of 12
Al-Cu-Fe alloys in the solar system: Going inside a Khatyrka-like micrometeorite
(KT01) from the Nubian Desert, Sudan
Chi MA
1
, Jinping HU
1
, Martin D. SUTTLE
2
, Yunbin GUAN
1
, Thomas G. SHARP
3
,
Paul D. ASIMOW
1
, Paul J. STEINHARDT
4
, and Luca BINDI
5
*
1
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
2
School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, UK
3
School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
4
Department of Physics, Princeton University, Princeton, New Jersey, USA
5
Dipartimento di Scienze della Terra, Universit

a di Firenze, Florence, Italy
*
Correspondence
Luca Bindi, Dipartimento di Scienze della Terra, Universit

a di Firenze, Via La Pira 4, Florence I-50121, Italy.
Email:
luca.bindi@unifi.it
(Received 05 June 2023; revision accepted 02 October 2023)
Abstract–
A recently described micrometeorite from the Nubian desert (Sudan) contains an
exotic Al-Cu-Fe assemblage closely resembling that observed in the Khatyrka chondrite
(Suttle et al., 2019;
Science Reports
9:12426). We here extend previous investigations of the
geochemical, mineralogical, and petrographic characteristics of the Sudan spherule by
measuring oxygen isotope ratios in the silicate components and by nano-scale transmission
electron microscopy study of a focused ion beam foil that samples the contact between Al-
Cu alloys and silicates. O-isotope work indicates an affinity to either OC or CR chondrites,
while ruling out a CO or CM precursor. When combined with petrographic evidence we
conclude that a CR chondrite parentage is the most likely origin for this micrometeorite.
SEM and TEM studies reveal that the Al-Cu alloys mainly consist of Al metal, stolperite
(CuAl), and khatyrkite (CuAl
2
) together with inclusions in stolperite of a new nanometric,
still unknown Al-Cu phase with a likely nominal Cu
3
Al
2
stoichiometry. At the interface
between the alloy assemblage and the surrounding silicate, there is a thin layer (200 nm) of
almost pure MgAl
2
O
4
spinel along with well-defined and almost perfectly spherical metallic
droplets, predominantly iron in composition. The study yields additional evidence that Al-
Cu alloys, the likely precursors to quasicrystals in Khatyrka, occur naturally. Moreover, it
implies the existence of multiple pathways leading to the association in reduced form of
these two elements, one highly lithophile and the other strongly chalcophile.
INTRODUCTION
Aluminum is a highly reactive, refractory lithophile
element, while Cu is a low-reactivity, moderately volatile
chalcophile element. Consequently, the union of Al-Cu as
a metallic alloy is a highly unusual combination. Prior to
1985, Al-Cu alloys were known exclusively as synthetic
materials created under controlled laboratory conditions.
However, Razin et al. (
1985
) provided the first report of
natural Al-Cu alloys recovered as detrital grains
dispersed in soils from eastern Russia. Ultimately, these
fragments were found to be part of the disaggregated
CV chondrite Khatyrka (Bindi & Steinhardt,
2014
;
MacPherson et al.,
2013
; Steinhardt & Bindi,
2012
)and,
therefore, extraterrestrial in origin. Microanalysis revealed
the presence of exotic quasicrystals intergrown with the
silicate minerals and Al-Cu alloys. This included
icosahedral Al-Cu-Fe quasicrystals (Bindi et al.,
2009
,
Meteoritics & Planetary Science
58, Nr 11, 1642–1653 (2023)
doi: 10.1111/maps.14089
1642
Ó
2023 The Authors.
Meteoritics & Planetary Science
published by Wiley Periodicals LLC on behalf of The Meteoritical Society.
This is an open access article under the terms of the
Creative Commons Attribution
License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
2011
) and decagonal Al-Ni-Fe quasicrystals (Bindi et al.,
2015a
,
2015b
), representing the first natural occurrence of
these quasiperiodic materials. This class of matter has
properties intermediate betw
een crystalline and amorphous
materials, being characterized
by atomic structures that are
ordered but aperiodic and, the
refore, lack translational
symmetry while retaining highe
r order symmetries (Levine
& Steinhardt,
1984
; Shechtman et al.,
1984
).
As the only natural example of Al-Cu alloys and
quasicrystals, the Khatyrka meteorite has received
significant research attention (Andronicos et al.,
2018
;
Bindi et al.,
2012
; Lin et al.,
2017
; Ma et al.,
2017
;
MacPherson et al.,
2013
; Meier et al.,
2018
; Tommasini
et al.,
2021
; Wieler,
2023
). This led to the hypothesis and
experimental testing of a hypervelocity impact-induced
origin for the Al-Cu alloys and their quasicrystal
components (Asimow et al.,
2016
; Hollister et al.,
2014
;
Hu et al.,
2020
; Oppenheim et al.,
2017a
,
2017b
).
However, skepticism regarding their authenticity
persisted (Ivanova et al.,
2017
), primarily due to the
exceptional nature of this find.
More recently, a second independent discovery of Al-
Cu alloys was identified within a micrometeorite recovered
from the Nubian desert, Sudan (Suttle et al.,
2019
).
Although it does not contain quasicrystals, this particle
(termed KT01) contains Al-Cu alloys and metallic Fe
intermixed with silicate minerals, and therefore, exhibits
close similarities to the Khatyrka meteorite. KT01 is a
cosmic spherule and its parent body mineralogy was
affected by flash heating during atmospheric entry (Genge
et al.,
2008
).
The presence of at least three intermixed Fe-bearing
Al-Cu alloy-bearing phases
aluminum, khatyrkite (CuAl
2
),
and stolperite (CuAl)
whose textures and composition
demonstrate rapid crystallization spanning at least from the
CuAl
2
+
CuAl peritectic (
~
590
°
C; Zobac et al.,
2019
)tothe
CuAl
2
+
Al eutectic (
~
550
°
C) demonstrates that melting and
quench cooling altered the pre-atmospheric mineralogy of
the Al-Cu assemblage. The KT01 micrometeorite is
otherwise a typical S-type cosmi
c spherule containing relict
and neoformed olivine phenocrysts embedded within a
mesostasis of Ca-bearing silicate glass. Particle textures
preserve cumulate layering
indicative of rapid deceleration
and high-orbital ec
centricity prior to
atmospheric entry
(Suttle et al.,
2019
).
Despite detailed petrographic analysis, the
crystallography of the Al-Cu alloys and the parent body
provenance of KT01 remain poorly constrained. We set
out to conduct a more detailed study of this particle. This
study reports new data on the O-isotope composition of
the silicate minerals in KT01 and a detailed transmission
electron microscopy (TEM) investigation of the
nano-scale mineralogy within the Al-Cu alloys and
silicates in this sample.
ANALYTICAL METHODS
The original mount containing the polished section
of KT01 particle was made available for study by one of
us (M.D.S.). Previously, as described in Suttle et al.
(
2019
), the loose particle had been embedded in epoxy
resin, sectioned, polished, and carbon coated before being
studied under scanning electron microscope (SEM) by
energy-dispersive X-ray (EDX) spectrometry, electron
backscattered diffraction (EBSD) and wavelength
dispersive spectrometry (WDS). Upon receipt of the
sample, we conducted further research, as outlined.
Oxygen isotope measurements were performed at
three locations deemed representative of the micrometeorite’s
silicate mineralogy; they sample
primarily mesostasis material
(Figure
1
). These were acquired with a Cameca NanoSIMS
50L based at the Microanaly
sis Center, Division of
Geological & Planetary Scie
nces, Caltech. A primary Cs
+
beam of
+
8keVand
~
30pAwasusedtosputterthesample
in a rastering mode (3
9
3
l
m). Secondary ion signals
of
16
O

,
17
O

,and
18
O

were simultaneously collected with
electron multipliers on the multi
collection system, under high
mass resolution conditions that
resolve any interferences to
the mass peaks of interest. Un
certainties on each data point
include its counting statist
ical errors and the standard
deviation of repeated measure
ments of the San Carlos olivine
standard.
High-resolution scanning electron microscopy
(SEM), energy-dispersive X-ray spectroscopy (EDS), and
electron backscatter diffraction (EBSD) analyses were
performed using a ZEISS 1550VP field emission SEM at
the Division Analytical Facility of GPS, Caltech, to
determine the composition and structure of the Al-Cu
alloy phases and associated minerals in the KT01 section.
Backscattered electron (BSE) imaging and quantitative
EDS analyses (with an Oxford X-Max SDD system and
an XPP correction procedure calibrated with Oxford
factory internal standards) were carried out at 10 kV in
order to reduce the excitation volume and increase spatial
resolution. EBSD analyses used an HKL system with the
SEM operating at 20 kV and 6 nA in focused-beam
mode with a 70
°
tilted stage and variable pressure mode
(25 Pa); the procedure is described in Ma and
Rossman (
2008
,
2009
). The EBSD system was calibrated
using a single-crystal silicon standard.
A focused ion beam (FIB) foil of the alloy assemblage
including the contact with its silicate host was extracted
using a FEI Nova 600 Nanolab DualBeam FIB and
SEM at the Kavli Nanoscience Institute of Caltech. The
foil was milled with a 30 kV Ga ion beam to 500 nm
thickness, placed on a Ted Pella molybdenum grid and
further thinned to 100 nm with an 8 kV Ga ion beam.
Transmission electron microscopy analysis was
performed on a FEI CM200-FEG in the Eyring Materials
Al-Cu-Fe alloys in the solar system
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Center at Arizona State University. The TEM was
operated at 200 kV. Bright field imaging and selected area
electron diffraction (SAED) were used to characterize the
texture and crystal structure of the alloy phases.
RESULTS
Oxygen Isotope Measurements
Three spot analyses returned data points ranging in
d
17
O from
+
2.1
&
to
+
2.8
&
(

1.5
&
[1
r
]), in
d
18
O from
+
3.7
&
to
+
5.9
&
(

1.0
&
[1
r
]), and in
D
17
O from

0.7
&
to
+
0.9
&
(

1.0
&
[1
r
]). The average composition is
d
17
O
=+
2.4
&
,
d
18
O
=+
4.4
&
,and
D
17
O
=+
0.1
&
,
plotting within error on the terrestrial fractionation line
(TFL; Figure
2
).
KT01 Metal Mineralogy
Contrast in SEM-BSE images demonstrates that the
metallic assemblage in KT01 includes four different
phases (Figures
3
and
4
). In order of increasing backscatter
brightness (and hence increasing mean atomic number):
Aluminum (
fcc
), khatyrkite (
h
,
I
4/
mcm
), stolperite (
b
,
Pm
-3
m
), and a new Al-Cu nanoinclusion (see next
subsection for structural info). The Al phase is present
in a eutectoid intergrowth with khatyrkite around some
of the metallic regions (Figure
4c
) but not those areas
studied in detail here. Although phase domains
1
l
min
size are considered challenging for SEM analysis, a field
emission instrument operated at low accelerating
voltage can obtain quality si
ngle-phase EDS analyses
using the
K
a
line of Al and the
L
a
line of Cu. EDS
results (Table
1
) show compositions closely resembling
stolperite (CuAl) and khatyrkite (CuAl
2
)andare
consistent with the previous electron microprobe data of
thesamemicrometeoritesample(Maetal.,
2017
; Suttle
et al.,
2019
). The new Al-Cu phase is found as inclusions
in stolperite up to 400 nm across. The composition of
the largest domain (Figure
4a
) matches best to a
nominal Cu
3
Al
2
formula (Table
1
), consistent with its
BSE brightness clearly exceeding that of stolperite.
FIGURE 1. SEM-BSE image (left) of the KT01 micrometeorite with three pits generated by nanoSIMS measurements in the
silicate mesostasis (red circle enlarged on the right in an SEM-BSE image to emphasize the topography). The green area is
enlarged in Figure
2
. The scale bar in the BSE image is 10
l
m. (Color figure can be viewed at
wileyonlinelibrary.com
.)
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C. Ma et al.
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Crystal Structures
The EBSD results are consistent with the EDS
results and unambiguously identify the stolperite (
b
)
and khatyrkite (
h
) structures (Figure
5
). TEM SAED
patterns again confirm these
structures and, moreover,
reveal superstructures in stolperite as well as a topotactic
relationship between coexist
ing stolperite and khatyrkite
(Figures
6
and
7
). The stolperite patte
rn includes diffraction
from the common 3
9
superstructures on
<
100
>
and
<
011
>
directions (Figure
6b
;Guietal.,
2001
), whereas the
diffraction on
<
111
>
direction is more complex, likely
indicating a non-periodic large-scale superlattice. Besides
the pronounced
<
111
>*
/3
9
superstructure, the apparent
d
-
spacings represent 12
9
and/or 6
9
superstructures even
though they are not expected
basedonpreviousstudiesof
the CsCl-type
b
phase (Gui et al.,
2001
)andsomeof
diffraction spots are missing (Figure
6b
). The observed
topotaxy of adjacent domains takes the form of SAED
patterns with the
<
012
>
zone of stolperite superimposed on
the
<
012
>
zone of khatyrkite, with stolperite (100)
||
khatyrkite (200) and
stolperite (021)
||
khatyrkite (042;
Figure
7d
). Although the
d
-spacings of diffraction spots
were calibrated against gold standard, given the limited
precision of TEM electron diffr
action, it is still difficult to
refine the unit-cell parameter of stolperite to better than
2.9

0.1

A. Nonetheless, in the superposed SAED pattern
it is clear that the (400) diffr
action of khatyrkite occurs at
slightly greater
d
-spacing than the (200) diffraction of
stolperite, confirming the coexistence of two structures and
their orientation relationship. The structure of the Al-Cu
nanoinclusion is still to be fu
lly determined, but current
EBSD data suggest a W-type
bcc
structure for the phase
(Figure
5c
).
FIGURE 2. The O-isotope composition of KT01 (labeled squares with 1
r
uncertainties) is plotted in
d
18
O/
D
17
O isotope space.
During atmospheric entry the pre-atmospheric O-isotope composition of micrometeorites is progressively altered by mass-
dependent evaporation and partial equilibration (mixing) with terrestrial oxygen. The composition of stratospheric oxygen,
shown by the large black cross was defined in Thiemens et al. (
1995
). For reference we also show the compositions of relevant
chondrite groups, labeled and shaded in gray. These include the OCs, the ECs, and several carbonaceous chondrite groups (CO,
CR, CM, CV, and CI). Data defining these regions were taken from Clayton & Mayeda (
1984
), Clayton et al. (
1991
), Clayton
and Mayeda (
1999
), Moriarty et al. (
2009
), Schrader et al. (
2011
), Schrader et al. (
2014
), Kimura et al. (
2020
). The solid gray
line labeled “TFL” represents the terrestrial fractionation line (
d
17
O
=
0.52
9
d
18
O) and the solid black line labeled “Y&R"
represents the Young and Russell line (Young & Russell,
1998
). (Color figure can be viewed at
wileyonlinelibrary.com
.)
Al-Cu-Fe alloys in the solar system
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Textures of Silicate-Metallic Phases
On SEM-BSE images, khatyrkite domains appear
homogeneous, whereas stolperite domains have
resolvable brightness variations and white nanoinclusions
(Figure
4
). In contrast, in TEM bright field images, the
khatyrkite domains commonly show a mottled texture
along random zone axes, the result of a high density of
defects (Figure
7b
). The stolperite domains are more
uniform under TEM imaging, although concentrated
defects may show up in dark field images taken along
specific reciprocal lattice vectors g (Figure
7c
). The Al-Cu
inclusions diffract more strongly than the surrounding
stolperite (Figure
7a
), consistent with a higher atomic
weight (Table
1
) and they commonly occur as elongated
slabs in the FIB foil.
A thin layer (200 nm) of almost pure MgAl
2
O
4
spinel
occurs at the interface between the alloy assemblage and
the surrounding silicate. The silicate side of the spinel
interface layer is also decorated by metallic droplets,
predominantly iron in composition (Figures
3
7
). The
droplets are well-defined and almost perfectly circular in
the FIB foil (and so presumably spherical in three
dimensions).
DISCUSSION
Modification of the Al-Cu Alloys During
Atmospheric Entry
The precursor of KT01 was extensively melted by
heating during atmospheric entry; in this way it followed
a very different thermodynamic path from the Khatyrka
meteorite which, while recovered only as small detrital
grains, was a larger mass at atmospheric entry and the
bulk of it therefore experienced negligible heating. KT01
likely formed two immiscible liquids, one silicate and one
metallic. These liquids reacted at their interface, as
demonstrated by the presence of both Al and Cu within
the quench-cooled silicate glass (Al: 8.7 wt%, Cu: 0.4 wt
%) and the neoformed olivine phenocrysts (Al: 0.5 wt%,
Cu: 0.2 wt%; Suttle et al.,
2019
). This exchange of
components is presumed to have occurred during their
short lifetime in the liquid phase before rapid cooling
drove crystallization of each. The Fe droplets on the
silicate side of the interface may represent Fe reduced
from the silicate melt, coupled to oxidation of a share of
the Al from the metal, leaving the metallic phase as a Cu-
enriched nearly binary Cu-Al melt. In fact, except for the
presence of
<
3 atom% of iron in the CuAl and CuAl
2
phases (Table
1
), we did not observe other characteristic
Al-Cu-Fe phases in the low-Fe ternary system (Zhang
et al.,
2005
; Zhang & L
uck,
2003
). Therefore, the Cu-Al
phase equilibrium is more relevant (Suttle et al.,
2019
). At
the interface, a layer of MgAl
2
O
4
grew from the two
liquids, drawing on Al oxidized from the metal and
MgO from the silicate. Upon crystallization, the silicate
liquid crystallized zoned olivines and a fine or glassy
mestostasis. The metallic melt, whose bulk composition
lies between CuAl
2
and CuAl, would
according to
available phase diagrams (Zobac et al.,
2019
)
begin by
crystallizing a more copper-rich phase, likely the
e
’ phase
with
~
Cu
3
Al
2
composition and the NiAs structure, at
around 800
°
C. The crystallization path would encounter
a first peritectic reaction at 625
°
C, Liquid
+
Cu
3
Al
2
?
CuAl, with the CuAl initially in a high-temperature
structure (
g
, orthorhombic). This reaction would likely
leave inclusions of Cu
3
Al
2
armored by CuAl material,
preventing the consumption of the Cu
3
Al
2
by reaction
with the liquid as it evolved toward Al-rich compositions.
Upon further cooling to 590
°
C, the liquid would
encounter another peritectic reaction, Liquid
+
CuAl
?
FIGURE 3. SEM-BSE image of the green area of Figure
1a
showing the metallic Al-Cu assemblage. Ol
olivine, Spl
spinel, and Fe
iron. The shaded rectangle indicates the
region where the FIB foil was sampled. (Color figure can be
viewed at
wileyonlinelibrary.com
.)
FIGURE 4. (a, b) High-resolution BSE images of the metallic assemblages shown in Figure
3
, including stolperite (Slp),
khatyrkite (Ktk), and a new Cu-Al phase. Contrast has been optimized for viewing the various metal alloys; hence the silicate
matrix appears black. The crystal size is generally in the range of 100
500 nm. The droplets in surrounding silicate glass are made
of iron (Fe). The blue dots mark the regions for EDS analyses reported in Table
1
. (c) BSE image showing a different metallic
assemblage in KT01 with an Al
+
khatyrkite eutectoid intergrowth rim. (Color figure can be viewed at
wileyonlinelibrary.com
.)
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Al-Cu-Fe alloys in the solar system
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CuAl
2
, which would consume more of the liquid and
form khatyrkite domains surrounding and enveloping the
CuAl domains. Meanwhile, also at about 590
°
C, the
e
0
phase of Cu
3
Al
2
would reach its lower stability limit
and convert to another structure. The phase diagram
does not show a W-type
bcc
phase in this region, but
subsequent investigation of the structure of the Cu
3
Al
2
inclusions and consideration of kinetics of rapid cooling
may resolve this issue. At 570
°
C, the high-temperature
orthorhombic CuAl would invert to low-temperature
monoclinic CuAl and ultimately transform to CsCl-type,
because of the presence of trivial iron in the system
(Table
1
) (Zhang et al.,
2005
). Previous studies show that
at the low-Fe end, the CsCl-type AlCu commonly shows
<
111
>*
/3
9
superstructure around 600
°
C and
<
110
>*
/
10
9
superstructure around 500
°
C (referred to as the
φ
phases; Zhao et al.,
2003
). In our sample, we observed the
coexistence of CsCl-type phases including stolperite (
b
)
with no superstructure (Figure
7d
); with pronounced 3
9
modulation superstructure on
<
100
>
,
<
110
>
, and
<
111
>
;
and with complex quasi-12
9
modulation superstructure
on
<
111
>
. Although the observed superstructures do not
exactly match the experimental phase equilibrium predictions,
the assemblage indicates the sample was quenched while
undergoing a series of phase transitions. Finally, residual Al-
rich liquid, isolated by peritectic rims from the Cu-rich
phases, would eventually reach the khatyrkite
+
Al eutectic
at 550
°
C and crystallize the fine intergrowth of these two
phases observed at the boundaries of some metallic domains.
This crystallization pathway explains the observed
assemblage of Cu
3
Al
2
inclusions in stolperite, intermixed in
turn with khatyrkite, and finally rimmed in some places by
Al
+
khatyrkite.
The Parent Body Affinity of the KT01 Micrometeorite
Oxygen isotopes can be used to link individual
micrometeorites to established meteorite groups and
thereby constrain their parent body provenance (Goderis
et al.,
2020
; Rudraswami et al.,
2022
; Suavet et al.,
2010
;
Suttle et al.,
2020
). However, before a direct comparison
between micrometeorite O-isotope data and bulk
chondrite data can be made, three factors must be
considered:
1. Chondritic meteorites are une
quilibrated, heterogeneous
assemblages containing
high-temperature,
16
O-rich solid
phases intermixed with lower temperature
16
O-poor
phases (Clayton & Mayeda,
1984
). Oxygen isotope
measurements on small sample volumes typically
reveal large intra-sample variation (e.g., Soens
et al.,
2020
; Suttle et al.,
2021
). Thus, when analyzing
micrometeorites their small size can result in an
unrepresentative sampling of the parent body. For
example, O-isotope data obtained from a
micrometeorite may be strongly affected by sampling
of a single mineral phase and not reflective of the bulk
sample. As a result, isotopic data should always be
considered alongside other diagnostic properties such
as petrographic and geochemical data.
2. The O-isotope composit
ion of cosmic spherules is
systematically altered dur
ing atmospheric entry by two
processes: evapor
ation and mixing. Evaporation induces
a mass-dependent fractionation effect, forcing the
micrometeorite’s bulk O-isotope to evolve along a 0.52
slope in
d
17
O/
d
18
O isotope space, parallel to the TFL,
and resulting in isotopicall
y heavier compositions.
Simultaneously, mixing between oxygen in the
micrometeorite and atmos
pheric oxygen r
esults in an
evolution of the particle’s bulk composition toward the
composition of stratospheric oxygen (
d
17
O
=+
12.1
&
,
d
18
O
=+
23.9
&
,and
D
17
O
=

0.32
&
;Packetal.,
2017
;
Thiemens et al.,
1995
). Notably, mixing and evaporative
fractionation are most effective once particles melt and
form cosmic spherules (as in the case of KT01).
3. NanoSIMS measurements of small sample volumes
provide benefits of high spatial resolution but with
the drawback of lower precision (relative to more
traditional SIMS instruments analyzing significantly
larger volumes). The data collected here have 1
r
uncertainties between 1.0
&
and 1.5
&
.
These factors (unrepresentative sampling, O-isotope
evolution during atmospheric entry and large analytical
uncertainties), make reconstructing the parent body affinity
of KT01 uncertain. The combin
ed sample heterogeneity and
uncertainties of the three datapoints produce an area which
straddles the TFL and overlaps the bulk compositional range
of the ordinary chondrites (OCs), the enstatite chondrites
(ECs) and the CR chondrites, suggesting that the parent
body provenance of KT01 is most likely related to one of
these groups.
An affinity to the ECs is unlikely based on the
presence of abundant, small relict Mg-rich olivine grains
(Fo82
89, Suttle et al.,
2019
; Table
1
) in KT01. In
contrast, olivine is both rare in ECs (
<
4.5 vol%) and
TABLE 1. Representative EDS chemical analyses of
metallic phases.
Al
Fe
Cu
Total
Cu-Al
a
38.14
n.d.
61.86
100.00
Stolperite
a
48.03
n.d.
51.97
100.00
Stolperite
b
48.22
1.63
50.15
100.00
Stolperite
c
47.95
2.60
49.45
100.00
Khatyrkite
b
59.21
0.50
40.29
100.00
Note
: Data in atomic percentage, normalized to 100%. Locations of
the analyses are shown in Figure
4
:
a
Figure
4a
;
b
Figure
4b
upper
portion;
c
Figure
4b
lower portion.
Abbreviation: n.d., not detected.
1648
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occurs almost exclusively as the forsterite endmember
(Fo
>
98; Weisberg & Kimura,
2012
). Additionally, the
kamacite in KT01 contains minimal Si (
~
0.1 wt% in
KT01) but Si is otherwise present at relatively high
concentrations in EC metal (
~
1.4 wt%; Weisberg &
Kimura,
2012
).
Distinguishing between the other two parent body
possibilities (OC vs. CR) requires further consideration
of the micrometeorite’s petrography. The absence of
magnetite (except along the immediate particle
perimeter), combined with the presence of droplets of
near-pure Fe metal and the appearance of reduced relict
olivine within the phenocryst cores demonstrates that the
entire melt, and not just the region proximal to the
intermetallic alloy, experienced sustained reducing
conditions. The pyrolysis of carbon is the most likely
driver of reducing conditions in micrometeorites
(Brownlee et al.,
1997
; Cordier & Folco,
2014
; Genge &
Grady,
1998
; Taylor et al.,
2005
). To maintain these
conditions during passage through an oxidizing
atmosphere requires high abundances of carbon, which
implies a carbonaceous chondrite, rather than an OC
precursor. Furthermore, the
l
-porphyritic texture
and presence of vesicles suggest that at least part of the
micrometeorite was composed of hydrated material,
which would be consistent carbonaceous chondrite
FIGURE 5. EBSD patterns (left column) with indexed structure solutions (right column) showing stolperite (a;
Pm
-3
m
),
khatyrkite (b;
I
4/
mcm
), and the new Cu-Al phase (c). The new phase is indexed best with the W-type
bcc
structure. The blue
cross indicates the center-point of the pattern. (Color figure can be viewed at
wileyonlinelibrary.com
.)
Al-Cu-Fe alloys in the solar system
1649
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matrix, as opposed to OC matrix (Van Ginneken
et al.,
2017
). Additionally, the NiO contents of
neoformed olivine microphenocryts in cosmic spherules
provide another mechanism of constrain the probable
precursor material of micrometeorites, with Ni-poor
(NiO
<
0.7 wt%) compositions representing hydrated
carbonaceous chondrites with high carbon contents (CM,
CI, CR) and Ni-rich (NiO
>
0.7 wt%) compositions
representing carbon-poor, metal-rich bodies (CO, CV,
CK, OC; Cordier et al.,
2011
). Wave dispersive EMPA
data from the neoformed olivines were previously
reported in Suttle et al. (
2019
; Table
1
) and showed Ni-
poor compositions (average NiO: 0.28

0.14 wt%,
n
=
7) consistent with a CR, CM, or CI parent body.
The previous study of Suttle et al. (
2019
) argued that
KT01 was derived from a carbonaceous chondrite
precursor, and most likely a CO chondrite. Their
conclusion was based on petrographic and geochemical
evidence, namely the particle’s
l
-porphyritic texture
(Van Ginneken et al.,
2017
) and the minor element
composition of relict olivines (which overlaps the CM
and CO compositional ranges). Although the new O-
isotope data presented here rule out CO parentage (and
the closely related CM chondrites), it leaves open the
possibility of EC, OC, or CR chondrite source. Our re-
evaluation of the petrographic data strongly suggests that
KT01 derives from a carbonaceous chondrite parent
body and, therefore, we conclude that a CR chondrite is
the most likely precursor for this micrometeorite.
CONCLUSIONS
Much of the controversy and mystery surrounding
the discovery of quasicrystals in the Khatyrka meteorite
centered on the fact that icosahedrite (Bindi et al.,
2011
),
i
-Phase II (Bindi et al.,
2016
), and several of the
crystalline phases found closely associated with them (Ma
et al.,
2017
; Razin et al.,
1985
) are alloys of aluminum (a
high-temperature refractory lithophile) and copper (a
low-temperature chalcophile). Native aluminum metal is
an extremely rare occurrence, and up to that point, no
other natural example of the co-occurrence of Al and Cu
was known. Moreover, it was unclear how two metals
with such different cosmochemistry could ever come
together in any natural setting.
The discovery of KT01 proved that Al-Cu alloys are
not unique to Khatyrka and not unique to the CV3 class
of meteorites generally. Notwithstanding, the fact that
the only two known examples of this type of material
both originate from carbonaceous chondrite parent
bodies seems to suggest that this material is unique to, or
at least preferentially generated in, the outer solar system.
However, in terms of explaining how Al-Cu alloys form
naturally, the evidence presented in this paper only
deepens the mystery because the evidence suggests a
different pathway leading to the Al-Cu alloys in KT01
and in Khatyrka. In Khatyrka, impact-induced shock
processing appears to have played an essential role in
forming the alloys in general and the quasicrystals in
particular, as shown by the presence of high-pressure
phases and supported by laboratory shock experiments
that reproduced them (e.g., Asimow et al.,
2016
). In
KT01, we found no evidence of high-pressure phases,
although the pronounced thermal processing experienced
during atmospheric entry may have removed evidence of
shock metamorphism. Also, KT01 is a cosmic spherule,
while the Khatyrka grains originate from a larger
“macroscopic” meteorite. In fact, its only similarity to
Khatyrka, aside from the presence of Al-Cu alloys, is the
presence of small Fe droplets at the silicate/metal
interface and the rind of spinel, features consistent with
the formation processes proposed by Lin et al. (
2017
)
involving the “thermite” reaction coupling the reduction
of FeO to the oxidation of metallic Al.
FIGURE 6. Images of a 100 nm FIB foil cut across a
boundary between the metallic and silicate domains. (a) A
BSE image of the FIB foil. The
h
(khatyrkite,
I
4/
mcm
) and
b
(stolperite,
Pm
-3
m
) phases are intergrown and the new Cu-Al
phase occurs as nanoinclusions fully entrained in the
b
domains. (b) Mosaic of TEM bright field images showing
elongated inclusions in
b
grains and the mottled textures from
local defects in
h
grains. The metal/silicate interface shows a
spinel layer and Fe droplets as in Figure
4
. Inset is the
diffraction pattern of
b
<
011
>
zone, with 3
9
superstructure on
<
100
>*
and
<
011
>*
plus a complex superlattice on the 111
plane (see text). The
d
-spacing measured on the stolperite 100
diffraction is 2.84

A.
1650
C. Ma et al.
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Understanding the unknown physical processes that
led to these alloys is potentially important because they
were probably present since the birth of the solar system
and may have continued to occur through various
mechanisms in later stages. Continued study of the
existing specimens and attempts to reproduce them in the
laboratory are leading to insights. Meanwhile, the search
for more examples, though difficult, likely offers the most
effective way to shed light on this mystery.
Acknowledgments
—SEM, EBSD, and EDS analyses were
carried out at the Caltech GPS Division Analytical
Facility, which is supported, in part, by NSF Grants
EAR-0318518 and DMR-0080065. TEM analyses were
carried out in the Eyring Materials Center at Arizona
State University, supported in part by NNCI-ECCS-
1542160. The research was funded by MIUR-PRIN2017,
project “TEOREM deciphering geological processes
using Terrestrial and Extraterrestrial ORE Minerals”,
prot. 2017AK8C32 (PI: Luca Bindi).
Conflict of Interest Statement
—The authors of this paper
declare they have no conflict of interest.
Data Availability Statement
—The data that support the
findings of this study are available from the
corresponding author upon reasonable request.
Editorial Handling
Dr. Donald E. Brownlee
REFERENCES
Andronicos, C., Bindi, L., Distler, V. V., Hollister, L. S., Lin,
C., MacPherson, G. J., Steinhardt, P. J., and Yudovskaya,
FIGURE 7. TEM images of the observed phases. (a) The alloy
silicate interface with spinel (spl) rim and Fe droplets (the same
area is visible at lower magnification in Figure
6b
). The Cu-Al nanophase is entrained in
b
(stolperite) domains. (b) The defect-
rich texture in
h
(khatyrkite) contrasts with the smooth-looking
b
in bright field image. (c) Dark field image of
b
with
g
vector
=
121 shows a moderate density of defects in stolperite. (d) SAED pattern indicates a superposition of the
h
and
b
patterns with a specific topotactic relationship involving parallel
<
012
>
zones of both phases. Splitting of
h
(200) and
b
(400)
reflections indicates that the (400)
d
-spacing of khatyrkite is slightly greater than the (200)
d
-spacing of stolperite. The
d
-spacing
of the stolperite 100 diffraction is 2.97

A. (Color figure can be viewed at
wileyonlinelibrary.com
.)
Al-Cu-Fe alloys in the solar system
1651
19455100, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maps.14089, Wiley Online Library on [23/07/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License