Recognizing Sulfate and Phosphate Complexes - USRA

46th Lunar and Planetary Science Conference (2015)
WEATHERING PRODUCTS ON MARS. E. B. Rampe1, R. V. Morris2, and P. D. Archer, Jr.3 1Aerodyne Industries, Jacobs-JETS contract, NASA-Johnson Space Center, Houston, TX 77058, [email protected],
Introduction: Nanophase weathering products
(i.e., secondary phases that lack long-range atomic order) have been recognized on the martian surface via
orbital observations and in-situ measurements from
landed missions. Allophane, a poorly crystalline, hydrated aluminosilicate, has been identified at the regional scale in models of thermal-infrared (TIR) data
from the Thermal Emission Spectrometer (TES) [1]
and at the local scale from visible/near-IR (VNIR) data
from the Compact Reconnaissance Impact Spectrometer for Mars (CRISM) instrument [2] and phase calculations of Alpha Particle X-ray Spectrometer (APXS)
data of rocks encountered by the Mars Exploration
Rovers (MER) Spirit and Opportunity [3,4].
Nanophase iron oxides (npOx) have been recognized in
rocks and soils measured by the Mössbauer Spectrometer on Spirit and Opportunity [5,6]. Furthermore, analyses of X-ray diffraction data measured by the CheMin
instrument onboard the Mars Science Laboratory rover
Curiosity indicate rock and soil samples are comprised
of ~20-50 wt.% X-ray amorphous materials [7-9].
Chemical measurements by landed missions indicate the presence of sulfur and phosphorus in martian
rocks in soils, and APXS data from Gusev crater
demonstrate abundances of up to ~5 wt.% P2O5 and
~30 wt.% SO3 [4]. However, the speciation of phosphorus and sulfur is not always evident. On Earth,
phosphate and sulfate anions can be chemisorbed onto
the surfaces of nanophase weathering products. This
process may also occur on Mars, and calculations of
the composition of the amorphous component at Gale
crater using CheMin mineral models and APXS data
show that amorphous material is enriched in volatiles,
including S [7-10]. Here, we examine the ability to
detect chemisorbed sulfate and phosphate complexes
by analyzing sulfate- and phosphate-adsorbed
nanophase weathering products using instruments similar to those on landed and orbital missions.
Methods: We synthesized two nanophase weathering products that are common in terrestrial volcanic
soils and have been identified on the martian surface:
allophane and ferrihydrite (a npOx). We adsorbed sulfate and phosphate anions separately onto the mineraloid surfaces using techniques outlined by [11-13].
We analyzed the untreated and ion-adsorbed materials
using instruments similar to those on landed and orbital
Mars missions, including X-ray diffraction, evolved
gas analysis (EGA), Mössbauer spectroscopy, and TIR
emission and VNIR reflectance spectroscopy to determine whether adsorbed sulfate and phosphate complexes are detectable in martian datasets. VNIR spectra
were measured under lab air and at room temperature
and also in a glove box purged with dry N2 gas at room
temperature, 110 °C, and 220 °C to measure spectral
signatures under desiccating conditions as might be
encountered on Mars.
Results: X-ray diffraction patterns of untreated
synthetic allophane and ferrihydrite demonstrate that
these materials lack long-range crystallographic order
because their patterns display a few broad peaks with
low intensities (data not shown). The patterns of the
synthetic materials are similar to those of natural allophane and ferrihydrite [14,15]. The adsorption of sulfate and phosphate anions onto allophane or ferrihydrite does not affect the position of the XRD peaks and
generally does not affect peak intensities.
Evolved gas analyses of untreated and ion-adsorbed
nanophase weathering products show the most intense
releases from mass/charge (m/z) 18 (i.e., H2O) and
show minor releases from adsorbed complexes (Fig. 1).
The releases of H2O from allophane at ~120 and ~275
°C are from the removal of adsorbed and structural
H2O, respectively. The release of H2O from ferrihydrite
at ~50-350 °C is from the loss of adsorbed and structural H2O as the ferrihydrite transforms to hematite
[14]. EGA of m/z 48 (i.e., SO) from sulfate-adsorbed
ferrihydrite and allophane display relatively hightemperature releases (>400 °C and >900 °C, respectively); however, analyses of phosphate-adsorbed ferrihydrite and allophane do not show definitive evidence
of the presence of phosphate because phosphate decomposition temperatures exceed our maximum measurement temperatures.
Results from Mössbauer spectroscopy show that the
untreated ferrihydrite spectrum is similar to the phosphate- and sulfate-adsorbed spectra (data not shown).
Mössbauer spectroscopy was not performed on the
allophane samples because allophane does not contain
Mössbauer active elements (e.g., Fe).
Near-IR spectra of untreated and ion-adsorbed
nanophase weathering products show broad spectral
bands from overtones and combinations of the fundamental vibrations of OH and H2O, from (Si,Al)OH
bending vibrations in the case of allophane, and from
Fe2OH bending vibrations in the case of ferrihydrite.
46th Lunar and Planetary Science Conference (2015)
Ion-adsorbed allophane and ferrihydrite spectra are
similar to those of the untreated phases (data not
TIR spectra of allophane samples have broad bands
from Si-O stretching and bending vibrations and from
Al-O-Si and Al-OH deformation vibrations (Fig. 2A).
Sulfate- and phosphate-adsorbed allophane spectra
have shoulders at ~1050-1200 cm-1 from S-O and P-O
stretching vibrations, respectively. TIR spectra of ferrihydrite samples have broad bands from Fe-O stretching
and bending vibrations, and spectra of ion-adsorbed
ferrihydrite have Christiansen features at higher wavenumbers than the untreated ferrihydrite (Fig. 2B). The
position of the Christiansen feature of the sulfateadsorbed ferrihydrite is similar to those of Fe-sulfate
minerals reported by [16].
Discussion: Our analyses of sulfate and phosphate
adsorbed onto allophane and ferrihydrite with laboratory instruments that correspond to those on martian orbiter and landed missions indicate a range in detectability of chemisorbed ionic species on nanophase weathering products on the martian surface. EGA measurements, which were implemented as a part of the Phoenix lander mission and the MSL SAM instrument
package on the Curiosity rover, easily distinguish allophane and ferrihydrite from their forms chemisorbed
with sulfate because the method directly detects
evolved S-bearing species. Manifestations of chemisorbed sulfate and phosphate anions are also observed
in TIR spectra. XRD powder patterns and VNIR and
Mössbauer spectra show no apparent evidence for
chemisorption at the concentration levels of our experiments.
References: [1] Rampe E. B. et al. (2012) Geology, 40, 995-998. [2] Bishop J. L. and Rampe E. B.
(2014) LPS XLV, #2068. [3] Clark B. (2005) Earth
Planet. Sci. Lett., 240, 73-94. [4] Ming D. W. et al.
(2006) JGR, 111. [5] Morris R. V. et al. (2006a) JGR,
111. [6] Morris R. V. et al. (2006b) JGR, 111. [7] Bish
D. L. et al. (2013) Science, 341. [8] Blake D. F. (2013)
Science, 341. [9] Vaniman D. T. et al. (2014) 343. [10]
Morris R. V. et al., this meeting. [11] Cichota R. et al.
(2007) Soil Sci. Soc. Am. J., 71, 703-710. [12] Jara A.
A. et al. (2006) Soil Sci. Soc. Am. J., 70, 337-346. [13]
Willet I. R. et al. (1988) J. Soil Sci., 39, 275-282. [14]
Schwertmann U. and Cornell R. M. (2000) WileyVCH. [15] Wada K. (1989) SSSA Book Series, no. 1.
[16] Lane M. D. et al. (2015) Am. Mineral., 100, 6682.
Figure 1. EGA data for untreated and ion-adsorbed A)
allophane and B) ferrihydrite.
Figure 2. TIR emission spectra of untreated and ionadsorbed A) allophane and B) ferrihydrite.