Radiation-Enhanced Aqueous Dissolution of Minerals.
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2010 MRS Fall Meeting
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Baragiola, Raul; University of Virginia, Materials Science and
Engineering
Dukes, Catherine; University of Virginia, Materials Science and
Engineering
radiation effects, ion-solid interactions, surface reaction
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Radiation-Enhanced Aqueous Dissolution of Minerals
Catherine A. Dukes1 and Raúl A. Baragiola1
1
Laboratory for Atomic and Surface Physics, University of Virginia, Materials Science and
Engineering, 395 McCormick Road, Charlottesville, VA 22904
ABSTRACT
Mineral samples of varying petrology, exposed to ion irradiation and subsequently
immersed in water or exposed to a humid environment, show up to 60% depletion of specific
surface atoms (Mg, Ca, K, and Na) — a depletion that is enhanced 26,000x compared to unirradiated surfaces. Surface depletions of irradiated minerals exposed to water were measured using
X-ray photoelectron spectroscopy. Irradiations were performed with 4 keV Ar+ ions at fluences
from 1014 – 1019 ion cm-2; samples were subsequently exposed to liquid water or humid air (35º
C and 70% RH). Analyses were done before irradiation, after irradiation, and after exposure to
water, allowing identification of changes in composition due solely to ion irradiation or
combined with water exposure. Before water exposure, we observe no significant change in
stoichiometry of the minerals for ion fluences <1018 ions cm-2. We find incongruent depletion of
60% Mg for forsterite after exposure to humidity or three minutes (or more) water immersion.
Augite undergoes reduction in the surface concentration of approximately 30% Mg, 40% Ca, and
55% Na after 1.9 x 1017 Ar cm-2 and immersion in HPLC water (pH: 6.8) for three minutes.
Depth profiles of the irradiated, water exposed, minerals show that the depth of the depleted
region is on the order of the ion range, ~15nm. In addition, preliminary results for albite, anorthoclase, and microcline in water show significant depletions of Na, Na and K, and K, respectively, from the mineral surface.
INTRODUCTION
This research originated in the laboratory simulation of space weathering of minerals in
space. Space weathering is the alteration of the chemical composition of surfaces of airless
bodies by the relentless impact of energetic photons, ions, and electrons from the sun, as well as
meteorites and cometary fragments. Space weathering can be studied by examining extraterrestrial materials from sample return missions or by laboratory simulations using lunar, meteoritic,
or planetary surface analogues. In such studies, it is critical to minimize alterations of the
surfaces by environmental factors during sample handling.
Space weathering effects on the structure and composition of minerals have been the focus
of a significant number of laboratory studies [1]. In particular, to correlate measurements of
VIS/NIR reflected light with surface chemistry and mineralogy. Bradley [2] measured the
compositions of interplanetary dust particles (IDPs) and chondritic meteorites using energy
dispersive X-ray analysis (EDX) on a transmission electron microscope (TEM). Cross sections
of IDPs revealed a depletion of magnesium and calcium relative to oxygen from the weathered
silicate surfaces with respect to the bulk. A similar Mg depletion of olivine from the Allende
meteorite was measured after 20 keV proton irradiation and attributed to non-stoichiometric
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sputtering of the mineral surface [2]. Demyk et al. [3] measured with EDX a 30% reduction in
the Mg:Si ratio of olivine due to bombardment with 4 and 10 keV He ions. Similar results were
reported by Toppani et al. [4] in silicate samples analyzed ex situ by EDX. In contrast, Dukes et
al. [5] showed that keV ion bombardment of olivine does not significantly change the relative
concentrations of Mg, Fe, O, and Si as measured by X-ray photoelectron spectroscopy (XPS) in
situ; i.e., without removing the sample from vacuum. We also note that Jӓger et al. [6] found no
change in chemical composition of enstatite (MgSiO3) due to 50 keV He+ bombardment in
contrast with the Mg depletion reported by Bradley [2].
This conundrum has important implications for the curation and analysis of returned planetary, cometary, or meteoritic samples. Planetary samples that have been exposed to solar wind,
magnetospheric or ionospheric ion bombardment may be, upon return to Earth, subject to atmospheric humidity during specimen analysis or to liquid water during laboratory processing and/or
analysis. With any exposure to water, returned samples will not maintain their intrinsic (extraterrestrial) surface stoichiometry.
In another context, nuclear waste management where glasses are used to contain radioactive
material from entering the biosphere, it is important to ascertain the effect of groundwater on
waste glasses. Numerous studies of aqueous alteration (leaching, dissolution) of complex oxides
subject to high-energy ion bombardment have been performed [e.g. 7]. These studies typically
employ very high-energy ions (MeV to 100s of MeV), and discussion of the results focuses on
the correlation between dissolution rate and amorphization rate, given the assumption that the
open structure of amorphized material favors the transport of atoms in the leaching process. A
wide range of responses has been reported, from essentially no effect to 10-100 enhancement in
dissolution rates for 840 MeV Kr ions [8].
EXPERIMENTS
We determined the surface chemical composition and stoichiometry of irradiated-minerals
exposed to water, using X-ray photoelectron spectroscopy (XPS). This surface sensitive
technique provides quantitative compositional information about the outermost 3-5 nm of a
surface. For these experiments Al X-rays (1486.6 eV) impinge upon the sample surface, penetrating to depths of about ~1micron, and interact with the surface atoms, ejecting photoelectrons.
The photoelectrons are detected by a cylindrical mirror electron energy analyzer (Physical
Electronics 560 ESCA/SAM). Spectra were taken in survey (ΔE = 3.2 eV) and high-resolution
modes (ΔE = 0.8 eV) to assess surface stoichiometry and changes in chemistry.
The analysis/processing chamber and analyzer were maintained at ultrahigh vacuum (10-9
Torr) throughout these series of experiments. Irradiations were performed at normal incidence in
situ with 4 keV Ar ions at fluences from 1014 to 1019 ion cm-2 in the XPS system. Differential
charging across the mineral surface was minimized using an electron flood gun, which provides
low energy electrons (< 2 eV) for charge neutralization during irradiation and analysis. Subsequent to irradiation, samples were removed from vacuum and immersed in liquid water or
exposed to humid air (35º C and 70% RH) in a regulated environmental chamber for varying
times.
After exposure, the samples were reintroduced into the analysis chamber. XPS measurements
were done before irradiation, after irradiation, and after exposing irradiated samples to water or
humidity, allowing identification of changes in composition due solely to ion irradiation and
those that required water exposure. We find that the stoichiometry of the minerals did not change
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significantly under ion irradiation for fluences <1018 ions cm-2 for 4 keV Ar+ [1]. However, even
at these low energies, changes in surface chemistry, i.e., Fe2+ → Fe0, have been observed [5]. A
small number of unirradiated samples were also exposed to aqueous or humid environments for
varying times to quantify the sole effect of water on a surface. We find no significant change in
the surface stoichiometry or chemistry due exclusively to water in unirradiated materials for
exposure periods less than one hour (aqueous and gaseous).
Fig. 1: XPS analysis and irradiations are done in the same ultra high vacuum (10-9 Torr)
chamber. Samples are removed from the chamber and immersed in liquid water or exposed to a
controlled humid environment in a dedicated environmental chamber.
RESULTS
We find that irradiated forsterite immersed in liquid water loses surface Mg relative to Si as
a strong function of ion fluence (Fig. 2). Olivine, irradiated with 1014 - 1018 Ar cm2, and subsequently immersed in water for 3 minutes loses up to 40% of its surface Mg relative to Si. This
implies that damage due to ion irradiation has a strong effect on the long-term degradation of
minerals, in a manner similar to the corrosion of glass --- a problem important within the nuclear
industry, which uses glass to enclose waste.
Fig. 3 shows that, after irradiation with ~1× 1017 Ar cm-2, the Mg:Si ratio decreases only
weakly with time in aqueous solution, reaching a steady state after 2 minutes. Further water
exposure does not affect the depleted Mg:Si ratio. However, even very rapid exposure to liquid
water (~1 s) causes ~10% Mg loss relative to Si. This implies that even rinsing irradiated olivine
will change the surface stoichiometry in a measureable manner.
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Fig. 2. Effect of Mg surface depletion (relative to Si) as a function of total ion fluence. All
samples were immersed in HPLC water for three minutes. The band gives the range of Mg:Si
ratios for unirradiated samples.
Fig. 3. Mg surface depletion (relative to Si) in forsterite irradiated with 1017 Ar cm-2 versus time
of subsequent immersion in liquid HPLC water.
Similar to the depletion observed in liquid water, we find the incongruent loss of 60% Mg
relative to Si in forsterite after exposure to humid air (70% RH; temp: 35 ̊C) for 20.8 days (Fig.
4a). To ascertain that the loss was catalyzed by irradiation, an analogous experiment was
conducted with unirradiated forsterite. We measure a less than 2% change in the Mg:Si ratio
after 28.2 day exposure to humid air (70% RH; temp: 35 ̊C), shown in Fig 4b. The effectiveness
of humid air is probably due to capillary condensation of water on the rough, sputtered surfaces.
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Fig. 4. (a) Effect of ion irradiation and ion irradiation plus humidity on irradiated natural
forsterite. No Mg loss is observed after 1017 Ar+ cm-2. Exposure of the irradiated sample to
humid air, produces a ~60% loss of Mg relative to Si from the mineral surface. (b) No significant
Mg loss relative to Si is observed in unirradiated forsterite exposed to humid air for 28.2 days.
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Fig. 5. After a five minute water immersion, irradiated (2 x 1017 Ar+ cm-2) augite shows loss of
Ca, Mg, and Na. Only Fe did not show a clear loss with respect to Si. Similar behavior was
observed for albite and anorthoclase.
We also performed measurements with augite [(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6], an inosilicate
found in many terrestrial igneous and metamorphic rocks as well as in lunar material, to ascertain
the combined effect of irradiation and water on the surface of a pyroxene. XPS spectra show
cation reductions in surface concentration of approximately 30% Mg, 40% Ca, and 55% Na after
1.9 x 1017 Ar/cm2 and five minutes immersion in HPLC water, pH: 6.8 (Fig. 5). Preliminary
results for water-exposed, irradiated albite and anorthoclase show preferential depletions of Na+,
and Na+ and K+, respectively, from the mineral surfaces.
DISCUSSION
Previous experiments by a number of investigators have shown contradictory evidence of
cation depletion at the surface of irradiated materials [2-5]. Toppani et al saw significant cation
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loss near the surface of IDP minerals. This contrasts with Dukes et al and Jäger et al, who saw no
stoichiometric change in the surface of their irradiated silicates. We suggest that the difference
between these measurements can be explained by water exposure — either atmospheric or
perhaps rinsing during sample preparation.
That water may preferentially remove ions from the surface of minerals is well known and has
been studied extensively within the geological community [ie.9]. Casey and Bunker have shown
that the dissolution rate depends strongly on pH as well as on the Mg++ linkage to the native
silicate bonds at the surface. One likely mechanism for this process is the exchange of protons
for surface cations, leached from the exposed layer, as 2H+→ Mg2+, which preserves the charge
neutrality of the surface. We suggest that the radiation damage produced by energetic ion irradiation enhances the penetration of protons into the mineral by breaking the bonds between the
Mg++ and the silicate tetrahedra. To substantiate this hypothesis, we measured the Mg content of
the rinse water, used to dip the forsterite sections, with ion chromatography. A distinct correlation between immersion time and Mg content in the HPLC was recorded in Fig. 6, supporting a
cation exchange mechanism, but not determining the mode (H+, H2O+, or H3O+).
Fig. 6. Ion chromatography data gives the ion concentration in a known volume of water. The
Mg concentration increases with olivine immersion time, as expected from a proton exchange
model.
Depth profiles of soda lime glass exposed to humidity but unirradiated, suggest that proton
exchange is a reasonable interpretation for Mg, Ca, Na, and K loss at the surface. Secondary ion
mass spectrometry (SIMS) measurements showed a depleted region at and just below the surface
for Na in unirradiated, humidity-exposed, soda lime glass and a corresponding increase in
hydrogen within the same region [10]. Additionally, these SIMS measurements, calibrated
against a standard glass, found that three Na atoms are removed for each proton. This number is
an upper limit, however, since other hydrogen bearing molecules were not measured. Nuclear
reaction analysis (NRA) also showed surface concentration enhancement of hydrogen in soda
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lime glass placed in a water bath [11], with three hydrogen atoms gained for each Na atom lost.
This suggests that either H2O begins to diffuse into the surface after an initial proton exchange or
that H3O+ diffusion inward occurs simultaneous with Na+ diffusion outward. We plan to investigate this mechanism in more detail in future studies, using additional surface science techniques.
The depth of the NRA measured depletions (for unirradiated glasses) depends strongly on
water immersion time and varies from <100 to >500 nm for bath times of 3 - 500 hours [11]. A
second study of water immersed float glass, exposed for 14 - 28 days, finds hydrogen penetration
to ranges of ~50nm [12]. In either case, these penetration depths are much larger than those
determined in irradiated forsterite (~15 nm), immersed for 3 minutes; however, this is reasonable
if diffusion, which depends strongly on petrology and exposure time, is an important mechanism
in the depletion process.
Comparison of depletion rates for Mg in forsterite between irradiated and unirradiated
material finds irradiated forsterite immersed in liquid water loses surface Mg relative to Si at a
rate 26,000x faster than unirradiated forsterite [1]. For unirradiated specimens, the steady state
Mg dissolution rate is ~1.1 × 10-14 moles cm-2 s-1 or ~6.6 × 109 atoms cm-2 s-1 in neutral solutions
at room temperature [13,14]. XPS depth profiles of the forsterite sections measure a depletion
region thickness of ~15 nm, roughly equivalent to the ion penetration range based on TRIM
calculations [15]. Thus, for our olivine samples, a 15 nm thick altered layer loses on average 2.0
× 1016 Mg cm-2 (half the initial value) in 2 min (Fig 2.). This gives a specific dissolution rate of
1.7 × 1014 atoms cm-2 s-1, enormously greater than for unirradiated samples.
CONCLUSIONS
The surface stoichiometry of irradiated minerals is altered by exposure to water, both in
liquid and gaseous form. Therefore, any irradiated material or returned samples exposed to the
solar wind must be stored in vacuum or in an inert (entirely anhydrous) atmosphere to retain
intrinsic surface composition. Any leak to air or slow water outgassing may alter the surface
chemistry of that particular sample. In addition, laboratory analysis of the surface of lunar
material exposed to the solar wind irradiation must be analyzed without exposure to the earth’s
atmosphere, and sample handling and preparation techniques must not include rinsing in water.
Likewise, it is important that laboratory simulations of extraterrestrial processes involving
irradiation are analyzed without subsequent exposure to air.
ACKNOWLEDGMENTS
The authors would like to thank Suzanne Maben (Environ. Sci. Dept., UVa) for her assistance with the ion chromatography data, and acknowledge the support of NASA’s Cosmochemistry (NNX08AG72G) and LASER (NNX08AX11G) programs.
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