahedral crystallographic sites. These absorp- 
tions remain unchanged after heating. Additionally, all spectra have an absorption centered 
near 3pm which is due to H20. This absorption changes in intensity, and in some cases shape, 
after heating. We believe these changes represent the removal of physically adsorbed H20 from 
the sample. The continued presence of the 3p.m band band in the spectra of all samples after 
heating implies that H20 bearing contaminants are present, or alternatively, the samples 
https://ntrs.nasa.gov/search.jsp?R=19870013988 2020-03-20T10:24:49+00:00Z
contain fluid inclusions. 
The results of the spectral measurements for the phyllosilicates are presented in Figures 4 and 
5. Kaolinite and pyrophyllite have types of structures which typically accommodate little 
cationic substitution and hence do not incorporate interlayer H20(10). Before and after heating, 
the spectra of both samples exhibit strong absorption bands located near 2.75pm, due to OH-, 
and the spectra after heating show minor changes due to the removal of physically adsorbed 
H20. This spectral behavior is illustrated in Figure 4 for kaolinite. 
The Na-Montmorillonite (Fig. 5) has a structure which has abundant cationic substitution and its 
open nature results in the presence of interlayer H20(10). The spectrum of the sample before 
heating exhibits a broad 3pm band and narrower but pronounced 1.9pm band both due to H 0 
and bands located near 1.4 and 2 . 8 ~  both due to OH-. After heating, the spectrum exhibits tge 
effects of removing the physically adsorbed, and possibly some of the interlayer, H20. The 1.9 
and 3.0pm bands have decreased in intensity, while the band near 2 . 8 ~  has become more pro- 
nounced. 
The palagonite spectrum (Fig. 6) before heating shows a strong, broad 3pm absorption and a 
weak absorption near 1.9pm, both indicate the presence of H 0. After heating, the spectrum 
shows virtually no 1 . 9 ~  band and the 3pm band although sfightly reduced in intensity still 
remains very prominent. We believe the physically adsorbed H 0 has been removed, so the con- 
tinued presence of the 3pm band indicates abundant H20. &e spectral behavior of the 3pm 
band is reminiscent of the plagioclase. 
CONCLUSIONS This study has provided valuable spectral reflectance information about mafic sili- 
cates and phyllosilicates in the 2.5 to 4 . 6 ~  wavelength region. In this wavelength region we 
have shown that the reflectance of these materials is strongly affected by the presence of H20 
and OH-. Therefore, the identification of these absorbing species is greatly enhanced. 
The reflectance behavior of m&c silicates beyond 2.5pm can provide additional information 
about the chemical composition of the pyroxenes by revealing the long wavelength edge of the 
clinopyroxene absorption band located near 2pm. For Mercury, the Moon, Earth, Mars, and the 
asteroids this provides additional informatipn which enhances our ability to map changes in the 
mineral composition and abundance across the surface of a planetary body. 
The reflectance of phyllosilicates beyond 2.5pm provides additional information concerning the 
potential for identifying compositional and water content variations of these minerals. Since OH- 
is structurally bound to octahedrally coordinated cations in phyllosilicates, the substitution of 
various cations should result in the shifting of absorptions due to the OH--metal vibrations. Addi- 
tionally, absorptions due to interlayer H20 can be expected to vary as a function of amount of 
water available on the surface. In the case of Mars, a map of water content variation could iden- 
tify possible sources and/or sinks of weathering products on a global scale and allow the study of 
water transport on the surface. 
The spectral reflectance behavior of the palagonite beyond 2.5pm implies that the water con- 
tained within the sample is not composed solely of physically adsorbed water. Thus, the palagon- 
ite, if located in the desiccated Martian environment, could provide an exchange mechanism for 
atmospheric water vapor by trapping and releasing physically adsorbed water. The palagonite, 
during its initial formation, also provides a more permanent mechanism for removal of atmos- 
pheric water from the MaI-tian environment. 
A C K N O G E 3 E N T S  This research was supported by NASA grants NSG 7590 and NSG 7312. 
m C E S  l)Hunt, G.R. and J.W. Salisbury, Mod. Geol., 1, 283-300, 1970. 2)Hunt G.R. and J.W. 
Salisbury, Hod. Geol., 2, 23-30, 1971. 3)Adams, J.B., J. Geophys. Res., 79, 4829-4836, 1974. 
$)Singer, R.B., J. Geophys. Res., 86, 7967-7982, 1981. 5)Clark, R.N., J. Geophys. Res., 86, 3074- 
3086, 1981. 6)Evans, D.L. and J.B. Adams, Proc. Lunar Planet. Sci. Conf. I l th ,  757-763, 1980. 
?)Singer, R.B. J. Gophys.  Res., 87, 10 159-10168, 1982. 8)Burns, R.G., Mineralogical Applications 
of Crystal Field Theory, 1970. 9)Hunt, G.R., Geophysics, 42, 501-513, 1977. 10)Deer, W.A., R.A. 
Howie, and J. Zussman, An Introduction to the Rock Forming Minerals, 1966. 
1.0 2.0  3.0 4.0 
Wavelength Urn) 
Figure 1. Reflectance spectra of olivine Poag) before (b) and 
after (a) heating. ')&e absorption centered near of lpm is due to 
the presence of Fe and is unaffected by heating. The decrease 
of the absorption located near 3pm upon heating is due to the 
loss of physically adsorbed H20. 
0 .0  
1 .0  2.0 3.0 4.0 
Wavelength Urn) 
Figure 2. Reflectance spectra of orthopyroxene @na6) before 
(b) and after (a) heating. The abso tions located near 1 and 
2pm are due to the presence of FeR and are not affected by 
heating. Because this sample contains tremolite as a contam- 
inant, after heating the 3p-n band depth decreases, due to the 
loss of physically adsorbed H 0, and the relatively narrow 
absorption looated near 2.75pn-1, %ue to OH-, is more pronounced. 
Figure 4. Reflectance spectra of kaolinite (A1 Si 0 (OH) before 
(b) and after (a) heating. The spectra e d b i f  dk vidational 
absorptions near 1.4. 2.2. and 2.75pm. The spectral changes due 
to heating indicate the loss of physically adsorbed HZO. 
" . L, 
1.0 2.0 3.0 4.0 
Wavelength (urn1 
Figure 5. Reflectance spectrum of Na-Montmorillonite (nomi- 
nally. (3 Ca.Na)o,7(A1.Mg.Fe)4[(Si.~)a0 01(OH)4 nH20) before (b) 
and after (a) heating. Before heating &e spectrum of this sam- 
ple exhibits absorptions due to HgO near 1.9 and 3.0pm and OH- 
near 1.4 and 2.2p-n. After heatmg the spectrum exhibits the 
effects of removing physically adsorbed, and potentially inter- 
layer H20, by the reduced intensity of the 1.9 and 3 . 0 ~  bands 
and the OH band near 2.8pm is more pronounced. 
0 .0  
1 .0  2 .0  3.0 4 . 0  
Wavelength Urn) 
Figure 3. Reflectance spectra of plagioclase (An5 ) before (b) 
and after (a) heating. The spectra of this minerd exhibit the 
strongest 3pm of all the mafic silicates due to the presence of 
fluid inclusions in the sample. Since the H20 in the fluid inclu- 
sions is not easily lost upon heating, the minor spectral changes 
indicate solely the loss of physically adsorbed HZO. 
1.0  2.0 3 .0  4.0 
Wavelength Urn; 
Figure 6. Reflectance spectrum of palagonite before (b) and 
after (a) heating. Before heating the spectrum of this sample 
exhibits a weak 1 . 9 ~  band and a strong 3pm band both indica- 
tive of HZO., After heating the spectrum exhibits a v e v  weak 
1.9pm band Implying removal of HZO. yet the 3pm band remains 
prominent. This spectral behavior 1s very reminiscent of the pla- 
gioclase. 


Reflectance spectra of mafic silicates and phyllosilicates from .6 to 4.6 microns

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