First synthesis of a unique icosahedral phase from the Khatyrka meteorite by shock-recovery experiment

research papers

434

https://doi.org/10.1107/S2052252520002729

IUCrJ

(2020).

, 434–444

IUCr

ISSN 2052-2525

CHEMISTRY

CRYSTENG

Received 19 December 2019

Accepted 26 February 2020

Edited by P. Lightfoot, University of St Andrews,

Scotland

Keywords:

shock-wave experiments; graded

density impactors; icosahedral quasicrystals;

Khatyrka meteorite; phase transitions; planetary

impacts; nanostructures.

Supporting information

this article has

supporting information at www.iucrj.org

First synthesis of a unique icosahedral phase from

the Khatyrka meteorite by shock-recovery

experiment

Jinping Hu,

* Paul D. Asimow,

Chi Ma

and Luca Bindi

b,c

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA,

Dipartimento di Scienze della Terra, Universita

degli Studi di Firenze, Firenze I-50121, Italy, and

Istituto di Geoscienze

e Georisorse, Consiglio Nazionale delle Ricerche, Firenze I-50121, Italy. *Correspondence e-mail: jinping@caltech.edu

Icosahedral quasicrystals (i-phases) in the Al–Cu–Fe system are of great interest

because of their perfect quasicrystalline structure and natural occurrences in the

Khatyrka meteorite. The natural quasicrystal of composition Al

referred to as i-phase II, is unique because it deviates significantly from the

stability field of i-phase and has not been synthesized in a laboratory setting to

date. Synthetic i-phases formed in shock-recovery experiments present a novel

strategy for exploring the stability of new quasicrystal compositions and prove

the impact origin of natural quasicrystals. In this study, an Al–Cu–W graded

density impactor (GDI, originally manufactured as a ramp-generating impactor

but here used as a target) disk was shocked to sample a full range of Al/Cu

starting ratios in an Fe-bearing 304 stainless-steel target chamber. In a strongly

deformed region of the recovered sample, reactions between the GDI and the

steel produced an assemblage of co-existing Al

61.5

30.3

6.8

1.4

i-phase II +

stolperite (

, AlCu) + khatyrkite (

,Al

Cu), an exact match to the natural

i-phase II assemblage in the meteorite. In a second experiment, the continuous

interface between the GDI and steel formed another more Fe-rich quinary

i-phase (Al

68.6

14.5

11.2

1.8

), together with stolperite and hollisterite (

), which is the expected assemblage at phase equilibrium. This study is

the first laboratory reproduction of i-phase II with its natural assemblage. It

suggests that the field of thermodynamically stable icosahedrite (Al

)

could separate into two disconnected fields under shock pressure above 20 GPa,

leading to the co-existence of Fe-rich and Fe-poor i-phases like the case in

Khatyrka. In light of this, shock-recovery experiments do indeed offer an

efficient method of constraining the impact conditions recorded by quasicrystal-

bearing meteorite, and exploring formation conditions and mechanisms leading

to quasicrystals.

1. Introduction

Quasicrystals (QCs) are a unique type of solid characterized

by quasiperiodic translational order (Lifshitz, 2003). The first

known QC, for example, has an Al–Mn binary composition

and icosahedral symmetry featuring fivefold, threefold and

twofold rotation axes (Shechtman

et al.

, 1984). Since the

discovery of the first QC, a number of QCs in Al–TM (tran-

sition metal) binary, ternary and quaternary systems have

been synthesized at ambient pressure (

e.g.

Tsai, 1999; Steurer

& Deloudi, 2009, and references therein). In the last decade,

the discovery of naturally occurring quasicrystalline phases

opened up new questions about QC formation mechanisms

under conditions very different from those of conventional

metallurgical processing and about the implications of such

processes in a geological context. To date, three natural

quasicrystalline phases have been identified, exclusively from

a single meteorite, the Khatyrka CV3 chondrite (MacPherson

et al.

, 2013). The first phase is an icosahedral QC (i-phase) with

composition Al

, officially named icosahedrite (also

referred to as i-phase I; Bindi

et al.

, 2009, 2011). The second is

decagonite (d-phase, Al

), named after its decagonal

symmetry (Bindi

et al.

, 2015

). Both icosahedrite and

decagonite are known to be thermodynamically stable at

subsolidus temperatures and were produced by Al-alloy

quenching experiments before their discovery in nature (Tsai

et al.

, 1987; Lemmerz

et al.

, 1994). Nevertheless, their natural

discovery presents a puzzle because the conditions and

procedures used in laboratory synthesis of QC from metallic

liquid, gas or glass (Tsai, 1999) hardly resemble any natural

rock-forming processes. The third phase is another icosahedral

QC but with composition Al

(Bindi

et al.

, 2016),

known as i-phase II. This composition is outside the stability

field of icosahedral QCs in the Al–Cu–Fe system and has not

previously been produced in any experimental study. It

appears that a special formation mechanism or synthesis

conditions different from that of classic rapid quenching are

needed to understand the occurrence of i-phase II.

The discovery of shock-induced high-pressure silicate

minerals in Khatyrka,

e.g.

ahrensite and stishovite, motivated

the idea of a planetary impact origin for natural QCs (Holl-

ister

et al.

, 2014). Subsequently, this idea has been unam-

biguously supported by successful syntheses of Al–Cu–Fe i-

phase and Al–Ni–Fe d-phase by impacting Al alloys in the

laboratory (Asimow

et al.

, 2016; Oppenheim

et al.

, 2017

Interestingly, the i-phases reported so far from shock-wave

recovery experiments have compositions of Al

68–73

11–

10–12

1–4

1–2

(Asimow

et al.

, 2016), close to but different

from any previously observed natural or synthetic i-phases.

Hence, although these experiments demonstrate that natural

decagonite and i-phases like icosahedrite can have an impact

origin, the shock experiments open up two new questions.

First, Al–Cu–Fe icosahedrite is of great interest because it has

the most perfect (

i.e.

least defective) structure among all

known QCs. It also has a quite narrow stability field in

composition space, even at the optimal temperature range

(550–730

C; Bancel, 1999). Very small deviations from the

stability field lead to complex transformations of the i-phase at

lower temperatures (Bancel, 1999). Nevertheless, the shock-

synthesized i-phase has a distinct composition beyond the

known stability field and still shows a perfect structure, indi-

cated by robust diffraction studies (Asimow

et al.

, 2016;

Oppenheim

et al.

, 2017

). The high pressure and differential

stress during shock events may plausibly affect either the

(meta)stability or formation mechanism of the i-phase and

cause this discrepancy. Second, the usage of stainless steel as a

sample chamber and an Fe source in the shock experiments

brings Cr and Ni into the system, which also probably affect

the stability relations (Oppenheim

et al.

, 2017

) and lead to a

quinary Al–Cu–Fe–Cr–Ni i-phase. These questions motivated

continued studies of QC formation and stabilization by

experimental shock compression. One goal is to find an

optimal match to the phase assemblages and phase composi-

tions observed in nature, in order to refine estimates of the

exact shock conditions that produced the natural QC-bearing

assemblage, hence the overall impact history of the Khatyrka

meteorite and the origin of extraterrestrial Al–Cu alloys.

In this study, we report two new shock-recovery experi-

ments that used Al–Cu–W graded density impactors (GDIs),

initially designed for quasi-isentropic impact loading, as

starting materials in the targets. The Al–Cu gradient in a GDI

allows sampling of a wide range of Al/Cu ratios and, ideally,

traversal of the full stability field of icosahedral Al–Cu–Fe

QCs in composition space. In one of the new experiments, we

successfully produced Al

i-phase II from reactions in

the region that started with a high Cu/Al ratio. We also

investigated in detail the associated intermetallic phases,

khatyrkite (

,Al

Cu), stolperite [

, Al(Cu,Fe)] and hollis-

terite (

,Al

), which were either not observed or not fully

characterized in previous shock experiments. This is the first

laboratory synthesis of i-phase II and the most exact repro-

duction so far of the complete phase assemblage associated

with QCs in the Khatyrka meteorite. The results further

reinforce the impact origin theory of natural QC formation

and constrain the shock conditions for creating i-phase II and

its associated intermetallic companion phases. A complicated

pressure–temperature–time path is apparently required to

explain the co-existence of i-phase I, i-phase II, decagonite

and the assemblage of silicate high-pressure phases observed

in Khatyrka.

2. Experimental methods

2.1. Graded density impactor

The GDI was originally manufactured as a component of

gas-gun projectiles to produce quasi-isentropic ramp loading

for shock experiments (

e.g.

Kelly

et al.

, 2019), but was instead

used as a target in this study. The Al–Cu–W GDI disk that we

used has graded composition from aluminium on top through

the full range of Al–Cu alloys to copper in the middle, then

through the range of Cu–W alloys to tungsten at the bottom

(Fig. 1). The graded composition is achieved by tape-casting

layers of powder mixtures of Al–Cu or Cu–W (Kelly

et al.

2019). The original Al, Cu and W particles are under 325

mesh,

i.e.

m

m. The observed particles in the finished fully

dense sintered products are mostly equant and

m

m in size

[Figs. 1(

) and 1(

)]. The overall thickness of the Al–Cu–W

GDI disk is 3.3 mm. The Al top is 1.4 mm thick and contains

5% of Cu [Fig. 1(

)]. The proportion of Cu particles

increases gradually from the Al top towards the 0.1 mm pure

Cu layer in the middle [Fig. 1(

)]. Behind the Cu–W transi-

tional zone, the 1 mm W layer on the rear also contains above

5% Cu particles [Fig. 1(

)].

2.2. Shock-recovery experiment setup

The shock-recovery experiments were performed in the

Lindhurst Laboratory for Experimental Geophysics at

Caltech. We sliced an Al–Cu–W GDI disk at an oblique angle

to produce a complementary pair of wedge samples. The

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First synthesis of a unique icosahedral phase from the Khatyrka meteorite

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original GDI plate was milled to a disk 8.2 mm in diameter

and then cut diagonally on a plane inclined 22

from normal to

the cylindrical axis into two wedge-shaped halves. The two

wedges were used for two separate shock experiments. For

each experiment, an identically shaped wedge of 304 stainless

steel (SS304) was made to back up the GDI wedge (Fig. 1) and

assemble into an overall right circular cylinder to fit in the

sample chamber. The Al top of the GDI faced toward the

impact surface of the recovery chamber. The sample assembly

was encased in a SS304 chamber and impacted by either a

tantalum or SS304 flyer. The chamber and steel wedge operate

both as a momentum trap to confine the sample and as an Fe

source to the system, which is needed to synthesize Al–Cu–Fe

phases.

Although the wedge geometry causes lateral sliding across

the wedge interface, the peak pressure experienced by the

sample is controlled by (and can be calculated from) the

material impedance and is not initially affected by sliding on

the wedge interface (Potter & Ahrens, 1994). The pressures

were calculated from the Hugoniots of the metals and alloys

using both analytical impedance matching and the

WONDY

1D hydrocode (Kipp & Lawrence, 1982). This calculation

directly incorporated the porosity, density, sound speed and

shock impedance of the GDI as reported in the work by Kelly

et al.

(2019).

The first experiment, shot 1253 (S1253), used the Al-rich

half of the GDI and a tantalum flyer. The impact velocity of

0.93 km s

produced an estimated peak shock pressure of

20–30 GPa for 800 ns in the sample (see Fig. S1 in the

supporting information). In the two-wedge sample geometry,

the pressure–time history varied both along the shock direc-

tion and perpendicular to it as the layer thicknesses changed.

The Al-rich thin end of the GDI wedge [left side of Fig. 1(

)]

has a lower shock impedance than the steel chamber and was

not backed by the Cu–W layers. It was initially compressed to

14 GPa by the first shock front and then reshocked to 23 GPa

by a wave reflected from the steel chamber. In contrast, the

thicker part of the wedge [middle right of the GDI in Fig. 1(

)]

had layers of Cu–Al (and Cu–W). This part was first shocked

21 GPa and gradually compressed to >25 GPa as the

shock wave traversed the Cu–W gradient (Fig. S1). Only the

thickest part of the wedge [right side of Fig. 1(

)], with full Cu–

W layers, experienced the 30 GPa peak pressure. Because the

Cu–W regions have higher impedances than the steel

chamber, there was no reshock; rather a partial rarefaction

wave propagated back into the sample after the first shock

front reached the rear sample-chamber interface. This rare-

faction wave subsequently intersected release waves from the

sidewall and from the back of the flyer to create a spatially and

temporally complex pressure-release pattern. In the thickest

part of the GDI, it took 1

m

s for the peak pressure to drop to

10 GPa and 1.7

m

s to release to 0 GPa (Fig. S1). The thinner

part of the wedge saw a peak-pressure pulse shorter than

800 ns and a faster release. The second experiment, S1255,

used the W-rich half GDI for a sample and a SS304 flyer with

an impact velocity of 1.28 km s

(Fig. 1). The corresponding

peak pressure was 30–35 GPa for 600 ns. Since this wedge had

a uniform W-rich back, the peak pressure was also relatively

uniform.

The finely graduated Al–Cu transition in the GDI provides

an efficient way to sample a full range of Al/Cu ratios in the

starting material in one experiment. The wedge sample is

designed to convert the different particle velocities across the

GDI/steel interface into a component of interface-parallel

sliding and thus create strong shear flow. The sheared zone is

expected to enable or enhance melting and simultaneous

reactions at the GDI/steel interface. We emphasize that,

contrary to many shock-recovery capsules, this experiment is

not designed either to maintain simple 1D particular motion

across a planar shock front or to allow enough time for full

reverberation across the sample to bring the pressure to a

value equilibrated with that in the chamber walls.

2.3. Sample preparation and analytical techniques

The recovered sample was cut through the mirror plane of

the GDI wedge [Fig. 2(

)]. The exposed surface was polished

on diamond lapping films and analyzed with a field-emission

scanning electron microscope (FE-SEM) in the Division of

Geological and Planetary Sciences at Caltech. Backscattered

and secondary electron (BSE and SE) images were employed

to observe the microtextures of the run product. Energy-

dispersive X-ray spectroscopy (EDS) with a silicon drift

detector was used to measure the chemical composition of the

intermetallic phases. To accurately measure the compositions

of submicrometre grains of interest, we collected and

compared spectra obtained by operating the SEM at 10, 12, 15

and 20 kV accelerating voltage and 4–6 nA beam current. The

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Figure 1

Starting material and target assemblage of the shock-recovery experi-

ments. (

), (

) The target assemblies of shots 1253 and 1255, respectively.

Two wedges cut from one Al–Cu–W GDI disk were used for the two

experiments. The size of the SS304 chamber and Ta/steel impactor plate

are schematic and are not proportional to the sample size. (

), (

) SEM

BSE image of the cross section of the starting GDI. The Al–Cu gradient is

shown in the middle of (

). A high-magnification image of the transitional

part is shown in (

corresponding X-ray excitation volumes in Al–Cu–Fe alloy

range from

0.5 to 1.8

m

m in both depth and diameter, indi-

cated by Monte Carlo simulations (Gauvin

et al.

, 2006). Each

EDS spectrum was collected for 10 s with more than 200

counts channel

and 40% dead time. Our previous study

indicates that EDS measurement of intermetallic phases in

this size range with this protocol is accurate and in good

agreement with electron-microprobe quantitative analysis

(Oppenheim

et al.

, 2017

). We employed electron backscatter

diffraction (EBSD) to determine the (quasi)crystal structure

of the phases. Kikuchi bands in the diffraction patterns of the

crystalline phases are indexed to lattice planes, with mean

angular-deviation values of less than 0.6

. The icosahedral QC

phases, which cannot be indexed by the

AZtec

EBSD

nanoanalysis software (Oxford instruments) despite high

pattern quality, are identified from the arrangement of five-

fold, threefold and twofold rotation axes in the diffraction

patterns.

3. Results of shock-wave experiment

3.1. Shock deformation in experiment S1253

Overall, shock-induced deformation is concentrated along

the ramp and at the corners of the GDI wedge and identified

from the significantly changed shape (Fig. 2) compared with

the starting wedge (Fig. 1). Shock-shear melting and reactions

between Al, Cu and steel are associated with the deformation

zones and produce new intermetallic phases [Fig. 2(

)]. The

long interface between the Al-rich top of the GDI and steel

driver is coherent and well defined without much reaction. In

contrast, the bottom portion of the wedge is deformed into an

L shape [Figs. 2(

) and 2(

)], significantly different from the

starting triangular cross section of the wedge [Fig. 1(

)]. In the

lower-right corner (Fig. 2), the sample has been squeezed and

extruded along the direction of the impact, making it normal

to the impact face and the coherent Al-rich top of the wedge.

In this region of high strain, both the GDI and the steel

chamber (or insert) underwent partial melting, with simulta-

neous reaction between them [Fig. 2(

)]. In Fig. 2(

), the

compositional gradients have been rotated to the horizontal

direction by the deformation. The right side is Al rich and

forms relatively Cu-deficient intermetallic phases by reaction

with steel. The left side has a higher Cu/Al ratio and forms Al–

Cu–Fe phases that resemble the natural metallic assemblage in

the Khatyrka meteorite, including i-phase II. Details of these

reacted assemblages are described in the following sections.

3.2. i-phase II assemblage in S1253

Reactions between the steel chamber and the Al–Cu

mixture on the Cu-rich side produced intermetallic phases in

association with deformed Cu grains [Fig. 3(

)]. Away from

the zone of melting and extensive reaction, the remnant Al

and Cu have been deformed into 10

m

m elongated grains

[Figs. 3(

) and S2(

)] from the starting

m

m equant grains

[Figs. 1(

) and 1(

)]. Reaction in this region is limited to

narrow (<1

m

m) bands along the Al–Cu grain boundaries. In

contrast, in the reaction zone, the aluminium is probably

consumed by eutectic melting (Lin

et al.

, 2017; Suttle

et al.

2019) and completely reacted with Cu into the intermetallic

phases [Fig. S2(

)], whereas Cu is partially remnant. The

stainless-steel chamber is locally incorporated in the reaction

as the only Fe source.

Al–Cu–Fe icosahedral QCs occur only in specific locations

with Cu/Al > 1 and are associated with reactions involving the

steel chamber. Fig. 3(

) demonstrates the texture of the

icosahedral QC and associated phases. The grains with

medium grey contrast and semi-radial patterns represent

i-phase II (see below). The EBSD pattern of these grains

[Fig. 3(

)] includes fivefold, threefold and twofold rotation

axes demonstrating icosahedral symmetry. High-contrast

Kikuchi bands in the pattern indicate the robustness of the QC

structure. The diffraction also indicates that the core and

petals of a given i-phase aggregate have the same crystal-

lographic orientation. That is, each contiguous i-phase grain is

a single QC domain. Because of the small domain size, we

used various accelerating voltages (10, 12, 15 and 20 kV) to

image and analyze nine well defined QC domains (Figs. S2 and

S3 and Table S1 in the supporting information). The formula

of the i-QC averaged from 10 kV analyses is Al

61.5

30.3

6.8

1.4

with an uncertainty of up to 2.8 atomic percent

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Figure 2

The overall image of the shocked GDI wedge of S1253 and its deformed

zones. (

), (

) represent reflected light and BSE images of the sample.

The ramp of the wedge is deformed into an L shape and reaction is

concentrated along the right wall. (

) A BSE image of the boxed area in

(

). Reactions between the GDI and the steel chamber occur along the

right and left edges of the GDI material, which began before deformation

as the Al top and Al–Cu ramp of the GDI, respectively. The boxed area is

shown in Fig. 3.

(Tables 1 and S1). The results from 12 and 15 kV analyses are

identical to 10 kV analyses within the uncertainty (Table S1),

whereas a few 20 kV analyses show clear interference from

surrounding phases. This composition is in good agreement

with natural i-phase II Al

and is distinct from the

i-phase I in the Khatyrka meteorite (Tables 1

and S4). The shock-synthesized i-phase II also contains minor

Cr because the Fe source in this experiment is the stainless-

steel chamber.

Two intermetallic phases are associated with i-phase II in

the reaction zone [Fig. 3(

)]. The phase with dark grey BSE

contrast, close to that of i-phase II, shows a tetragonal

symmetry matching the

mcm

space group in its EBSD

pattern [Fig. 3(

)], identified as khatyrkite (also referred to as

the

phase in the Al–Cu–Fe system). The 1

m

m grain size of

khatyrkite is sufficient for robust EDS analysis (Table 1). Its

formula of Al

65.1

34.6

0.3

also matches with natural khatyr-

kite, with slight Al deficiency relative to the ideal formula

Cu of the

phase. Khatyrkite is not a significant Fe host

but it contains a few atomic percent more Cu than i-phase II

(Table 1), leading to a similar mean atomic number and an

almost identical BSE contrast. The

light grey phase in the BSE images is

stolperite [



, known as the

phase, ideal formula Al(Cu,Fe)], iden-

tified by EBSD [Fig. 3(

)] and EDS

analysis (Table 1). Stolperite generally

provides very good band quality in

EBSD patterns that allows unambig-

uous differentiation of the primitive

lattice from the body-centred cubic

lattice (Oppenheim

et al.

, 2017

). Its

formula of Al

57.3

40.6

1.6

0.4

0.1

again nearly identical to the natural

occurrence in the Khatyrka meteorite,

except for minor Cr and Ni from the

steel (Table 1). Besides the i-phase II +

khatyrkite + stolperite assemblage,

there are fragments in the reaction

zone with bright white BSE contrast

[Figs. 3(

) and S3]. These are remnant

stainless-steel fragments, proving the

inflow of the chamber material to the

reaction zone.

In this i-phase II + khatyrkite +

stolperite assemblage, none of the

mineral grains are completely euhedral

except that the petal texture of the i-

phase II can be considered as a hint of

its symmetry [Fig. 3(

)]. The stolperite

occurs as subhedral rounded grains

that show a subtle tendency to enclose

the i-phase II grains, suggested by the

concavity of stolperite-QC interfaces.

In contrast, the khatyrkite is mostly

anhedral and interstitial to the i-phase

II and stolperite. This is noticeably

different from the khatyrkite predominant assemblage

described in the following section.

3.3. Non-QC assemblage in S1253

Apart from the i-phase II assemblage described above, the

majority of the reaction zones in S1253 [Figs. 2(

) and S4(

)]

contains only Al–Cu–Fe intermetallic phases without QCs.

Figs. 3(

) and S2(

) show a typical reacted area adjacent to the

i-phase II assemblage. In this area, submicrometre grains

occur in a matrix of finer-grained material of darker BSE

contrast. EBSD and EDS analyses indicate that the coarser

grains are khatyrkite (Table S2). The formula Al

67–71

29–33

matches with the ideal

phase and no Fe can be detected.

Equant, elongated and irregular grains of khatyrkite are all

common in this region, with subhedral shapes surrounded by

the matrix [Fig. S2(

)]. The matrix contains grains smaller than

100 nm, which is difficult to analyze with the SEM. Based on

their BSE contrast, the matrix phases are inferred to be pure

Al plus another Al-rich alloy. It is worth noting that neither

steel fragments nor measurable concentrations of Fe, Cr or Ni

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Figure 3

BSE images of the QC-bearing assemblage from the reaction zone in S1253. (

) The region from the

boxed area in Fig. 2(

). The right side of the image presents deformed and elongated Al–Cu grains

from the GDI. The left side shows the texture of reaction products between the GDI and the steel

chamber. The QC assemblage is shown in (

) and the intermetallic assemblage is shown in Fig. S2. (

)

The assemblage of i-phase II, khatyrkite (

), and stolperite (

). The i-phase II forms petal-like

domains. Stolperite is brighter and more euhedral than khatyrkite. (

) EBSD patterns of the

identified phases. The coloured numbers mark the rotation axes of icosahedral symmetry in the i-

phase II pattern. Coloured lines denote the indexed poles of the Kikuchi bands. (

) The same i-phase

II assemblage in a natural sample, Khatyrka meteorite grain 126A [after the work by Bindi

et al.

(2016)]. The i-phase II in this case is enclosed by stolperite.

occur in these khatyrkite predominant regions, in contrast

with the i-phase II region [Figs. 3(

) and S2].

As shown in the overall image of the sample (Fig. 2), the

right side of the GDI was deformed along the impact direction

and both top and bottom [correspondingly right and left in

Fig. 2(

)] of the GDI melted and reacted with the steel insert

or chamber. Although deformation in the top and bottom part

is equally strong, the resulting reactions and product phases

are quite different. The QCs only occur along the Cu-rich

bottom of the ramp. Fig. S4 shows the textures of the

complementary Cu-deficient phases along the Al-rich top. In

this reaction region [Fig. S4(

)] there are portions with

distinctively high and low BSE contrast, corresponding to high

and low iron content. The low-contrast portion has two phases,

a black granular phase and a dark grey dendritic phase, in the

BSE image [Fig. S4(

)]. The phase with black contrast is pure

Al, probably recrystallized to submicrometre domains from

the large starting Al grains. The dendrites are hollisterite

(

, referred to as the

phase) based on EBSD and EDS

analyses

(Table

S2).

The

formula

64.7

18.7

8.1

5.5

2.5

0.5

includes significant Cu compared with the ideal

formula Al

(that is, Al

76.5

23.5

on a 100-atom basis) but is

in good agreement with the natural hollisterite in Khatyrka

(Ma

et al.

, 2017). Distinctively, the portion with high BSE

contrast is dominated by petal-textured grains [Fig. S4(

)].

This is another

phase with a more Fe-rich formula

(Al

7.6

30.3

3.2

0.5

0.3

, Table S2) than the stol-

perite in the i-phase II region; we refer to this as Fe-stolperite.

The aggregates of the petals are 3–5

m

m in size and each petal/

dendrite is submicrometre. In this area, Fe-stolperite also

occurs as equant grains without forming aggregates [Fig.

S4(

)]. There are small voids spreading through the Fe-stol-

perite region, appearing as black dots in the BSE image.

Generally, the reaction zone along the Al-rich top of the GDI

incorporated a larger fraction of the steel chamber than the

i-phase II assemblage along the opposite chamber wall, as

shown by positive correlation between Cr, Fe and Ni. Mn and

Si can also be detected by EDS in reacted phases in this area

(Table S2).

3.4. Deformation and icosahedral QC in S1255

The GDI wedge used in S1255 has a ramp with an Al–Cu

gradient and a W base, which provides a higher shock pressure

than S1253 [30 GPa (Fig. S1)]. It is easy to distinguish the run

product of S1255 by the continuous reaction zone along the

Al–Cu ramp [Fig. 4(

)]. The concave shape of the deformed

ramp matches the impact direction, whereas the top corner

preserves the ramp angle. This suggests weaker local defor-

mation than in S1253, where part of the ramp was fully

transposed to the vertical direction.

The phase assemblage in the continuous reaction zone of

S1255 is relatively consistent (except at the Cu-free pure Al

tip). Unlike the limited occurrence of i-phase II in S1253,

icosahedral QC is a major constituent of the reaction zone of

S1255. Fig. 4(

) demonstrates a representative zoomed-in

image of the reaction region. The i-phase is identified by

EBSD patterns matching icosahedral symmetry. In this case,

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Table 1

Compositions of the intermetallic phases in S1253 and the Khatyrka

meteorite.

Data from this study (S1253) and the Khatyrka meteorite are presented in

atomic percent and normalized to a total of 100%. Numbers of analyses are in

parentheses. Absent numbers are for elements below the detection limit or not

reported. Variance of the analyses is presented using standard deviation of the

mean (

). The data for i-phase II are averaged from analyses under 10 kV

accelerating voltage using the SEM. Results for individual analyses are shown

in Table S1.

i-phase II

Khatyrkite

Stolperite

Element S1253 (9) Khatyrka†‡ S1253 (3) Khatyrka§ S1253 (7) Khatyrka§

61.47

61.92

65.09

68.48

57.35

57.34

30.30

31.23

34.63

30.83

40.56

40.58

6.80

6.78

0.28

0.69

1.57

2.08

1.42

0.08

—

0.40

—

Ni—— —— 0.12—

1.33

0.99

1.00

1.78

1.26

1.33

2.82

0.80

1.03

0.80

1.21

0.94

1.69

0.37

0.24

0.02

0.61

0.05

0.34

0.02

—

0.21

—

Ni—— —— 0.20—

† Bindi

et al.

(2016).

‡ Lin

et al.

(2017).

§ Ma

et al.

(2017).

Figure 4

BSE images of S1255. (

) The overall image of the shock-deformed GDI.

The grey smooth background is the steel chamber. Reactions occur

continuously along the Al–Cu ramp. (

) A representative zoomed-in

image of the phase assemblage in the reaction zone in (

). The i-phase

and hollisterite (

) occur as subhedral grains, while stolperite (

) is more

anhedral.

the i-phase mostly occurs in angular but equant grains up to

m

m in size. QC aggregates are not observed in the S1255

reaction zone. The i-phase formula, Al

68.6

11.2

14.5

1.8

(Table S3) is quite distinct from either the i-phase II in S1253

(Al

61.5

30.3

6.8

1.4

) or the optimal i-phase (icosahedrite,

), but is similar to the previously shock-synthe-

sized i-phase (Fig. 5 and Table S4). The reaction zone contains

two more major phases in direct contact with the i-phase. First,

the phase with very slightly darker BSE contrast than the

i-phase [Fig. 4(

)] is identified as hollisterite (

,Al

again by EBSD and EDS (Table S3). The grains are mostly

equant, granular and subhedral, but some rectangular grains

of hollisterite manifest the prismatic euhedral crystal form.

The formula, Al

70.4

10.6

12.7

3.9

1.4

, generally matches

with the hollisterite in Khatyrka (Ma

et al.

, 2017) and S1253,

with some Fe deficiency (Table S3). Second, EBSD and EDS

indicate that the light grey phase in the BSE image [Fig. 4(

)]

is a

phase. The grains are mostly angular and anhedral. The

composition of this

phase in S1255 varies by up to 20 at.% in

Cu and Fe, resulting in a formula range of Al

47–58

14–34

12–

3–7

1–3

, whose projection into the Al–Cu–Fe ternary plots

along a binary join between ideal AlCu (stolperite) and ideal

AlFe. Despite the compositional variation, the quality of the

EBSD patterns of this phase are consistently robust, matching

the primitive cubic structure of the

phase.

4. Discussion

4.1. Composition and stability of quaternary/quinary

shock-synthesized i-phases

The i-phase II in S1253 is the first shock-synthesized QC

that almost exactly matches the composition of a natural i-

phase (Table 1 and Fig. 5). This is a composition never seen

before in laboratory syntheses and is outside the well defined

stability field of the i-phase at ambient pressure. The natural i-

phase II in Khatyrka occurs in domains that are >1

m

m in size

[Fig. 3(

)], allowing for robust

in situ

chemical analysis, even

though the occurrences are limited to only a few areas of a

single fragment of the meteorite. In the case of Khatyrka,

more than ten electron-microprobe analyses provide a

consistent composition of Al

61.9

31.2

6.8

0.1

, with less than

1 at.% uncertainty (Table 1; Bindi

et al.

, 2016; Lin

et al.

, 2017).

The i-phase II in S1253, however, is more challenging to

analyze because of its submicrometre domain size [Figs. 3(

)

and S3]. The low accelerating voltage (10 kV) FE-SEM-EDS

analysis helps by reducing the excitation volume significantly

0.5

m

m in depth and diameter. The average composition

from low-voltage analysis is Al

61.5

30.3

6.8

1.4

with uncer-

tainty (standard deviation) on the four elemental atomic

percentages of 1.33, 2.82, 1.69 and 0.34 at.%, respectively

(Table 1). Measurements on the same individual grains with

up to 20 kV voltage (up to 1.8

m

m in depth and diameter of the

excitation volume) result in the same level of uncertainty. This

suggests that the deviation is not primarily caused by random

error in the low voltage/current measurement but rather

records small compositional variation among the analyzed i-

phase II grains (Table S1). Nevertheless, the 15–20 kV

analyses, with excitation volumes larger than the grains, start

to show moderate interference from surrounding khatyrkite

and stolperite [Figs. S2(

) and S3]. This is consistent with the

fact that the 20 kV BSE image shows blurred grain boundaries

(Fig. S3). However, in the 10 kV BSE image the grain

boundaries are sharp and clear. Because the majority of the

10–12 kV analyses show consistent results, we take the 10 kV

average as the best estimate of the composition of synthetic i-

phase II in S1253 (Table 1). This formula, Al

61.5

30.3

6.8

1.4

, is a good

match

with

natural

i-phase

61.9

31.2

6.8

0.1

and distinct from the thermodynamically

stable icosahedrite (Al

; Bindi

et al.

, 2011), even

when considering the 1–2 at.% uncertainty (Fig. 5).

Another distinctive feature of the shock-synthesized

i-phase is the incorporation of Cr and Ni from the stainless-

steel chamber (Fig. 5). The i-phase II in S1253 contains 1.4

at.% Cr versus 0.1% in natural i-phase II. Generally, the Cr

and Ni content of the S1253 and S1255 i-phases analyses

increase with Fe content and can contain up to 4% Cr and

1.8% Ni (Fig. 5 and Table S4). Their corresponding Cr/Fe

ratios fall between 0.2 and 0.3, and the Ni/Cr ratios are

0.5

(Table S4). These ratios agree with the 18/8 (18% Cr and 8%

Ni) composition of SS304 from the sample chamber, which is

the only Fe source. This suggests that Cr and Ni are neither

selectively taken up by the QC in preference to Fe nor

selectively excluded. Nevertheless, the Cr + Ni content may

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Figure 5

Compositions of the natural and synthetic i-phases. Corresponding EDS

analysis results are shown in Table S4. The error bar is the standard

deviation of multiple individual analyses. When there is no symbol for a

certain element it means the element is not detected in the analysis. When

there is no visible error bar it means it is smaller than the size of the

symbol. References: natural i-phase II in Khatyrka, Bindi

et al.

(2016);

natural thermodynamically stable icosahedrite (i-phase I), Bindi

et al.

(2011); a previous shock-synthetic i-phase (second from the right),

Asimow

et al.

(2016); and a previous shock-synthetic i-phase (far right),

Oppenheim

et al.

(2017

affect the Al/Cu ratio in the Al–Cu binary cluster in the i-

phases. Oppenheim

et al.

(2017

) studied the stability of the

Al–Cu–Fe–Cr–Ni quinary i-phase, using the Hume–Rothery

theory based on a valence electrons per atom criterion. This

theory suggests that >70% Al and equal Cu–Fe contents are

preferred if the i-phase is to absorb >5% Cr + Ni. This helps

explain the unexpectedly high Al content (>68 at.%) and

nearly equal Fe and Cu contents of all the shock-synthesized i-

phases in S1255 and in previous studies (Fig. 5). In contrast,

the Cr and Ni in the measured i-phase II are low or even

undetectable in some grains (Fig. 5), which may be one of the

reasons that its Al and Cu/Fe content are such a close match to

natural i-phase II. Moreover, Wolf

et al.

(2019) analyzed semi-

equilibrated Al–Cu–Fe–Cr samples made by co-sputtering

followed by quick annealing and observed a high Cr i-phase

with Al

63–65

12–22

5–11

6–15

composition. This suggests that

metastable i-phases can incorporate significant Cr. The stabi-

lity of i-phase II in contact with intermetallic assemblages in

the Al–Cu–Fe system will be discussed in the next section.

4.2. Formation and stability of i-phase and intermetallic

co-existing phase assemblage

There have been a number of studies on the phase

boundaries of i-phases in the Al–Cu–Fe ternary system since

its first synthesis (Tsai

et al.

, 1987). Previous work investigated

the stability field for both subsolidus (

e.g.

Bancel, 1999) and

liquidus (

e.g.

Zhang & Lu

ck, 2002, 2003; Stagno

et al.

, 2017)

conditions. It has been found that only a very narrow range of

composition in the vicinity of the optimal Al

thermodynamically stable through the whole subsolidus

temperature range (Bancel, 1999). In this sense, it is not

surprising that the first discovered natural i-phase, induced by

impact, sits right on this optimal composition and survived a

complex series of planetary and terrestrial processes (Bindi

al.

, 2009, 2012; Lin

et al.

, 2017). In contrast, this restricted

stability range for the i-phase also makes it challenging to

understand the conditions of formation of i-phase II and other

shock-synthesized i-phases.

Since S1253 almost exactly reproduced the phase assem-

blage, composition and texture of the i-phase II in the

Khatyrka meteorite (Fig. 3; Table 1), their conditions and

sequence of formation can be discussed together. The i-phase

II can occur either as a stable or metastable phase; evidently,

the former would require a thermodynamic stability field. A

simple but important fact is that a typical shock-recovery

experiment produces a high-pressure pulse of

m

s duration

(Fig. S1) and a shock temperature of a couple of 100

Cin

dense metal samples at 20–30 GPa (Oppenheim

et al.

, 2017

This temperature–time condition cannot drive long-range

diffusion or reconstructive solid-state transformations in

metals and alloys (Porter

et al.

, 2009). Therefore, reaction and

new intermetallic phases need to occur either in areas of local

melting by strong shear deformation and collapse of pre-

existing pores and voids from shock compression (Oppenheim

et al.

, 2017

) or by a much slower process after release from

the shock state. The latter is not favoured because, in shots

1253 and 1255, the melting and reaction zones are exactly

coincident with the regions of strongest deformation, which

are localized at ramp corners and edges. The central coherent

part of the GDI is compressed and released uniaxially without

generating new phases (Fig. 2). In this scenario, the liquidus

phases are the most likely to occur in the reaction zones and

could then quench through heat transfer to the colder

surroundings with the possibility of undergoing subsequent

peritectic or subsolidus transformations as they cool. There-

fore, both liquidus and solidus phase relations are relevant in

explaining the stability of the i-phases.

Fig. 6 shows a representative isothermal section of the Al–

Cu–Fe system at subsolidus temperature (<740

C) and

ambient pressure (Bancel, 1999). In this diagram, icosahedrite,

i-phase II, hollisterite (

), stolperite (

) and khatyrkite (

)

have been observed in the QC assemblage in the Khatyrka

meteorite and in this study. A section of the liquidus diagram

indicated by the brown triangle in Fig. 6 is also shown as an

inset (Zhang & Lu

ck, 2003). The liquidus triangle includes the

distributory peritectic four-phase reaction point, stolperite +

hollisterite + melt

icosahedrite, at 882

C. This is in good

agreement with the occurrence of icosahedrite and directly

associated hollisterite + stolperite in Khatyrka (Bindi

et al.

2016). Because the liquidus temperatures of hollisterite and

stolperite are higher than that of icosahedrite (Bancel, 1999),

icosahedrite rarely crystallizes directly from the melt. Instead,

it forms by peritectic reaction between stolperite + hollisterite

and evolving residual melt. In Khatyrka, both icosahedrite and

i-phase II are enclosed within stolperite grains [Fig. 3(

); Bindi

et al.

, 2016]. Since i-phases are not supposed to form prior to

stolperite, it is inferred that stolperite-associated icosahedrite

and i-phase II both formed by peritectic reactions involving

stolperite. For the i-phase II assemblage in S1253 [Fig. 3(

)],

although i-phase II is not completely enclosed, it is partially

surrounded by stolperite, commonly with concave boundaries.

It is possible that the i-phase II in S1253 also formed by

peritectic reaction with melt and stolperite. Now, unlike the

pervasive stolperite, hollisterite is quite rare in the vicinity of

both natural and shock-synthetic i-phase II [Figs. 3(

) and

)]. In fact, khatyrkite is the other major constituent in the

QC assemblages. This can best be explained by the fact that

the i-phase II region has relatively low Fe content and

hollisterite is almost completely consumed to provide the

needed Fe for i-phase II (Table 1 and Fig. 6). That is, hollis-

terite appears to be the limiting reactant for the extent of

formation of i-phase II. Subsequently, at lower temperature,

<591

C, the residual melt reacts with stolperite to form

khatyrkite through another peritectic reaction (Zhang &

ck, 2003; Suttle

et al.

, 2019). The khatyrkite predominant

region in S1253 [Figs. 3 and S2(

)], only a few

m

m away from

the i-phase II, is evidence that local Al–Cu melts with no Fe

source can be supercooled to 591

C to make khatyrkite before

any significant crystallization.

This two-stage peritectic reaction sequence is proposed to

explain the texture and assemblage of i-phase II occurrences.

It is still a question whether the observed assemblage can co-

exist stably or if instead it is an evolving assemblage preserved

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metastably. In previous studies of subsolidus equilibrium at

ambient pressure, the most ‘i-phase II-like’ compositions are

62.3

28.6

9.1

annealed from 660

C (Zhang

et al.

, 2005) and

67–56

24–31

9–13

annealed from 700

C (Zhu

et al.

, 2020).

Nevertheless, these compositions are still not as Fe deficient as

i-phase II. Moreover, the authors inferred a lower limit to

their Fe content of i-phase co-existing with stolperite and

khatyrkite assemblage from the topology of the phase diagram

(Zhu

et al.

, 2020). In other words, if one draws a tie-line

connecting i-phase II and hollisterite (instead of khatyrkite)

on, for example, Fig. 6, the line would traverse the icosahedrite

region, preventing these two phases from co-existing.

However, the inferred khatyrkite-bearing assemblage was not

actually observed in the vicinity of the Fe-deficient i-phases in

previous experiments and the stability field of such an i-phase

was not fully defined (Fig. 6). In this study and in Khatyrka

(Bindi

et al.

, 2009, 2011, 2016), two assemblages are indeed

observed that perfectly separate into two fields and do not

interact with each other: (1) icosahedrite + hollisterite +

stolperite and (2) i-phase II + stolperite + khatyrkite (Fig. 6).

A simple explanation for their co-existence is that either or

both the strong shear-stress field and >20 GPa

pressure during a shock event cause the

stability field of the i-phase to separate or shift

towards Fe-deficient compositions and enables

a peritectic crystallization sequence leading to

the Fe-deficient i-phase assemblage, given a

suitable starting composition. The composi-

tional range for making i-phase II can be very

narrow. The GDI was a successful starting

material here because it traverses the full

range of Al–Cu binary compositions. Even so,

the extent of inflow of the steel chamber

material needs to be coincidently right in order

to avoid crossing into the other, more Fe-rich,

i-phase field. It is also worth noting the possi-

bility of i-phase II being a metastable phase at

the relevant pressure–temperature conditions;

its formation could still cause the separation of

iron-rich and iron-deficient assemblages. In

this scenario, the shock pressure may lower the

activation energy for nucleation and growth of

i-phase II and make it easier to form than

under ambient-pressure conditions. Although

there are not enough thermodynamic data on

high-pressure Al–Cu–Fe i-phases to assess this

hypothesis directly, experiments on a loosely

analogous

7.5

metallic

glass indicate that the energy barrier for QC

formation decreases by 40% at 0.86 GPa

compared with ambient pressure (Jiang

et al.

2001). Subsequent fast quenching during a

shock event would then be advantageous for

preserving the metastable assemblage.

In contrast to S1253, the i-phase + hollis-

terite + stolperite assemblage in S1255 shows a

texture of simple crystallization, more like an

equilibrium

rather

than

peritectic

assemblage.

The

68.6

11.2

14.5

1.8

composition of the i-phase matches

with the Al

68–73

11–16

10–12

1–4

1–2

composition seen in

previous experiments on CuAl

alloys. The current data are

consistent with a separation of the stability field of i-phase into

two fields: Fe deficient as in S1253 and Fe rich as in S1255.

However, considering that the composition of icosahedrite

does not change much with the application of static high

pressure (Stagno

et al.

, 2014, 2015), a possible alternative is

that i-phase I is stabilized by the presence of Cr + Ni whereas

i-phase II forms and quenches metastably through the

proposed two-stage peritectic reaction sequence.

4.3. Impact conditions recorded by the Khatyrka meteorite

The near-exact reproduction of the natural i-phase II

assemblage in S1253 helps to better constrain the impact

conditions experienced and recorded by the Khatyrka

meteorite. The finding of ahrensite (from transformation) and

stishovite (crystallized from melt) in the silicate lithology of

the Khatyrka meteorite has supported inference of a shock-

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Figure 6

Representative subsolidus phase diagram of the tenary Al–Cu–Fe system at ambient

pressure after the work by Bancel (1999). Light grey marks one-phase fields, and tie-lines

delimit two-phase regions and three-phase triangles. Dark grey marks the field of

icosahedrite. The light brown dash-line triangle indicates the area of the composition space

shown in the upper-right inset for a section of the liquidus phase diagram at 882

C after the

work by Zhang & Lu

ck (2003). Against this background, data points are shown for the

synthetic and natural phase assemblages discussed in this work; symbol shape represents the

phase type and colour represents its origin. The contents of minor elements such as Ni and

Cr are not reflected in this diagram. The coloured fields connect the co-existing phases in the

icosahedrite and i-phase II assemblages. In the Khatyrka meteorite, icosahedrite co-exists

with hollisterite and stolperite and defines the dark cyan obtuse triangle. In the same

meteorite, i-phase II co-exists with khatyrkite and hollisterite and defines the light cyan

triangle. The red field constrained by the shock-synthesized i-phase II assemblage in S1253

is close to that from Khatyrka. The i-phases from other shock experiments are concentrated

on the more Cu-deficient side of the diagram.

pressure well above 5 GPa, likely to be

20 GPa for one

shock event (Hollister

et al.

, 2014; Ma

et al.

, 2016). The shock

pressure for the reaction zone of interest in S1253 is 20–

25 GPa (Fig. S1). The 5 GPa difference between these results

is not a discrepancy because the experiment has not placed a

lower bound on the shock pressure for formation of the i-

phase II assemblage. Nevertheless, if high shock pressure and

shear stress truly shifts or separates the stability field of the i-

phase and determines whether the Fe-rich or the Fe-deficient

assemblage forms, then icosahedrite would tend not to form at

the same conditions as i-phase II. In a single shock scenario, i-

phase II and its assemblage would occur from material

quenched during the high-pressure pulse, whereas material

that remains as hot Al–Cu–Fe melt until after the shock-

pressure release could crystallize icosahedrite and its co-

existing assemblage. In contrast, the formation of the Al–Cu

metal precursors to all the shock-induced reaction chemistry

described here probably requires a separate earlier event, as

shown by the petrographic evidence reported in the work of

Lin

et al.

(2017) and by uranium Th–He dating of olivine in

Khatyrka (Meier

et al.

, 2018).

5. Conclusions

Al–Cu–W disks with graded composition (GDIs) are useful

for sampling a wide range of Al/Cu ratios and efficiently

exploring composition space for conditions that may yield

variant Al–Cu–Fe quasicrystals in shock-recovery experi-

ments. With our experiments, the naturally occurring i-phase

II quasicrystal (Al

) found in the Khatyrka meteorite

has been reproduced for the first time in a laboratory setting,

along with the associated natural assemblage of stolperite plus

khatyrkite.

Another

Fe-rich

quinary

i-phase

(Al

68.6

14.5

11.2

1.8

) is also produced together with

stolperite and hollisterite. The shock synthesis of the i-phase II

+ stolperite + khatyrkite assemblage suggests that the ther-

modynamic stability field of icosahedrite (Al

) could

shift and/or separate under shock conditions above 20 GPa,

leading to the co-existence of Fe-rich and Fe-poor i-phases

like the case in Khatyrka. Future experiments, for example

with Cr- and Ni-free starting material, will better resolve the

phase boundaries of Al–Cu–Fe i-phases under impact condi-

tions.

Acknowledgements

We are grateful to Jeffrey Nguyen from Lawrence Livermore

National Lab for providing the GDI. We thank Matthias Ebert

and

two

anonymous

reviewers

for

their

constructive

comments.

Funding information

thank

NASA

Solar

System

Workings

grant

80NSSC18K0532 for supporting JH and this research. The

Lindhurst Laboratory for Experimental Geophysics at

Caltech is also supported by NSF awards EAR-1725349 and

1829277. LB is funded

by MIUR-PRIN2017,

project

‘TEOREM - deciphering geological processes using terrestrial

and extraterrestrial ORE minerals’, prot. 2017AK8C32 (PI:

Luca Bindi). Analyses were carried out at the Caltech GPS

Division Analytical Facility, which is supported, in part, by

NSF Grants EAR-0318518 and DMR-0080065.

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