text:
11institutetext: Institute for Geological and Geochemical Research, Research
Centre for Astronomy and Earth Sciences, HUN-REN, Budapest, Hungary
22institutetext: HUN-REN, Institute for Nuclear Research Debrecen, Hungary
33institutetext: Konkoly Thege Astronomical Institute, Research Centre for
Astronomy and Earth Sciences, HUN-REN, Budapest, Hungary
44institutetext: CSFK, MTA Centre of Excellence Budapest, Konkoly Thege Miklós
út 15-17., H-1121, Hungary
55institutetext: Eotvos Lorand University of Sciences (ELTE) Budapest, Hungary
66institutetext: Geographical Institute, Research Centre for Astronomy and
Earth Sciences, HUN-REN, Budapest, Hungary
77institutetext: Department of Systems Innovation, School of Engineering, The
University of Tokyo, Japan
88institutetext: Atomki, Institute of Nuclear Research, 4026, Debrecen,
Hungary

# Irradiation induced mineral changes of NWA10580 meteorite determined by
infrared analysis

I. Gyollai 1144 S. Biri 22 Z. Juhász 22 Cs. Király 4466 B. D. Pál 773344 R.
Rácz 88 D. Rezes 334455 B. Sulik 22 M. Szabó 1144 Z. Szalai 4466 P. Szávai
4466 T. Szklenár 3344 Á. Kereszturi , Corresponding author:
<EMAIL_ADDRESS>
(Received September 15, 1996; accepted March 16, 1997)

###### Abstract

Context.
Identifying minerals on asteroid surfaces is difficult as space weathering
modifies the minerals’ infrared spectra. This should be better understood for
proper interpretation.

Aims. We simulated the space weathering effects on a meteorite and recorded
the alterations of the crystalline structure, such as the change in peak
positions and full width at half maximum values.

Methods. We used proton irradiation to simulate the effects of solar wind on a
sample of NWA 10580 CO3 chondrite meteorites. After irradiation in three
gradually increased steps with 1 keV ion energy, we used infrared microscopic
reflectance and diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) to identify and understand the consequences of irradiation.

Results. We find negative peak shifts after the first and second irradiations
at pyroxene and feldspar minerals, similarly to the literature, and this shift
was attributed to Mg loss. However, after the third irradiation a positive
change in values in wavenumber emerged for silicates, which could come from
the distortion of SiO4 tetrahedra, resembling shock deformation. The full
width at half maximum values of major bands show a positive (increasing) trend
after irradiations in the case of feldspars, using IR reflection measurements.
Comparing DRIFTS and reflection infrared data, the peak positions of major
mineral bands were at similar wavenumbers, but differences can be observed in
minor bands .

Conclusions. We measured the spectral changes of meteorite minerals after high
doses of proton irradiation for several minerals. We show the first of these
measurements for feldspars; previous works only presented pyroxene, olivine,
and phyllosilicates.

###### Key Words.:

techniques: imaging spectroscopy – meteorites, meteors, meteoroids – solar
wind

##  1 Introduction

Space weathering effects on airless body surfaces include irradiation,
implantation, sputtering processes due to galactic and solar energetic
particles, UV irradiation, meteorite impacts, and temperature fluctuations.
These processes modify the appearance and crystalline structure of minerals as
well as the composition of asteroid surfaces due to mechanical fracturing,
melting, ablation, ion implantation, and destruction of the crystalline
lattice, leading to amorphization.

The speed of the solar wind strongly influences the consequences of space
weathering; it also influences the potential energy of the solar wind ions and
the total witnessed dose. The fluence values are representative for solar wind
exposure timescales (e.g., Dukes et al. 1999; Vernazza et al. 2013; Lantz et
al. 2015; Lazzarin et al. 2006; Brunetto et al. 2014; Lantz et al. 2017).
Since the timescales of laboratory tests are substantially shorter than
typical astrophysical exposure durations, higher doses and more intense
radiations are used to gain realistic results. In laboratory experiments the
irradiations can be characterized by various physical quantities. The flux,
which is the number of ions per surface area deposited during the unit time,
determines the rapidity of the surface processing. The induced change is a
function of the total fluence, which is the total number of ions per surface
area deposited during the irradiation. Alternatively, the total deposited
energy per surface area can be used, or the dose, which is the absorbed energy
by a unit mass of the irradiated material. The recorded infrared spectral
curves before and after irradiation tests indicate modifications caused by
mineral changes, which we discuss in this work.

We summarize earlier irradiation test experiments below to provide context.
Both infrared and Raman spectroscopy techniques provide information on the
mineral structures of the targets, usually different sample materials were
measured (e.g., olivine, Lantz et al. (2017); polystyrol and olivine,
Kaňuchová et al. (2017)). The OH loss from minerals can be better detected by
infrared spectroscopy, and less crystallized minerals (clay minerals,
ferrihydrite) have a more extended literature background using infrared
spectroscopy. With Fourier transform infrared (FTIR) spectroscopy using the
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method,
almost bulk (millimeter- to centimeter-sized) meteorite chips were usually
measured (Lantz et al., 2017; Brunetto et al., 2014, 2020). The FTIR spectra
of Frontier Mountain 95002 and Lancée meteorites are similar, and they both
showed typical olivine bands (830–860 cm -1) corresponding to a composition of
Fo-50–60 (Hamilton, 2010) before irradiation, which shifted to Fo-30–35 after
irradiation (Brunetto et al., 2020). This was interpreted as Mg loss from
olivine after irradiation caused by amorphization of the structure. The
doublets observed at about 500 cm -1 (20 $\mu$m) and 400 cm -1 (25 $\mu$m) in
CO/CV and at about 450 cm -1 (22.2 $\mu$m) (saponite) in CI/C2 are modified by
irradiation (Brunetto et al., 2020). Ion irradiation experiments on Tagish
Lake using 1 $\times 10^{16}$ He+ cm-2 at 200 keV reported (Vernazza et al.,
2013) Mg-rich phylosilicate band shifts of about 0.3 $\mu$m for the 9.8 $\mu$m
(10-20 cm -1), and about 0.15 $\mu$m for the 22.3 $\mu$m (448 cm -1) band
(Vernazza et al., 2013). The main band at about 445–448 cm -1 (22.32–22.47
$\mu$m), measured on the Mighei meteorite, shows the largest spectral shift
(Brunetto et al., 2020) due to irradiation.

Ion irradiation is known to amorphize silicates, as shown by IR-spectroscopy
analysis (Demyk et al., 2004). Dukes et al. (2015) observed an increase in the
Fe/Si ratio on their Tagish Lake sample after ion irradiation. Lantz et al.
(2015) found shifts of the phyllosilicate and olivine bands near 2.7–10 $\mu$m
toward the Fe-rich spectral region, suggesting a loss of the element Mg,
probably due to a preferential replacement by Fe. The spectral shift of the IR
bands could also be due to a loss of magnesium, as a result of preferential
sputtering (when the somewhat more volatile Mg is more easily sputtered
backward than the heavier Fe) (Hapke et al., 1975), possibly leading to
amorphization indicated by the broadening of the bands, seen and confirmed by
the Raman method (Brucato et al., 2004; Demyk et al., 2004).

Here our aim is to identify and to understand the consequences of artificial
solar wind simulating proton irradiation on a sample of NWA 10580 CO3
chondrite meteorites, and the infrared analysis based mineral changes. Since
the cosmic weathering has a substantial influence on the spectra of meteorite
host asteroids to a large extent, we focused on artificial irradiation to
understand the spectral modifications, and applied three gradually increased
irradiation levels. In addition, the survey of the observability of different
bands at different spectral resolutions also provides information on the
observational capabilities that are needed for an infrared detector on board
any future asteroid mission that aims to identify the consequences of space
weathering. Although several works on mineral changes caused by artificial
irradiation have been published, few of them apply gradually increasing
irradiation as we did to follow the alteration process closely. In addition,
while in most of the previous works bulk infrared analysis after irradiation
was performed, in this work separate minerals were surveyed.

##  2 Methods

We used two basic strategies for this work. The first was to irradiate the
meteorite by gradually increased doses of solar wind simulating artificial
particle bombardment, and the second was to follow the changes of its infrared
spectral characteristics by comparing the measurements both before and after
each irradiation action.

We analyzed a 11x8x2 mm section of the NWA 10580 CO3 chondrite meteorite in
this work. This is a poorly altered primitive meteorite with unweathered
material, mainly composed of forsteritic olivine, enstatite, and diopside,
with many small chondrules, very few CAIs, sulphide blobs, all embedded in a
fine-grained matrix. The surface was rough plain shaped by cutting process but
not polished, resulting a moderately flat surface with submillimeter
undulations. The same side of this meteorite slice was irradiated and measured
subsequently in three steps. The first measurement was made before irradiation
to see its intact infrared spectra; after each irradiation the same sequence
of measurements was realized with DRIFTS and reflection methods. By using
DRIFTS the bulk spectra were recorded. Measurements using the microscope-based
reflection method were realized of an $\sim$0.1$\times$0.1 mm subsection of
the meteorite sample.

For infrared spectroscopy and microscopy we used a Vertex 70 FTIR spectrometer
and a Hyperion 2000 microscope with 15x IR objective in reflection mode (Biri
et al., 2012). For the measurements we did 32 scans in the 400-4000 cm-1
range, with 30 s duration at 4 cm-1 spectral resolution. We used the Bruker
Optics’ Opus 5.5. software to manipulate the spectra (e.g., baseline
correction, atmospheric compensation). The absorbance maximum observed in this
work is at the position in wavenumber corresponding to the strongest v1 SiO4
vibration. The areal view of the IR 15x objective is 200 $\mu$m and this
method is applicable for polished rock samples. Minerals were identified by
the RUFF Database and the Crystal Sleuth software (Lafuente et al., 2016), as
well as by the relevant publications (see Fig. 1. for example peaks).

Figure 1: Example spectra of meteorite NWA 10580 with the main peaks (labels
in bold) and some of the minor peaks before irradiation.

For the DRIFTS measurements we also used the Vertex 70 FTIR spectrometer with
a Praying Mantis diffuse reflectance accessory. The following parameters were
usually used: a spectral resolution of 4 cm-1, a sample scan time of 512 s,
and a covered wavelength range of 4000–400 cm-1. The DRIFTS instrument
operates by directing the IR radiation into a sample cup, where it interacts
with the sample grains. The IR light is reflected or scattered off as it scans
the surfaces, and becomes diffused (Drochner & Vogel, 2012). The spatial
resolution of the measurements was about 1 mm, but definitely below 1.5 mm due
to the infrared spot size on the sample. The IR radiation is reflected from
the sample surface in all directions, and therefore DRIFTS requires a special
mirror arrangement (Mitchell, 1993; Fraser & Griffiths, 1990; Korte, 1988;
Kortum & Delfs, 1964). The output mirror directs this scattered radiation to
the detector in the spectrometer to record the collected IR light as an
interferogram signal, which is used to generate a spectrum. Typically, a
background is collected with the empty cup DRIFTS accessory or filled with
just potassium bromide (KBr).

The full width at halm maximum (FWHM) values or the absorption bands were
determined by manually measuring the width at half height. The peak shifts
were determined in these cases, where the bands appeared before and after
irradiations as well (Tables SOM II/1-2, 5). The irradiation effects of space
weathering were simulated by 1 keV protons produced by the electron cyclotron
resonance (ECR) ion source at ATOMKI with three different levels of fluence:
1011 ion/cm2, 1014 ion/cm2, and 1017 ion/cm2. The samples were irradiated
under vacuum conditions; the pressure was below 1$\times$10-7 mbar in the
experimental chamber. The first irradiation lasted only for a few seconds; the
third lasted for almost one day. In order to perform the different irradiation
steps in reasonable time durations, the ion beam current was set at different
values from 20 nA to 3 $\mu$A. This corresponds to fluxes ranging from
3$\times$1010 to 4.5$\times$1012 ions/cm2/s on the irradiated area of 4 cm2.

##  3 Results

In this section we first present the mineral identifications, then we
summarize the changes in peak positions related to the irradiation series, and
finally we describe the changes in FWHM values caused by the irradiation. In
several cases the measurement series after the irradiation actions did not
show a similar and strong trend, but presented a certain variability in the
measured quantities. Thus, we focus on the general characteristic changes,
which were strong and coherent in the observed cases, and describe them at the
beginning of each subsection. We mention the more stochastic and diverse
changes, which are more difficult to interpret, at the end of each subsection.
We include the related numerical tables as supporting online materials.

###  3.1 Mineral identification

The following main mineral bands and FWHM values were used for statistics:
849, 880 cm-1 for olivine; 1049 cm-1 for pyroxene; 1150 cm-1 for feldspar.
Certain bands can occur in several minerals: 664 in spinel and pyroxene; 1000
and 980 in olivine and feldspar. The FWHM is also influenced by the possible
occurrence of different mineral phases with nearby or overlapping peak
positions. Merged peaks from other mineral phases produce lower FWHM values;
for example, the 670 cm-1 band of spinel has a lower FWHM value, due to the
occurrence of bands of other minerals such as the 700 cm-1 band of pyroxene,
and the 635 cm-1 and 745 cm-1 bands of feldspar.

Figure 2: Reflection spectra of the A2_1 area: before irradiation (0), and
after the first (1), second (2), and third (3) irradiations.

Depending on the amorphization level, the minerals may or may not be detected
by minor bands in addition to the major bands. For example, olivine might show
only one band from a major doublet at a high amorphization level. In general,
the minerals (olivine, pyroxene) had fewer bands after the third (strongest)
irradiation than before (Fig. 2). The first and second irradiations provided
smaller doses and resulted in smaller alterations, and hence the changes in
peak positions could be minor and happen along crystal lattice weakness, along
with possibly increased noise as the changes are partly stochastic. The
consequences of each irradiation action (from before the first irradiation to
after the third) can be seen in the example figures in the subsections below;
in some cases the original trend changed from the early meta-stable state
toward the strongest last irradiation action.

####  3.1.1 Olivine

Before irradiation the bands of olivine appear at 884-894, 913, 940, and 978
cm-1. The same bands appear only after the second irradiation (c1 area spectra
1-6). In some cases (at certain locations) all of the bands disappear after
the irradiation actions (c1 area spectra 7-9, a2 area spectrum 2), and in
other cases the olivine appears only after the irradiation (R1 area spectrum
6, b1 area spectra 4-5, d1 area spectra 5-8) (Table SOM I/5). The 884 cm-1
band has a shift of +6 cm-1 after the first irradiation (d1/1-4), -3 cm-1
after the second irradiation (d1/1-4), and -4 cm-1 after the third irradiation
(b1/5, d1/1-2) (Tables 3,4, Tables SOM I/1-4). In the a2 chondrule the 885
band is detected only after the first irradiation, with a peak shift of -2
cm-1; after the second and third irradiations this band disappears (Tables
3,4). In the a3 chondrule the 849 band shifts by -15 cm-1 after the first
irradiation, increases by 15cm-1 after the second irradiation; the peak shift
is -4 cm-1 after the third irradiation. To summarize, the peak shift could be
observed where the above-described peak positions appear more than once after
the irradiation and do not disappear immediately. Some examples of the changes
can be seen in Fig. 3 and Fig. 4.

Figure 3: Olivine 849 cm-1 peak position changes (blue line, left) and FWHM
changes (red line, right), with the number of irradiations on the Y-axis.
Figure 4: Olivine 890 cm-1 peak position changes (blue line, left) and FWHM
changes (red line, right), with the number of irradiations on the Y-axis.

Depending on the structure and composition of olivine, in most cases the major
bands (between 894 and 837 cm-1) appear. The 890 cm-1 band appears before and
after irradiation as follows: before irradiation, 14 times; after the first
irradiation, 9 times; after the second irradiation, 6 times; after the third
irradiation, 8 times. In general, the appearance of the 890 cm-1 band of
olivine becomes rarer after the irradiations. The 845 cm-1 band shows more
stochastic changes; its appearance before and after irradiation shows the
following numbers: before irradiation, 6 times; after the first irradiation, 3
times; after the second irradiation, 16 times; after the third irradiation, 6
times.

Some minor bands show fluctuations in their appearance before and after
irradiation. The 950 cm-1 band occurs three times before irradiation, four
times after the first irradiation, and nine times after the second
irradiation. The 970 cm-1 band occurs four times before irradiation and five
times after the second and third irradiations. The 910 cm-1 band occurs two
times before irradiation, four times after the first and second irradiations,
and two times after the third irradiation.

####  3.1.2 Feldspar

In feldspar more bands appear after the third irradiation than before the
irradiations, due to polymerization of SiO2 tetrahedra (R1/1-2, 4-7; c1/7-8,
b1/-3-5, d1/6-7, a2/2). At some measuring points the bands disappear after the
first irradiation and appear after the second irradiation (R1/3-6, b1/4-5,
d1/4-6), or only after the third irradiation (c1/7-8, a4b/1-3) (Table SOM
II/3). In some cases (at some measurement locations), the bands of feldspar
appear after the second and the third irradiations (a1/4-8; see example in
Fig. 5). The peak shifts are described in these areas, where the bands appear
before and after irradiation (Tables SOM II/1-2, Table 5.)

Figure 5: Feldspar 1149 cm-1 peak position changes (blue line, left) and FWHM
changes (red line, right), with the number of irradiations on the Y-axis.

Both the major and minor bands appear mostly before irradiation and after the
third irradiation. The disappearance of the minor bands (1000, 980 cm-1) is
seen after the first and the second irradiations. However, the minor band at
750 cm-1 appears more times after the first and the second irradiation, and
disappears after the third irradiation. The band at 630-650 cm-1 disappears
after the first and second irradiations, but appears after the third
irradiation. The 950 and 1000 cm-1 bands appear again after the third
irradiation (Tables SOM II/1-3,Table 5)

####  3.1.3 Pyroxene

Pyroxenes contain generally more bands before irradiation, of which 30%
disappear by the end of the irradiation series (two bands: R1/1-2, d1/1-3; all
bands: (R1/3-7, c1/1, a4b/1-3). Most bands of pyroxene appear more times after
than before the second irradiation (see Table SOM III/3 (c1/1, 4, 5, 6, b1/2 ,
d1/7, a2/1), and in some cases more appear after than before the third
irradiation (see Table SOM III/3 b1/4-5), indicating the formation of a new
mineral phase (probably after olivine) (Delvigne et al., 1979). After the
final irradiation, in most cases fewer pyroxene bands emerge than before
irradiation, indicating the degradation of the crystal lattice after
irradiation (Table SOM III/3 , last column). This feature indicates a
distortion of the crystal lattice, depolimerization of SiO4 tetrahedra (Sharp
& de Carli, 2006). Only the band at 700 cm-1 of all pyroxene bands shows a
continuously disappearing trend: before irradiation it appears 13 times (30%),
but after the third irradiation this band appears only 3 times (0.6%).
However, in several cases the pyroxene appears more times after the third
irradiation (Table SOM III/3, a1); this indicates that a new mineral phase is
formed, but is less crystallized than before the irradiation, indicated by the
minor bands appearing fewer times after the irradiation (Tables SOM III/1-3,
Table 6 last column, e.g., c1/2-3, d1/1-4, a2/1-2 Fig. 2 (a2/1)).

Figure 6: Pyroxene 1050 cm-1 peak position changes (blue line, left) and FWHM
changes (red line, right), with the number of irradiations on the Y-axis.

At some measurement locations all bands disappear after the first irradiation:
(d1/6-10, a4b/1-3, R1/8-10). In other cases, only one band appears before
irradiation, and then disappears after the first irradiation. However, more
bands appear after the second irradiation (R1/4-6, b1/2, d1/7), due to the
dimerization of SiO4 tetrahedra. The bands of pyroxene (1050, 910, 670 cm-1)
appear fewer times after the first irradiation, but the disappearing bands
partly reappear after the second irradiation (see 1050 cm-1 in Fig. 6).

####  3.1.4 Spinel

The band of spinel appears in the range 664-680 cm-1. In some cases, spinel is
detected only after irradiation (Table SOM IV/3, R1/1-8, a2-1,), but in other
cases it does not appear after the third irradiation (c1/1-3). Spinel occurs
both before and after irradiation in the d1 chondrule (d1/1-5), and in the
a5/1 area (Table 7).

Figure 7: Spinel 668 cm-1 peak position changes (blue line, left) and FWHM
changes (red line, right), with the number of irradiations on the Y-axis.

The major band of spinel at 670 cm-1 appears 38 times before irradiation, 18
times after the first irradiation, 35 times after the second irradiation, and
41 times after the third irradiation (Table SOM IV/3). The peak shift of
spinel shows mostly negative values after the first irradiation, but the peak
shift changes to a positive trend after the second and third irradiations
(Fig. 7, Tables SOM IV/1-2 Table 7 ). Spinel shows a decreased FWHM after the
first irradiation (in more measuring points, see Table 1 and the SOM tables),
which changes to a positive trend after the second and third irradiations.

###  3.2 Spectral curve pair examples

Figure 8: Spectral curve pair examples demonstrating the changes before (blue,
bottom) and after (red, top) the strongest irradiation. The main mineral peaks
are labeled with letter pairs.

Several examples based on the spectral curves of the observed irradiation-
induced changes are visible in Figure 8, where the blue (lower) curve is the
original (nonirradiated) state, while the upper (red) curve is the most
heavily irradiated state. The differences between the original and irradiated
states can be seen with the following features:

  * •

disappearance of feldspar, and substantial weakening of pyroxene band (A21
location);

  * •

disappearance of feldspar and pyroxene, weakening of olivine and spinel bands
(A21 location);

  * •

peak shift and increased FWHM of pyroxene (A41 location);

  * •

increased FWHM of spinel (A41 location);

  * •

merging of two major bands of feldspar into a single band, and a shift of the
pyroxene major band to a higher wavenumber with increased FWHM after
irradiation (A52 location);

  * •

disappearance of olivine main bands, disappearence of the 700 cm-1 minor band
of pyroxene, weakening of spinel after irradiation (A52 location).

###  3.3 FWHM observations

Only feldspar shows an increased FWHM of the major bands after all the
irradiations (Table 1), while the other minerals show different and variable
trends. Pyroxene shows decreased FWHM values after the third irradiation in
the case of the 1050 cm-1 band (SOM tables), whereas after the first and the
second irradiations it shows rather increased FWHM values. Olivine, at the 887
cm-1 band, shows a decreased FWHM after the first and the second irradiations,
changing to a moderate increase after the third irradiation. However, at the
other major band of olivine: 849 cm-1 the FWHM shows an increase after the
first irradiation, changing to a decrease after the second and the third
irradiations. Consequently, a clear signature of disordering of the
crystalline lattice can be observed only in feldspar and in the case of the
other minerals the FWHM related changes are not enough for a firm conclusion.

Table 1: Summary of FWHM main bands mineral | spectrum | band | before irr. | 1st irr. | 2nd irr. | 3rd irr. | avg. after irr.
---|---|---|---|---|---|---|---
spinel | average | 668 | 56.3 | 43.2 | 57.7 | 79.2 | 60
spinel | min | 660 | 7 | 7 | 5 | 17 | 9.7
spinel | max | 679 | 130 | 140 | 150 | 130 | 140
spinel | median | 667 | 70 | 15.5 | 50 | 80 | 48.5
| average | 668.5 | 65.8 | 51.4 | 65.7 | 76.6 | 64.6
pyroxene | average | 1047.5 | 27.7 | 21.4 | 21.5 | 20.3 | 21.1
pyroxene | min | 1035 | 5 | 7 | 4 | 5 | 5.3
pyroxene | max | 1065 | 54 | 40 | 40 | 40 | 40
pyroxene | median | 1046 | 30 | 20 | 23.5 | 20 | 21.2
| average | 1048.4 | 29.2 | 22.1 | 22.3 | 21.3 | 21.9
feldspar | average | 1152.12 | 78.7 | 88.2 | 92.1 | 99.2 | 93.2
feldspar | min | 1116 | 12 | 42 | 7 | 70 | 39.7
feldspar | max | 1183 | 130 | 130 | 150 | 130 | 136.7
feldspar | median | 1148 | 85 | 85 | 90 | 100 | 91.7
| average | 1149.8 | 76.4 | 86.3 | 84.8 | 99.8 | 90.3
olivine | average | 887.75 | 11.9 | 8.4 | 6.3 | 12 | 8.9
olivine | min | 883 | 7 | 5 | 5 | 3 | 4.3
olivine | max | 894 | 18 | 15 | 8 | 30 | 17.7
olivine | median | 885.5 | 11 | 5 | 6.5 | 3 | 4.8
| average | 887.6 | 12 | 8.4 | 6.5 | 12 | 8.9
olivine | average | 849.2 | 9.7 | 17.5 | 6.1 | 4.3 | 9.3
olivine | min | 849 | 5 | 16 | 4 | 3 | 7.7
olivine | max | 850 | 16 | 19 | 12 | 5 | 12
olivine | median | 849 | 8.5 | 17.5 | 5 | 5 | 9.2
| average | 849.3 | 9.8 | 17.5 | 6.8 | 4.3 | 9.5

Spinel shows a decreased FWHM after the first irradiation, changing to a
positive trend after the second and the third irradiations. The Fe-bearing
minerals (spinel, olivine, pyroxene) show a decreasing trend on average after
the irradiations compared to pre-irradiation states. Only feldspar shows an
increased FWHM of its major band by calculation of the average FWHM of all of
irradiations (Table 1). Pyroxene shows a continuous negative trend in FWHM
values. Feldspar shows a positive trend in FWHM due to the deformation of
crystal structure after irradiations. The 887 cm-1 band of olivine shows a
negative trend after the first and second irradiations, changing to a moderate
increase after the third irradiation. However, for the other major band at 849
cm-1 the FWHM shows an increase after the first irradiation, which changes to
a decrease after the second and third irradiations.

The major bands of olivine (844 and 888 cm-1) and feldspar appear before and
after the first and second irradiations. The major band of pyroxene (1050)
appears before and after all of the irradiations, except spectrum 2, which
does not appear after the second irradiation (Table SOM 0/1, Tables SOM
V/1-2).

###  3.4 General trends

Below we evaluate the peak position and FWHM changes together of several
minerals. The Fe-bearing minerals (spinel, olivine, pyroxene) show an average
decreasing trend in band positions after irradiation. Not all mineral bands
appear after all of the irradiation actions, and so the calculated average
peak positions and FWHM values are indicated only in those cases where the
given peaks emerged after all the irradiations.

Olivine: The major 844 cm-1 band shows a negative trend in the peak shift, but
an increased FWHM, while the other major band at 888 cm-1 appears only after
the second irradiation for most DRIFTS data. The olivine minor band at 987
cm-1 appears only before irradiation. The other minor bands of olivine below
600 cm-1 (which could be detected only by DRIFTS) occur after all of the
irradiations. The 474 cm-1 band shows a decreasing peak shift and an
increasing FWHM after the first irradiation, but after the second irradiation
this trend inverts (showing negative correlation, i.e., increasing peak shift
and decreasing FWHM). The 499 cm-1 band shows a positive trend in peak shift
and FWHM, but after the second irradiation a negative trend can be observed in
both values (Tables SOM V/1-2, Table SOM 0/1).

The pyroxene major band around 1070 cm-1 appears after all of the irradiation
actions, and the peak shift continuously decreases after each irradiation. The
change in FWHM shows a positive trend after the first and second irradiations
(increasing FWHM values), changing to a negative trend after the third
irradiation (decreasing FWHM values). Near this position the band of feldspar
at 1161 cm-1 splits into two bands (1155 and 1188 cm-1) after the third
irradiation, which may influence the major band of pyroxene at 1070 cm-1
because of the sloped baseline. The minor band at 952 cm-1 appears only after
the irradiations. This band shows a lower peak position (wavenumber) and FWHM
shift after the second irradiation than after the first irradiation. It is
characterized by a lower peak shift and higher FWHM value after the third
irradiation compared with the second irradiation.

The major band of feldspar shows a split of peaks into two bands: 1151 and
1184 cm-1 before and after the third irradiation. After the first and the
second irradiations the band appears near 1149 cm-1. The band at 1150 cm-1
shows a negative trend, whereas the 1184 cm-1 band shows a positive trend in
peak and FWHM shift after the third irradiation. The minor bands at 1000 and
640 cm-1 appear only after the first and the third irradiations. Near the 1000
cm-1 band, the 950 cm-1 band (pyroxene) and the 987 cm-1 band (olivine) occur,
which may disturb and overlap this band. The 640 cm-1 band may be disturbed by
the Fe-Mg band of pyroxene at 665 cm-1 causing the disappearance of the 640
cm-1 band. The peak position and FWHM of the 640 cm-1 band after the third
irradiation increases, whereas in the case of 1000 cm-1 band only the FWHM
shows an increase after the third irradiation.

Using DRIFTS observations, the highest number of spectral changes can be
observed after the first irradiation (decrease in intensity, and shift of
bands; see Table SOM 0/1 yellow rows, Tables SOM V/1-2). The pyroxene band at
1050 m-1, the feldspar band at 1150 cm-1, and the olivine band at 844 cm-1
decrease in wavenumber. The major bands of olivine show increasing FWHM
values, whereas the FWHM of feldspar and pyroxene show variable trends. After
the first irradiation, alteration to a metastable phase may have happened, a
partly similar situation to that after a shock effect when the loading time is
too short (a few seconds), which does not produce a final structural
disordering, but only a metastable structure (Sharp & de Carli, 2006).

##  4 Discussion

In this section first we outline how mineral identification was possible
during the irradiation series according to the major bands (see Table 8); the
minor bands were more changeable and appeared or disappeared in a somewhat
stochastic manner. Overlapping of the bands also caused some difficulties: the
major band of spinel overlaps the 670 cm-1 band of pyroxene, and the minor
970-980 cm-1 band of olivine coincides with pyroxene and feldspar. The problem
of overlapping bands was described by Brunetto et al. (2009) in the range
630-680 cm-1. Where mineral bands are very close to each other in wavenumber,
they may merge and modify the FWHM values, which may increase the difficulty
of peak position determination.

###  4.1 Overview of results

Table 8 below shows the appearance of mineral bands for all measurements,
where the ratio of the mineral appearance for each measurement series is
indicated as a percentage. The major band of feldspar always appears before
and after the irradiations. The band of pyroxene appears on average 65% before
and after the first and the third irradiations, while this value for the
second irradiation is 76%. The appearance of the 887 cm-1 band in olivine
weakens continuously after all the irradiations (38 $\rightarrow$ 27
$\rightarrow$ 12%). The 849 cm-1 band of olivine shows a negative trend
(decrease of peak position) after the first and the third irradiations, but
this occurs more frequently after the second irradiation. The band of spinel
(666 cm-1) overlaps the Fe-Mg band of olivine, and hence it is difficult to
observe and shows a variable trend. It mostly appears after the second
irradiation, and less frequently after the first irradiation. Because of the
changes and variability in the observability of certain peaks, in the rest of
this work we only consider those cases where all of the analyzed bands
remained observable during the whole irradiation and measurement series at the
given targeted location, and thus where no close position to other bands,
merging of bands, or other unfavorable situations influenced the results.

For the identification of minerals, the major bands are applicable. The major
band of olivine at 849 cm-1 almost disappeared after the first irradiation,
but after the second irradiation became more frequent. In general, the
appearance of bands from feldspar, pyroxene, and spinel changed during the
irradiations. According to the last column of Table 8, smaller peak changes
occurred in the case of feldspar, and the major band at 1150 cm-1 occurs in
all of the measurements. The strongest change in spectra can be observed in
the case of the 887 cm-1 band of olivine, which shows a negative trend (shift
of bands to lower wavenumber) until after the third irradiation. The change in
the cases of pyroxene and spinel is smaller, but these occur more frequently
in the measurement series after the second irradiation than earlier.

In A2-1 (Fig. 2), after the first irradiation the spectrum is similar to the
spectrum before the irradiation, but after the second and third irradiations
the band of feldspar near 1150 cm-1 disappears, and the intensity of bands
between 600 and 750 cm-1 decreases after the second irradiation, and finally
disappears after the third. In spectra D1-2 (Table SOMII/3)), new bands appear
at around 1000 cm-1 after the second and third irradiations due to
depolimerization of SiO4 tetrahedra (Johnson et al., 2003, 2007). Johnson et
al. (2003) documented changes in the appearance and position of spectral
features with increasing shock impact pressure due to depolymerization
(disordering of SiO4 tetrahedra from the crystal lattice forming SiO2
molecules) of the silica tetrahedra, which agrees with our observation as
well. In spectra A4-1 (Table SOMIII/3) the 1050 cm-1 band shifts to a lower
wavenumber after the first and second irradiations (metastable disordering;
Johnson et al. 2003, 2007), but after the third irradiation this band appears
at peak positions similar to before the irradiation series (reorganizing
structure, observed by Johnson et al. 2003, 2007). Feldspar and pyroxene occur
in most spectra. Spinel appears mostly later during the irradiation series, or
shows a cyclic appearance (Table 9).

  * •

Olivine: The 849 cm-1 band shows a peak position decrease after the first
irradiation (negative shift), whereas after the second and third irradiations
positive peak shifts were observed. The other major 887 cm-1 band shows a
higher peak shift (positive trend) after the first and third irradiations, but
a decrease after the second irradiation. Thus, a variable peak shift change
occurs for the 849 cm-1 band, whereas the 884 cm-1 band shows a negative
(decreasing peak shift) trend after the second and third irradiations. The 849
cm-1 and 884 cm-1 bands unify to one band near 890 cm-1 (d1/1-4 after the
first irradiation, b1/5, d1/1-2 third irradiation) due to the distortion of
crystal lattice after the irradiations. The increase in peak shift of the main
doublet of olivine after the third irradiation indicates Fe loss from the
crystalline structure (Lantz et al., 2017). This increase in wavenumber is
variable in our measurements. The 890 cm-1 band disappears in some cases, but
the band at 850 cm-1 shows both positive and negative trends in peak shifts.
The positive trend of peak shift mainly seen at the end of the irradiation
series of this work could arise from the deformation of SiO4 tetrahedra of
olivine, in a similar fashion to the observed peak shift in the G band (C=C of
graphite) to higher wavenumber, which was described by Brunetto et al. (2009).
The negative peak shift occurs because of Mg loss from Fe-Mg bearing minerals
at the irradiated surface (Brunetto et al., 2014, 2020; Lantz et al., 2015,
2017).

  * •

Feldspar shows a decrease in peak position after the first and second
irradiations (on average by -2 cm-1, but the lowest values are between -37 and
-46 cm-1), and this peak shift is accompanied by an increase in FWHM (Table
11) after the second irradiation, averaging 86 cm-1), due to crystal lattice
defects.

  * •

The main band of pyroxene (1047 cm-1) shows an average decrease in peak shift
(negative trend) after the first irradiation, which becomes an increase
(positive trend) after the second and third irradiations.

  * •

Spinel shows a decrease in peak shift (negative trend) after the first
irradiation, and an increase in peak shift (positive trend) after the third
irradiation.

After the first irradiation, a negative peak shift was present in Fe-bearing
minerals (olivine, pyroxene, spinel) based on peak changes of bands between
630-680 cm-1. The positive peak shift of Fe-Mg-bearing minerals after the
third irradiation indicates Fe loss. According to Klima et al. (2007) the IR
bands of M1-M2 (Fe2+) positions are located at 1.2 $\mu$m, 1 $\mu$m, and 2
$\mu$m for silicates (especially pyroxene), but these wavenumbers are too high
to be analyzed by the instrument used in this work. Hanna et al. (2020)
indicate that the Mg bands of silicates are in the range 715-550 cm-1. In our
case, this band is at 660-680 cm-1, which is the major band of spinels, but it
occurs in olivine and pyroxenes as well. Hence, Table 2 shows not only the
peak shift of spinels, but also the peak shift of the Fe-Mg band of pyroxene
and olivine in general.

Peak shifts of olivine for the 887 cm-1 band are negative on average after the
first and second irradiations (-4.3 – -4.8 cm-1 with variation between -15 and
+6 cm-1), turning to positive after the third irradiation (on average 4.3
cm-1)(Table 2). Hence, the negative peak shifts indicate Mg-loss from the
crystal structure after the first and second irradiations. The negative trend
of the peak shift after irradiation is mentioned by Lantz et al. (2013, 2015,
2017); Lantz & Brunetto (2014)after the first and second irradiation.

Cation loss (olivine, feldspar, pyroxene, spinel after the first irradiation;
see Table 10, column 1, negative peak shifts) also causes the disappearance of
the major doublet of olivine by the merging of the doublet band to a single
band. This one direction change in IR bands was observed in several cases in
our measurements after the irradiations (see Table SOM I/5 d1/2 4; second
irradiation, b1/5 third irradiation, d1/1-2 third irradiation, a1/6 after the
third irradiation). The reverse transformation (splitting to a doublet) could
be not observed.

Table 2: Summary peak shift table of the minerals

mineral | spectrum | band | 1st irr. | 2nd irr. | 3rd irr. | avg. peak shift cm-1
---|---|---|---|---|---|---
spinel | average | 668 | -3.5 | -3.21 | 5.15 | -0.52
spinel | min | 660 | -15 | -19 | -3 | -12.333
spinel | max | 679 | 3 | 3 | 11 | 5.666
spinel | median | 667 | -2 | 0 | 4 | 0.666
| average | 668.5 | -4.3 | -4.803 | 4.29 | -1.6300
pyroxene | average | 1047.5 | -2.5 | 0.333 | 2.13 | -0.0296
pyroxene | min | 1035 | -30 | -8 | -23 | -20.333
pyroxene | max | 1065 | 16 | 38 | 30 | 28
pyroxene | median | 1046 | -1 | -6 | 0 | -2.333
| average | 1048.3 | -4.38 | 6.08 | 2.28 | 1.326
feldspar | average | 1152.1 | -2 | -2 | -3.92 | -2.64
feldspar | min | 1116 | -46 | -37 | -42 | -41.666
feldspar | max | 1183 | 29 | 33 | 26 | 29.333
feldspar | median | 1148 | -7 | 3 | 0 | -1.333
| average | 1149.8 | -6.5 | -0.75 | -4.98 | -4.076
olivine | average | 887.7 | 4.4 | -4 | 0.6 | 0.333
olivine | min | 883 | -2 | -7 | -4 | -4.333
olivine | max | 894 | 6 | 3 | 7 | 5.333
olivine | median | 885.5 | 6 | -4 | 0 | 0.666
| average | 887.56 | 3.6 | -3 | 0.9 | 0.5
olivine | average | 849.2 | -11.3 | 2 | 1.8 | -2.511
olivine | min | 849 | -15 | -15 | -4 | -11.333
olivine | max | 850 | -4 | 15 | 11 | 7.333
olivine | median | 849 | -15 | 4 | 0 | -3.666
| average | 849.3 | -11.33 | 1.5 | 2.2 | -2.544
111Min is the lowest value; max is the highest value.

The peak position of spinel decreases after the first and second irradiations
and increases after the third irradiation (on average -3.5, -3.1 cm-1 after
the first and second irradiation, 5 cm-1 after the third irradiation; see
Table 2). Similar trends can be observed in the case of pyroxenes: -2.5, -0.5
cm-1 peak position change after the first and second irradiations, 2 cm-1
after the third irradiation. On average a similar trend was observed in
feldspar, pyroxene, and spinel, but with a certain deviation (Table SOM II/1,
Table SOM III/1). The peak shifts of feldspar show a continuous trend after
all of the irradiations, presenting a decreasing wavenumber (negative) trend.
The 887 cm-1 band of olivine shows a higher peak shift after the first
irradiation, that changes to a negative trend after the second irradiation.
The 849 cm-1 band shows a -11 cm-1 peak shift after the first irradiation,
which changes to a positive trend after the second and third irradiations.

Consequently, negative peak shifts occurred after the first irradiation in Fe-
Mg bearing minerals. After the second irradiation this trend was variable, but
finally after the third irradiation the average values of peak position shift
of olivine turned to positive. All of these happened in the top less than
micrometer thick layer of the sample, where the maximum damage occurs at a
shallow penetration depth (Demyk et al., 2004). In the case of feldspar, on
average the negative peak shift changed to positive after the second
irradiation, but after the third irradiation these values turned negative
again. Pyroxene shows negative values in peak shift after the first and second
irradiations, which change to positive after the third irradiation.

###  4.2 Comparison of the results with other works

In this section we present the discussion of the mineral changes in the
following order: 1) summary of the related results of other published research
projects (there might be a small overlap with those that are listed in the
Introduction; however, they are presented here as well for comparison); 2)
comparison of our results with those of others regarding the peak changes
(position shift and FWHM); 3) presentation of the possible reasons for these
changes.

Published works by other authors present observations in the far-IR and mid-IR
regions after the irradiation of minerals, focusing on measured peak shifts,
but not FWHM values. In general, other works show a decrease in intensity and
increase in spectral slope (known as darkening) in the IR spectrum (Strazzulla
et al., 2005). Peak shifts of olivine and pyroxene are characteristically in
the range 0.4-0.12 $\mu$m (1-25 cm-1) (Lantz et al., 2015, 2017) after a flux
of $\sim$1013 ions (sec-1 cm-2) up to a total fluence of $\sim$6 $\times$1016
ions cm-2, indicating a variation in peak shift with increasing irradiation
energy. Peak shifts are negative when they are defined in wavenumbers, and
each band is influenced differently, but all of the spectra are affected
(Brunetto et al., 2014). They observed that the shift of the $\sim$870 cm-1
band is much larger (12 cm-1) than the shifts possibly caused by the
heterogeneity of the matrix olivines. Incorporation of nuclei due to
bombarding with bigger ions (H+, He+, Ar+, Ar2+) at higher energies (60-400
kev) distorts the crystal lattice more. In our case, the irradiation energy is
1 keV with increasing fluences after each irradiation, and the ion size is
smaller as we used H+ ion with fluences of 1011, 1014, and 1017 ions/sec/cm-2.
Instead, the flux of the second irradiation was similar to several tests
described in the literature. The peak shift of the olivine band (887 cm-1) was
a negative value (-4 cm-1), but with high variability (-15 and +15 cm-1).
According to Brunetto et al. (2006), the silicates (pyroxenes and olivine)
show progressive amorphization due to elastic collision of ions, causing a
shift of bands to lower wavenumbers and decrease in band intensity
(amorphization depends on ion size: amorphization is higher by bombardment of
Ar+ than H+). Similar band changes were observed in our olivine, pyroxene, and
spinel spectra, but with a higher variation: between -19 and 11 for spinel;
between -23 and 30 for pyroxene; between -7 and +7 (887cm-1) and between -15
and +15 (850 cm-1) for olivine.

Our results on peak shift of the 849 cm-1 olivine band were negative after the
first irradiation, which turned to a positive trend after the third and second
irradiations (Table 10). This observed negative peak shift is similar to the
results of Lantz et al. (2015, 2017) due to Mg-loss from the crystal lattice
on the irradiated surface. The other major band of olivine (887 cm-1) shows a
partly opposite trend: on average a positive value (increase in wavenumber)
after the first irradiation, which turns negative after the second
irradiation, but after the third irradiation on average remains almost zero.
The negative peak shift after the first and second irradiations is similar to
that found by Lantz et al. (2015, 2017). The peak shift of feldspar is
negative after the first and third irradiations, and turns positive after the
second irradiation, which is difficult to explain. Pyroxene and spinel have
negative peak shift values after the first irradiation, turning positive after
the second and third irradiations. The negative peak shift of pyroxene is
similar to other data (Lantz et al., 2015, 2017), but the positive peak shift
has not been described by others after irradiation. However, Johnson et al.
(2003) observed a peak shift due to distortion of SiO4 tetrahedra of feldspar
due to shock deformation of the crystal lattice, and hence in our case the
positive peak shift could be interpreted as crystal lattice defects due to
irradiation.

Considering the IR spectroscopic data presented in the literature from others
(Lantz et al., 2015, 2017; Brunetto et al., 2018, 2020), the spectral changes
after irradiation are rarely studied by Raman spectroscopy. However, the
disordering of meteorite minerals after shock metamorphism is usually studied
by this Raman spectroscopy, which may be used as a reference for this work
evaluating crystalline structural changes. Brunetto et al. (2014) observed the
merging of olivine Raman doublet bands (820, 850 cm-1) to a single band at
around 840 cm-1 due to irradiation. Similarly to their results, after the
irradiation we observed one unified band near 890 cm-1. In our data the other
major olivine band disappeared at 850 cm-1 after the first irradiation
(d1/2-4) and the third irradiation (b1/5, d1/1-2), and after the second
irradiation (c1/2-3). The olivine disappeared in most cases (a5/2, an4b/1-3,
a2/2, a3/2, c1/7-9) after all of the irradiations. Brunetto et al. (2014)
explains the disappearance of the olivine major band by the deformation of
crystal structure due to the irradiation, when the depletion of Mg ions from
the irradiated surface layer of olivine occurred.

An important problem in the interpretation of band changes is the possibility
of overlapping IR bands affecting the FWHM values, mostly observed in the
region around 650 cm-1. This effect is also observed by Brunetto et al. (2014)
who suggested surveying with more detectors or at higher spectral resolution
in the future. In our case, the Fe-Mg bands of olivine, pyroxene and spinel
are near to each other, and in the reflection spectra the FWHM is also
influenced by the characteristic of the baseline (Fig. 2) making the FWHM
interpretation difficult in certain cases.

Brunetto et al. (2020) studied bulk meteorite samples using He ion irradiation
(fluences -20-40 keV 6$\times$1016 \- 1013/ions/sec), but they had FTIR bands
in the far IR region, below 600 cm-1 (a region which cannot be detected by the
Vertex Hyperion 2000 IR-reflection microscope used in this work). They
measured peak shift values varying between -1.5 and -13.5 cm-1. Our bands in
DRIFTS spectra appear at 462-478 cm-1, and in the range 489-503 cm-1, but
characterized by variable peak shift values (-45-37 cm-1 after the first
irradiation and between -34 and -7 cm-1 after the second irradiation). Only
after the third irradiation do the emerged peak shifts differ slightly from
those of Brunetto et al. (2020) (1 and -7 cm-1). The negative peak shifts are
similar to those found by Brunetto et al. (2020), but the positive peak shifts
we observed are not described in the literature. However, their measured
negative shift is characterized by a smaller range (-1.5 and -13.5) than our
data. According to Lantz et al. (2015, 2017), gradually increasing negative
peak shift values (decreasing trend) occur in the MIR region due to Mg loss
from the structure, with band position versus band shift as follows: 980 cm-1
with -54 cm-1, 960 cm-1 with -92 cm-1, 950 cm-1 with -60 cm-1, 1020 cm-1 with
-40 cm-1, and 867 cm-1 with -2 cm-1. In our case the peak shift values are
variable, showing both negative and positive values (between -12 and +6 cm-1
for olivine, -46 and +33 cm-1 for feldspar, and -30 and 38 cm-1 for pyroxene).
Average peak shift values are negative after the first and second
irradiations, but turn positive after the third irradiation. In summary, we
observed trends in peak shifts similar to Lantz et al. (2015, 2017) indicating
Mg loss from the irradiated surface of the crystal lattice at the first and
second (weaker) irradiations; later at the much harder third irradiation, the
positive peak shift arises from the amorphization (i.e., falling apart) of the
tetrahedra structures.

Because the published works do not present FWHM values after irradiations, we
used shock metamorphism induced mineral deformation for comparison, as the
crystalline lattice was deformed there as well. In general, the increase in
FWHM in both Raman and IR spectroscopy indicates crystal lattice deformation
by shock effects (Sharp & de Carli, 2006), and thus deformation of the
original well-ordered crystalline structure. Unfortunately, only a small
amount of Raman data are available from other works after irradiation
experiments. In relation to Raman spectroscopy, a small amount of data is
mentioned by Lantz et al. (2017) and Brunetto et al. (2009) on the FWHM of IR
G band (C=C) of graphite showing a poorly ordered pristine structure after the
irradiation, indicating disordering of the crystalline structure.

###  4.3 Suggestions for future tests

Although several published works present IR peak position change by
irradiation (Lantz et al., 2015, 2017; Brunetto et al., 2014, 2020),
unfortunately FWHM calculations are not presented by other authors. We provide
several missing data points, mainly related to Fe-Mg bearing minerals
(pyroxene, olivine) and feldspar. In future tests the level of compositional
change should be followed by more focused analysis.

Because the variation in the appearance of minor bands is high (variable
appearance and disappearance), we present the major bands (845, 887 cm-1 for
olivine, 1050 cm-1 for pyroxene, 1150 cm-1 for feldspar), which characterize
the changes. In the case of spinel, only the major band between 660-680 cm-1
appeared, and hence only this band can be presented. It is reasonable to
further analyze the main band positions, partly as they appear better (longer)
preserved even during strong irradiation and might be identifiable even at
heavily space weathered asteroid surfaces as well.

Because of the overlapping of mineral bands, further measurements of spinel by
other methods (microXRD, Raman) is important. New microXRD measurements could
also help to distinguish the appearance of new phases and disappearance of
minerals after irradiation.

##  5 Conclusion

In this work IR reflection and DRIFTS measurements were carried out on the
NWA10580 CO3 meteorite after three proton irradiation actions to see the
alteration of crystal structure (change in peak positions and FWHM values) in
order to simulate space weathering effects. This work used proton irradiation
with an average 1 keV solar wind ion energy in three gradually increased
doses, using 1011 ion/cm2, 1014 ion/cm2, and 1017 ion/cm2 fluent values.

The changes observed after the irradiations and the listed peak positions are
also important themselves as many of them have not been published previously.
Minor bands presented variable appearance and disappearance from the
irradiations. Thus, we focused on the more obvious major bands for mineral
identification in the cases where the same minerals were identified firmly
after all irradiations. These observations mostly presented a negative peak
shift after the first and second irradiations in pyroxene and feldspar (-31
and -45 cm-1), as in the literature (Brunetto et al., 2014; Lantz et al.,
2015, 2017), caused mainly by Mg loss. However, after the third irradiation,
positive values in wavenumber change emerged for all silicates, which may
arise from the distortion and breaking down of SiO4 tetrahedra, as was
described in the case of shock deformation by Johnson et al. (2003). We also
presented the first published tests of meteorite-based feldspar irradiation
analysis, with the splitting of the major band into two bands.

The FWHM values of the major bands show a positive (increasing) trend after
irradiations in the case of feldspars using IR reflection. The increase in
FWHM of spinel also indicates that the main structural deformation occurred
after the third irradiation. However, the FWHM change in pyroxene is negative
(decreases) after the first irradiation, but is positive (increases) after the
second and third irradiations. This situation indicates that spinel is more
sensitive to irradiation as the crystalline lattice deformation starts already
at the weakest first irradiation. The change in FWHM of olivine is negative
(decreases) after the first irradiation, and changes to positive values after
the second and third irradiations, similarly to pyroxene.

By the comparison of DRIFTS and reflection IR data, we observed the following
trends. The peak position of major mineral bands were at similar wavenumbers
in the DRIFTS and reflection methods, but in minor bands differences could be
observed. Spinel could be better identified by IR reflection than by DRIFTS.
The olivine band at lower wavenumbers was better detected by DRIFTS because of
the limited wavelength coverage (up to 400 cm-1 vs 600 cm-1).

The overlapping of the bands influences the band identification and also the
FWHM measurements. Overlapping mainly occurred in the regions 650-680 cm-1 and
1150-1180 cm-1 using IR reflection measurements, while in DRIFTS, the minor
bands of olivine (987 cm-1) and pyroxene (952 cm-1) could disturb each other.
The overlap of mineral bands in the region 630-680 cm-1 influences the FWHM
data in reflection IR measurements: the co-existence of bands of more types of
minerals (spinel: 660-680 cm-1, feldspar: 640 cm-1, Fe-Mg vibration of
olivine: 660 cm-1, and pyroxene: 670 cm-1) decreases the FWHM values (Fig. 2).

The overlapping of bands near 650 cm-1 can influence the band identification
(decreasing the FWHM values in the case of the co-existence of more Fe-Mg
minerals). In reflection IR spectra feldspar has a peak position at 631-640
cm-1, while the Fe-Mg vibration of olivine is in the region near 670 cm-1,
while the Si-O band of spinel varies in the range 660-680 cm-1 in the
reflection IR spectra. With increasing irradiation fluences, the doublet of
olivine (849 cm-1 and 880 cm-1) merged into a single band at 887 cm-1 due to
disordering of SiO4 tetrahedra, in agreement with earlier results (Van de
Moortèle et al., 2007).

Comparing our results with those of other authors, it is worth mentioning that
previous tests used mainly higher energy protons around 30 keV, including
Brunetto et al. (2018) and Strazzulla et al. (2005), while our irradiation was
at 1 keV. However, the fluences 5$\times$1014, 2$\times$1015, 1$\times
10^{16}$, and 3$\times 10^{16}$ ions/cm2 used by Brunetto et al. (2018) were
lower than our third irradiation flux (1017 ions/cm2). Our first irradiation
was lower than in the above-indicated reference, but the flux of the second
irradiation was in range similar to that used by Brunetto et al. (2018). Our
results (negative peak shift values in wavenumber) after the first and second
irradiations were similar to published references (produced by Mg loss), but
the average data of the third irradiation (positive values in wavenumber) is a
new finding.

Considering all our measurements, a negative peak shift (caused by Mg loss)
was first observed, followed by amorphization of the crystal lattice. However
distortion of SiO4 tetrahedra was not specifically presented in other works,
only a description of amorphization in general. Our results indicate that with
further analysis it might be possible to separate different levels of
destruction of the crystalline structure and link such information to the
spectral properties of asteroids. The observed positive peak shift after our
third irradiation was also not mentioned in the references, because in
previous works the irradiations were done with lower fluences. With further
analysis such IR data might be linked to exposure durations of asteroid
surfaces, might confirm or fine-tune space weathering durations, and will help
to calculate breakup events using high spatial resolution spacecraft IR data
in the future.

###### Acknowledgements.

This project was supported by the K_138594 project of NKFIH. Z.J. is grateful
for the support of the Hungarian Academy of Sciences through the János Bolyai
Research Scholarship. This work was also supported in part by Europlanet 2024
RI which has received funding from the European Union’s Horizon 2020 Research
Innovation Programme under Grant Agreement No. 871149.

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##  Appendix A Peak shift and summary tables

###  A.1 Olivine

Table 3: Peak shift of the major 849 cm-1 band after the irradiations. area | spectrum | band | 1st irr. | 2nd irr. | 3rd irr.
---|---|---|---|---|---
a3 | 1 | 849 | -15 | 15 | -4
a4 | 1 | 849 | -15 | 8 | 3
a4 | 2 | 849 | -4 | 0 | 0
a5 | 1 | 849 |  | -15 | 11
a1 | 4 | 850 |  |  | -1
| average | 849.2 | -11.33 | 2 | 1.8
| min | 849 | -15 | -15 | -4
| max | 850 | -4 | 15 | 11
| median | 849 | -15 | 4 | 0
Table 4: Peak shift of the major 849 cm-1 band of olivine with statistic calculations area | spectrum | band | 1st irr. | 2nd irr. | 3rd irr.
---|---|---|---|---|---
c1 | 2 | 894 |  | -7 |
c1 | 3 | 894 |  | 3 |
c1 | 4 | 894 |  | -7 |
c1 | 5 | 894 |  | -7 |
b1 | 5 | 887 |  | - | 4
d1 | 1 | 884 | 6 | -3 | -4
d1 | 2 | 884 | 6 | -3 | -4
d1 | 3 | 884 | 6 |  |
d1 | 4 | 884 | 6 |  |
a2 | 1 | 883 | -2 |  |
a1 | 3 | 884 |  | -4 | 7
a1 | 6 | 887 |  |  | 0
| average | 887.75 | 4.4 | -4 | 0.6
| min | 883 | -2 | -7 | -4
| max | 894 | 6 | 3 | 7
| median | 885.5 | 6 | -4 | 0

###  A.2 Feldspar

Table 5: Peak shift values of the major feldspar band after irradiation area | spectrum | band | 1st irr. | 2nd irr. | 3rd irr.
---|---|---|---|---|---
c1 | 2 | 894 |  | -7 |
c1 | 3 | 894 |  | 3 |
c1 | 4 | 894 |  | -7 |
c1 | 5 | 894 |  | -7 |
b1 | 5 | 887 |  | - | 4
d1 | 1 | 884 | 6 | -3 | -4
d1 | 2 | 884 | 6 | -3 | -4
d1 | 3 | 884 | 6 |  |
d1 | 4 | 884 | 6 |  |
a2 | 1 | 883 | -2 |  |
a1 | 3 | 884 |  | -4 | 7
a1 | 6 | 887 |  |  | 0
| average | 887.75 | 4.4 | -4 | 0.6
| min | 883 | -2 | -7 | -4
| max | 894 | 6 | 3 | 7
| median | 885.5 | 6 | -4 | 0

###  A.3 Pyroxene

Table 6: Peak shift of the major band of pyroxene after all irradiations area | spectrum | band | 1st irr. | 2nd irr. | 3rd irr.
---|---|---|---|---|---
d1 | 1 | 1039 | 6 | -6 | 0
d1 | 2 | 1039 | 6 | -6 | 0
d1 | 3 | 1054 | -9 | -6 | 0
a2 | 1 | 1065 | -30 | 15 | 11
a4 | 2 | 1039 | -12 | 38 | -4
| average | 1047.2 | -7.8 | 7 | 1.4
| min | 1039 | -30 | -6 | -4
| max | 1065 | 6 | 38 | 11
| median | 1039 | -9 | -6 | 0

###  A.4 Spinel

Table 7: Peak shift of band of spinel after all irradiations. area | spectrum | band | 1st irr. | 2nd irr. | 3rd irr.
---|---|---|---|---|---
b1 | 1 | 679 | -15 | 3 | 4
d1 | 1 | 664 | -1 | -3 | 7
d1 | 2 | 660 | 3 | 1 | 3
d1 | 3 | 660 | 3 | 1 | 3
d1 | 4 | 664 | -1 | 1 | 3
d1 | 5 | 664 | -1 | 1 | 3
a4 | 1 | 668 | -8 | -3 | 7
a5 | 1 | 668 | -8 | 0 | 4
a5 | 2 | 668 | -1 | 0 | -3
| average | 666 | -3.22 | 0.11 | 3.44
| min | 660 | -15 | -3 | -3
| max | 679 | 3 | 3 | 7
| median | 664 | -1 | 1 | 3
Table 8: Number of spectra with identified minerals according to major bands before the first and after the three irradiations. mineral | Band | before irr. | ratio(%) | 1st irr. | ratio (%) | 2nd irr. | ratio(%) | 3rd irr. | ratio (%) | ratio range
---|---|---|---|---|---|---|---|---|---|---
no. of measurements |  | 46 |  | 26 |  | 37 |  | 49 |  |
olivine | 849.2 | 6 | 13 | 2 | 8 | 15 | 41 | 6 | 12 | 8-13
olivine | 887.7 | 21 | 46 | 10 | 38 | 10 | 27 | 6 | 12 | 12-46
feldspar | 1152.1 | 46 | 100 | 26 | 100 | 37 | 100 | 49 | 100 | 100
pyroxene | 1047.2 | 30 | 65 | 16 | 62 | 28 | 76 | 33 | 67 | 62-76
spinel | 666.1 | 38 | 83 | 18 | 69 | 35 | 95 | 41 | 84 | 69-95
Table 9: Summary of the appearance and changes of minerals (main peaks) before
and after the irradiations.

mineral | appear later | disappeared later | cyclic appearance | occur always
---|---|---|---|---
spinel | 17 (11%) | 5 (3%) | 17 (11%) | 9 (6%)
pyroxene | 11 (7%) | 15 (9.5%) | 15 (9.5%) | 16 (10%)
feldspar | 10 (6%) | 8 (5%) | 18 (11%) | 22 (14%)
olivine | 16 (10%) | 11 (7%) | 7 (4%) | 5 (3%)
222(Before irradiation: 46 identified mineral spectra, after first
irradiation: 26 spectra, after second irradiation: 37 spectra, after third
irradiation: 49 spectra.) In the above cases, all of the band observations
were successful both after and before the irradiations, while the cyclic band
appearance indicates that the given band emerged earlier, later disappeared,
and finally emerged again.

Table 10: Average peak position shift summary by major bands of minerals.

mineral | band | 1st irr. | deviation | 2nd irr. | deviation | 3rd irr. | deviation | avg. deviation after irr.
---|---|---|---|---|---|---|---|---
olivine | 849.2 | -11.33 | 6.5 | 2 | 12.8 | 1.8 | 5.7 | 8.3
olivine | 887.8 | 4.4 | 3.6 | -4 | 3.6 | 0.6 | 4.8 | 4
feldspar | 1152.1 | -1.58 | 20.4 | 2.16 | 19.9 | -1.4 | 15.7 | 18.7
pyroxene | 1047.2 | -7.8 | 14.9 | 7 | 19.6 | 1.4 | 5.6 | 13.4
spinel | 666.1 | -3.22 | 5.9 | 0.11 | 1.9 | 3.4 | 2.9 | 3.6

333Average peak position shifts, where minerals occur after all of the
irradiations (spinel, pyroxene, feldspar, but not olivine because of less
data). The standard deviation of peak shift is the difference from the average
values of peak shift position.

Table 11: Average FWHM values of major mineral bands with standard deviations.

mineral | band | 1st irr. | deviation | 2nd irr. | deviation | 3rd irr. | deviation | avg. deviation after irr. |  |
---|---|---|---|---|---|---|---|---|---|---
olivine | 849.3 | 9.8 | 4.4 | 17.5 | 4.5 | 6.8 | 1 | 4.3 | 1.03 | 2.2
olivine | 887.6 | 12 | 3.1 | 8.4 | 2.2 | 6.5 | 2.5 | 12 | 13.9 | 6.2
feldspar | 1149.8 | 76.4 | 31.4 | 86.3 | 25.2 | 84.8 | 30.4 | 99.8 | 19.8 | 25.1
pyroxene | 1048.4 | 29.2 | 14 | 22.1 | 11.2 | 22.3 | 20.5 | 21.3 | 9.03 | 13.6
spinel | 668.5 | 65.8 | 40.7 | 51.4 | 46.3 | 65.6 | 41.9 | 76.6 | 33.09 | 40.4

444At the end of the table the average deviation after the irradiations was
calculated. This table contains these measurements, where the given bands
appeared before and after all irradiation tests.

authors:
I. Gyollai, S. Biri, Z. Juh\'asz, Cs. Kir\'aly, B. D. P\'al, R.
  R\'acz, D. Rezes, B. Sulik, M. Szab\'o, Z. Szalai, P. Sz\'avai, T.
  Szklen\'ar, \'A. Kereszturi

submitter: Bernadett P\'al