E. Murad, D-95615 Marktredwitz, Germany

Martian Phyllosilicates: Recorders of Aqueous Processes (2008) 7020.pdf
Transmission (%)
100
98
96
100
99
100
98
96
94
92
Nontronite
Kaolinite
Illite
-5 -2.5 0
Velocity (mm/s)
2.5 5
Fig. 2. Room-temperature Mössbauer spectra of
nontronite (top), kaolinite (center) and illite (bottom).
A parameter which is readily revealed by
Mössbauer spectroscopy is the Fe 2+ /Fe 3+ ratio, which
can provide important clues regarding the redox
environment of a sample. The determination of this
ratio by Mössbauer spectroscopy is relatively
straightforward: thus Fig. 2 shows the illite to contain
a minor proportion of Fe 2+ , indicated by a resonant line
at ~ 2.4 mm/s. Such a line is missing in the other two
spectra, indicating Fe in these samples to be
exclusively trivalent.
Applications to natural clays of complex
mineralogy on Earth and Mars: a basic difference
between clay minerals s.s. (clay-sized phyllosilicates)
and Fe oxides is that almost all of the former are
paramagnetic to ~ 10 K, whereas many Fe oxides are
magnetically ordered at 300 K and all are magnetically
ordered at ~ 20 K [7]. Natural clays (and sediments
and soils) are usually more or less complex mixtures of
clay minerals, Fe oxides, and other (Fe-bearing and
Fe-free) minerals. Their room-temperature Mössbauer
spectra thus generally comprise three sets of
components: (1) Fe 3+ doublets from paramagnetic clay
minerals and other silicates, and superparamagnetic Fe
oxides and related minerals (e.g. schwertmannite),
(2) Fe 2+ doublets that, if present, would in most cases
originate from clay minerals, and (3) Fe 3+ sextets from
magnetically ordered Fe oxides. These features have
been used to characterize clays from a variety of
environments using Mössbauer spectroscopy in
combination with other techniques such as X-ray
diffraction, visible, infrared and Raman
spectroscopies, selective chemical extraction
procedures, optical and electron microscopies, and
thermal analysis [8,9,10]. As an example, Fig. 3 shows
how the selective removal of Fe oxides affects the
room-temperature Mössbauer spectrum of a bauxite.
This treatment not only allows a better characterization
of the paramagnetic components (residual minerals,
some or all of which could contribute to the Fe 3+
doublet, have been identified by X ray diffraction as
boehmite, kaolinite, anatase and crandallite), but also
shows the residual hematite to be identical to that
which has been extracted [11].
Transmittance (%)
100
98
96
100
99
98
BX-N
3 x DCB 295 K
-10 -5 0 5 10
Velocity (mm/s)
Fig. 3. Mössbauer spectra of a bauxite before (top)
and after extraction with Na dithionite (bottom).
A constraint on remote work, e.g. on Mars, is that
only minimal sample manipulation will be possible.
Although Mössbauer work has been instrumental in
identifying a variety of Fe oxides on Mars [12,13,14],
a combination of various techniques including
Mössbauer spectroscopy holds promise for a more
comprehensive characterization of clay mineralogy
than Mössbauer spectroscopy alone [15,16].
References: [1] Pollak et al (1962) phys. stat. sol.,
2, 1653, [2] Malden P.J. and Meads R.E. (1967)
Nature, 215, 844, [3] Stevens J.G. et al. (1988)
Mössbauer Mineral Handbook, MEDC, Asheville,
N.C., [4] Cardile C.M. & Johnston J.H. (1985) Clays
Clay Min., 33, 295, [5] Murad E. & Wagner U. (1991)
N. Jb. Min. Abh., 162, 281, [6] Murad E. & Wagner U.
(1994) Clay Min., 29, 1, [7] Murad E. & Cashion J.
(2004) Mössbauer Spectroscopy of Environmental
Materials, Springer, [8] Murad E. & Wagner U.
(1989) Hyp. Int., 45, 161, [9] Murad E. and Rojík P.
(2005) Clay Min., 40, 427, [10] Bishop J.L. et al. (2007)
Clays Clay Min., 55, 1, [11] Murad E. (200x) Min Eng.,
18, 984, [12] Klingelhöfer G. et al. (2005) Hyp. Int., 166,
549, [13], Morris R.V. et al. (2006) JGR, 111, E02S13,
[14], Ming D.W. et al. (2006) JGR, 111, E02S12, [15]
Bishop J.L. et al. (2004) IJA, 3, 275, [16] Lane M.D. et
al. (2004) GRL, 31, L17902.