Lunar and Planetary Science XXXII (2001) 1199.pdf
SPECTRAL-COMPOSITIONAL PROPERTIES OF IRON-BEARING CLAYS: REMOTE SENSING IMPLICATIONS. Dionne Marcino1, Edward Cloutis1, Pranoti Asher2, Johnathon Strong3, Brad Russell1, and Doug Goltz3. 1Department of Geography, University of Winnipeg, 515 Portage Ave., Winnipeg, MB, Canada R3B 2E9 ([email protected]; [email protected]; [email protected]; ), 2Department of Geology and Geography, Georgia Southern University, P.O. Box 8149, Statesboro, GA, USA 30460-8149 ([email protected]). 3Department of Chemistry, University of Winnipeg, 515 Portage Ave., Winnipeg, MB, Canada R3B 2E9 ([email protected]; [email protected] )
+2 Introduction:The spectral reflectance properties of ([(Ca/2,Na)0.3(Mg,Fe )3(Si,Al)4O10(OH)2·4H2O]), +2 a number of iron-bearing clay species have been exam- serpentine ([A3Si2O5(OH)4]; A=Mg,Fe ,Ni.), and +2 ined. The purpose of this study is to develop quantita- vermiculite ([(Mg,Fe ,Al)3(Al, Si)4O10(OH)2·4H2O]). tive relationships between composition and structure, Figure 1 shows the spectra of berthierine which and reflectance spectra. Because clays are potentially a exhibits a band at 0.35µm which is probably a metal-O volatile sink on Mars [1], these results will have great charge transfer [6]. The serpentine spectra show a significance in unravelling the geological history of the narrow absorption band near 0.38µm which is attributed surface minerals of Mars and certain groups of asteroids to ferric iron [6]. Serpentine, cronstedtite (Figure 3) and [2,3,4], through the identification of types, abundances, glauconite exhibit an absorption feature at 0.43µm 6 Ú 4 4 3+ and composition of iron-bearing clay minerals. This attributed to A1 T1, E(G) Fe spin-forbidden crystal paper focuses on the 0.3-1.2 µm region. field transition [7, 8], which becomes increasingly
intense with increasing Mn content. Berthierine, Experimental Procedure: A range of iron-bearing chamosite (Figure2), saponite and glauconite share an clays including berthierine, chamosite, cronstedtite, inflection point near 0.48µm which is due to a ferric- glauconite, saponite, serpentine, and vermiculite have ferrous intervalence charge transfer transition (IVCT)[9]. been analysed by reflectance spectroscopy, ICP-MS, wet chemistry, atomic absorption spectroscopy, X-ray In the saponite spectra, the slight inflection found at diffraction, and thermal analysis. The samples were 0.54 µm, is also credited to IVCT [2]. The feature which acquired from various sources, primarily the Smithso- is located at 0.65µm in the serpentine spectra is caused nian Institution National Museum of Natural History. by transitions within Fe2+ ions [10]. The absorptions at The samples were crushed manually and dry sieved to 0.7µm in berthierine, 0.75µm in cronstedtite, and 0.7 and <45 µm. Reflectance spectra were measured at the 0.75µm in the glauconite spectra [8], are all attributed to 2+ 3+ NASA-supported RELAB facility at Brown University a Fe Ú Fe charge transfer transition [7]. At 0.67µm in [5]. Spectra were measured from 0.3%2.6 µm relative to the saponite spectra, there is an inflection which Vilas 6 Ú4 3+ halon at i=30° and e=0° and 5 nm spectral resolution and [2] attributes to a A1 T2(G) Fe charge transfer from 2.5-26 µm relative to brushed gold at i=30° and transition. Absorption features found in chamosite at e=30° with 4 cm-1 spectral resolution. The spectra were 0.72µm and cronstedtite at 0.82µm are both attributed to merged in the 2.5-2.6µm region. Absorption band the ferric-ferrous intervalence charge transfers [7]. minimum wavelength positions were determined by fitting a second order polynomial to 5-10 data points on Discussion: The 0.43 and 0.7 bands seen in many both sides of a visually established minimum. X-ray dark asteroid spectra [7] are also seen in the reflectance diffraction was done at the University of Manitoba. spectra of a number of our samples, including Thermal analysis, wet chemistry, ICP-MS, and atomic cronstedtite, glauconite, serpentine, and berthierine. The 6 Ú 4 4 absorption were performed at the University of 0.43µm band is attributed to A1 T1, E(G) Fe , while 2+Ú 3+ Winnipeg. the 0.7µm band is attributed to a Fe Fe charge transfer transition in phyllosilicates [7]. Jarvis attributes Results: The reflectance spectra of some iron- a 0.7µm absorption feature in asteroid spectra, coupled bearing clays are shown in Figures 1, 2, and 3. These with low albedo, shallow UV/blue IVCT, and a slight slope in the VNIR, to be indicative of the presence of spectra exhibit a number of absorption features in the iron-bearing phyllosilicates.[9]. Jarvis also credits the 0.3-1.2 micron region. The clay minerals which exhibit 0.7µm feature found in some medium albedo M-class iron-related absorption features include berthierine asteroids to the occurrence of iron-bearing phyllosilicates ([(Fe+2, Fe+3,Mg) (Si,Al) O (OH) ]), chamosite 2-3 2 5 4 [9]. ([(Fe+2,Mg,Fe+3) Al(Si ,Al)O (OH,O) ]), cronstedtite 5 3 10 8 Vilas found that the spectra of asteroids 102 Miriam, ([Fe+2 ,Fe+3(SiFe+3)O (OH) ]), glauconite 2 5 4 and 1467 Mashona, as well as certain meteorites, are ([(K,Na)(Fe+3,Al,Mg) (Si,Al) O (OH) ]), saponite 2 4 10 2 compatible with that of iron-bearing serpentines. A Lunar and Planetary Science XXXII (2001) 1199.pdf
Iron-Bearing Clay Spectra: D. Marcino et al.
comparison of available spectra of these asteroids chamosite (Figure 2; middle), and cronstedtite (Figure 3, (wavelength range of 0.5µm to 1.0µm) with our spectra lower). of serpentines does show similarities at approximately 0.65µm. Both 1467 Mashona and 102 Miriam shows
features at approximately 0.65 to 0.67µm, 0.7µm, and 0.3 0.82µm [4]. Cronstedtite, saponite, glauconite, and berthierine also exhibit these features. 0.2 It should be noted that no single clay exhibits every feature found in the asteroid spectra. The wavelength 0.1 position of specific absorption bands due to various Absolute reflectance Berthierines electronic transitions varies between different clay species. Therefore we should be able to narrow down the 0 300 400 500 600 700 800 900 1000 1100 1200 range of possible clay minerals present on the surface of Wavelength (nm) asteroids on the basis of absorption band wavelength positions.
Conclusion: Iron-bearing clays exhibit a large number of absorption features attributable to both ferrous and ferric iron. The wavelength positions of these bands vary from species to species and hence are 0.6 potentially diagnostic for remote sensing detection of 0.5 Chamosites specific iron-bearing clay minerals 0.4
0.3 References: [1] Huguenin R.L.. (1974) JGR, 79, 0.2 3895-3905. [2] Vilas F. et al. (1994) Icarus, 109, 274- Absolute reflectance 283. [3] Vilas F. et al. (1993) Icarus, 102, 225-231. [4] 0.1 Vilas F. and Gaffey M.J. (1989) Science, 246, 790-792. 0 300 400 500 600 700 800 900 1000 1100 1200 [5] RELAB User’s Manual (1999) Brown U., Wavelength (nm) Providence, RI. [6] Gaffey S.J. et al. (1993) Remote Geochemical Analysis, Cambridge U. Press, pp. 43-77. [7] Jarvis K.S. et al. (2000) Icarus 146, 125-132. [8] Burns R.G. et al. (1971) Conf. Lunar Geophys., 93-102. [9] Jarvis K. S. et al. (2000) Icarus, 145, 445-453. [10]
Burns R.G. (1970) Am. Mineral., 55, 1608-1633. 0.11
Acknowledgements: Thanks to Carlé Pieters and 0.09 Takahiro Hiroi at Brown University for providing access to the NASA-supported RELAB spectrometer facility, 0.07 Dr. Jeffrey Post of the Smithsonian Institution’s National
Museum of Natural History for providing many of the Absolute reflectance 0.05 Cronstedtit clay samples, and Dr. Frank Hawthorne and Mr. Neil Ball at U of M for the X-ray diffraction analyses. This 0.03 300 400 500 600 700 800 900 1000 1100 1200 study was supported by research grants from the Natural Wavelength (nm) Sciences and Engineering Research Council and the Canadian Space Agency Space Science Program, and a discretionary grant from the University of Winnipeg (to EAC).
Figures: Reflectance spectra (0.3-1.2µm) of <45µm size fractions of : berthierine (Figure 1; upper),