Nishi et al.
Progress in Earth and Planetary Science(2023) 10:41
https://doi.org/10.1186/s40645-023-00572-0

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Effect of grain size on amorphization
mechanism and kinetics of bridgmanite
in shocked meteorites
Masayuki Nishi1* , Si Jin1, Katsutoshi Kawano1, Hideharu Kuwahara2, Akihiro Yamada3,4, Shogo Kawaguchi5,
Yuki Mori5, Tatsuhiro Sakaiya1 and Tadashi Kondo1

Abstract
Bridgmanite formation and amorphization in shocked meteorites constrain the pressure and temperature conditions
during planetary impact. However, the effect of the bridgmanite grain size on its amorphization kinetics is still unclear.
Here, the amorphization mechanism and kinetics of fine-grained polycrystalline bridgmanite were studied at hightemperatures up to 1080 K. High-temperature time-resolved synchrotron X-ray diffraction measurements showedthat significant volume expansion due to temperature-induced amorphization caused static stress, which then hindered amorphization progress. Further, the temperature required for the amorphization of fine-grained bridgmanite(~ 1 μm) was found to be approximately 100 K higher than that required for the amorphization of coarse-grained samples (> 10 μm). We also noted that amorphization preferentially commenced at the twin planes and subgrain boundaries of bridgmanite grains, resulting in lower amorphization temperatures for the coarse-grained samples. The limited
number of such specific locations in fine-grained natural bridgmanite suggested that grain boundary amorphization
may be the dominant mechanism for bridgmanite amorphization in shocked meteorites. This unique amorphization
kinetics would support the preservation of bridgmanite during the post-shock annealing in the shocked meteorite.
Although bridgmanite amorphization starts easily at temperatures above ~ 420 K, a small amount of bridgmanite
grains can survive at temperatures above 800 K by the effect of amorphization-induced stress.
Keywords Bridgmanite, High pressure, Shocked meteorite, Amorphization kinetics*Correspondence:
Masayuki Nishi
nishimasa@ess.sci.osaka-u.ac.jp
1
Department of Earth and Space Science, Osaka University, 1‑1
Machikaneyama‑cho, Toyonaka, Osaka 560‑0043, Japan
2
Geodynamics Research Center, Ehime University, 2‑5 Bunkyo‑cho,Matsuyama, Ehime 790‑8577, Japan3
Department of Material Science, The University of Shiga Prefecture,
Hikone, Shiga, Japan
4
Center for Glass Science and Technology, The University of Shiga
Prefecture, 2500, Hassaka‑cho, Hikone, Shiga 522‑8533, Japan
5
Japan Synchrotron Radiation Research Institute, Sayo‑gun,
Hyogo 679‑5198, Japan

1 Introduction
High-pressure and high-temperature generationinduced by planetary impact processes can cause constituent minerals to undergo phase transitions, forming
their high-pressure polymorphs (Gillet and El Goresy
2013; Hu and Sharp 2022; Stöffler et al. 2018). Further,high-pressure minerals, such as stishovite, majorite,
ringwoodite, and bridgmanite, can be found in shocked
meteorites that have fallen on Earth (Binns et al. 1969;
El Goresy et al. 2004; Ohtani et al. 2011; Smith and
Mason 1970; Tomioka and Miyahara 2017; Tschauneret al. 2014). These high-pressure minerals can beformed within a few seconds, corresponding to thetimescale of shock-induced high-pressure pulses during impact (Beck et al. 2005; Gillet and El Goresy 2013;© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Nishi et al. Progress in Earth and Planetary Science(2023) 10:41

Ohtani et al. 2004; Tiwari et al. 2022). However, they
become metastable immediately after pressure release.
Since the high temperature after pressure release (hereafter, residual post-shock temperature) is maintained
over several hours, high-pressure minerals often disappear owing to back transformation or amorphization(Hu and Sharp 2017; Kimura et al. 2004). Reportedly,
the kinetics of these reactions strongly depend on temperature, and possibly, the residual post-shock temperature is a key parameter that determines whether
high-pressure minerals can survive in shocked meteorites (Kubo et al. 2022; Nishi et al. 2022).
The occurrence of bridgmanites in shocked meteorites (Ghosh et al. 2021; Miyahara et al. 2011; Tiwari
et al. 2021; Tomioka and Fujino 1997; Tschauner et al.
2014) suggests that pressures above 23 GPa are generated during the impact process. Additionally, theresidual post-shock temperature should be sufficiently
low to preserve the structure of bridgmanite, which
is known to be easily destroyed at high temperatures
above 400 K and atmospheric pressure (Durben andWolf 1992; Wang et al. 1992). At much higher temperatures above ~ 800 K, bridgmanite transforms toenstatite (Knittle and Jeanloz 1987). A recent experimental study based on time-resolved in situ X-ray
diffraction (XRD) measurements showed that theamorphization of polycrystalline bridgmanite withgrain size > 10 μm starts at temperatures above ~ 400 K
and ends at ~ 700 K, irrespective of the annealing time
(Nishi et al. 2022). It has also been observed that amorphization progress at a constant temperature is strongly
prevented by static stress, which is induced by volume
expansion via amorphization. This stress effect enables
the preservation of bridgmanite during exposure to
the residual post-shock temperature, which continues
for several hours. Additionally, based on microtexturalobservations, it has been inferred that intracrystalline
amorphization is initiated at specific weak locations
within bridgmanite grains, such as twin planes and subgrain boundaries. However, the grain size corresponding to natural samples is below ~ 1 μm and conspicuous
dislocations are absent in small grains (Ghosh et al.
2021; Tomioka and Fujino 1997; Tschauner et al. 2014).
Therefore, intracrystalline amorphization may not be
the dominant mechanism of the amorphization of natural bridgmanite.
Here, we subjected bridgmanite to time-resolved
in situ XRD measurements during amorphization toclarify the effect of grain size on bridgmanite amorphization at high temperatures. To this end, we usedbridgmanite with a grain size one order of magnitude
smaller than that used in a previous study (Nishi et al.
2022).

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2 Methods
Polycrystalline bridgmanite was synthesized from
­MgSiO3 glass at 25 GPa and 1400 °C using a 3000-ton
multi-anvil apparatus (Orange-3000) at the Geodynamics
Research Center, Ehime University, Japan. First, ­MgSiO3
glass was synthesized from a mixture of reagent-grade
MgO and ­SiO2 via melting in a high-temperature furnace. The recovered bulk glass was then shaped into a rod(diameter = 2.5 mm) using an ultrasonic machining tool.
To avoid moisture adsorption, which causes rapid grain
growth, we used bulk glass without crushing. The cell
assembly and anvils were almost the same as that used in
our previous study (Nishi et al. 2022).
Time-resolved high-temperature XRD measurementson the polycrystalline bridgmanite were performed atbeamline BL02B2, SPring-8, Japan (Kawaguchi et al.
2017). Specifically, the synthesized polycrystalline bridgmanite was crushed into several pieces and filled in silica
glass capillaries (diameter = 0.5 mm). The incident X-ray
beam, size 1.5 × 0.5 mm, larger than that of the polycrystalline sample, was monochromatized to 25 keV using
Si(111) double crystals. The wavelength was determined
to be 0.496431 Å using a standard reference material,­CeO2. XRD patterns were then obtained every 10 s during heating. The fluctuation of incident X-ray intensity
monitored during the measurements was within 0.1%.
The temperature was controlled using an ­N2 gas blower.
In runs FBRD07-09, the temperature was raised to thetarget value at a heating rate of 80 K/min and maintained for 2–30 min to observe the amorphization kinetics at a constant temperature (Table 1). The heating rates
employed in the other runs were controlled at 10–30 K/
min, and changes in peak intensity and bridgmanite lattice volume as a function of temperature were noted. The
integrated intensities of the XRD and cell volumes of thecrystals during heating were calculated using diffractionlines 020, 112, 200, 210, 211, 022, 122, 221, and 204 relative to the initial intensities before heating. Secondary
electron images, obtained using a field-emission scanning electron microscope (JSM-IT500HR), were used toobserve the texture of the recovered bridgmanite.
3 Results
We synthesized fine-grained polycrystalline bridgmanite with grain size (G) ~ 0.9 μm (Fig. 1a), much smaller
than that of bridgmanite synthesized in a previous study(Nishi et al. 2022). G was estimated from the relationship:
G = cL, where c is a constant with a value of 1.56, and L
is the average intercept length, which is the line length
divided by the number of grain boundaries (Mendelson1969). The experimental conditions and results are presented in Table 1. The XRD patterns observed before
heating (300 K) showed the presence of bridgmanite andNishi et al. Progress in Earth and Planetary Science(2023) 10:41

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Table 1 Experimental conditions and run products
Run noTemperature(K)

Heating rate
(K/min)Keeping time(minutes)

Cooling time(minutes)

Relative peakintensity

Apparentpressure(GPa)

0.37 (2)0.51 (6)

Gradual heating
FBRD06

300–54720––

FBRD05300–59210––

FBRD04

300–61310–10FBRD10300–1079

30

–

–

80

20

80

Constant temperatureFBRD09554

FBRD07

576

80

30

80

0.26 (1)0.51 (6)FBRD08

613

80

2*

80

0.24 (1)0.56 (4)*Glass capillaries broken during storage time

Fig. 1 Secondary electron images of bridgmanite before and after amorphization. a Synthesized polycrystalline bridgmanite used as starting
material. b Partially amorphized bridgmanite after high-temperature annealing at 554 K (FBRD09). The darker regions surrounding the bridgmanite
grains represent amorphous phases

trace amounts of MgO periclase (Fig. 2a). Amorphization
at temperatures above 400 K induced a decrease in XRD
peak intensities and also brought about the appearanceof an amorphous halo (Fig. 2b), similar to the observation made in the previous study using a coarse-grained
sample (Nishi et al. 2022). The XRD peaks of our synthesized fine-grained polycrystalline bridgmanite survived
at temperatures up to ~ 850 K. In the previous study, it
was observed that the peaks disappeared at lower temperatures (~ 700 K) (Nishi et al. 2022). Thus, the crystal
structure of fine-grained bridgmanite is more robust at
high temperatures.

The relative peak intensities and relative cell volumes(V/V0) of bridgmanite as a function of temperature areshown in Fig. 3a and b, respectively. Data corresponding
to coarse-grained bridgmanite polycrystalline (> 10 μm)
obtained in the previous study are shown for comparison (Nishi et al. 2022). The decrease in the integrated
peak intensities at temperatures above ~ 420 K indicatedthe partial disappearance of the bridgmanite structure owing to amorphization. The data showed that thefine-grained sample required a higher temperature foramorphization than for coarse-grained samples. Thus,
the fine-grained polycrystalline bridgmanite survived
at temperatures up to ~ 850 K, ~ 150 K higher than the

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Fig. 2 a Selected XRD patterns during heating (FBRD10). The heating rate was 30 K/min. The vertical bars indicate peak positions calculated
for bridgmanite under ambient conditions. The numbers are Miller indices. Pe, periclase. b Magnified XRD pattern of the near amorphous halothat appeared at a 2θ range of 3–20°. The amorphous halo signal from the glass capillary was subtracted, taking background measurements
into accountmaximum temperature the coarse-grained sample could
survive (Fig. 3a). As previously observed, the thermal
expansivity of bridgmanite decreased discontinuously
following amorphization (Fig. 3b). This change could be
attributed to the occurrence of stress during the amorphization process. The large difference in cell volume(~ 32%) between bridgmanite and the amorphous glass
phase (Fei et al. 2021) could be attributed to the apparent pressure on bridgmanite grains experienced during amorphization. Following the previously reported
method and assuming that bridgmanite has a constantthermal expansivity (2.1 × ­10–5 ­K−1) (Nishi et al. 2022),
we estimated the apparent static pressure on bridgmanite during amorphization based on the compressibility ofits structure as shown in Fig. 3c. The apparent pressure
on bridgmanite increased due to increasing temperature, reaching ~ 1.1 GPa at 750 K. All lattice axes showed
the stress effect (Fig. 4). We also observed that the peakintensities and apparent pressure were maintained during the cooling process (Fig. 5). Thus, the amorphization-induced stress could be maintained at ambient
temperature.

The amorphization of bridgmanite in meteorites should
proceed continuously as long as the residual post-shock
residual temperature is maintained. Figure 6 shows therelative peak intensities during the heating process as afunction of time. In these runs, the temperature was rapidly increased to the target value at a heating rate of 80 K/
min and maintained for 2–30 min. The short heating run
at 613 K was due to the breakdown of the glass capillary
during heating. As previously observed, the peak intensity suddenly decreased before the target temperature
was attained and did not change significantly when thetemperature was kept constant. The kinetic data wereanalyzed using the Avrami rate equation:

X = 1 − exp −kt n(1)where X represents the transformed (amorphous) volume
fraction, k and n are constants, and t represents the duration over which the temperature was maintained. The k
and n values obtained are listed in Table 2. The n values
obtained in this study (very low n values, less than ~ 0.1)
were consistent with that reported by Nishi et al. (2022)
for coarse-grained polycrystalline bridgmanite.

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Fig. 4 Unit cell parameters of bridgmanite as a functionof the temperature (FBRD10). The thermal expansivity decreased
discontinuously following amorphization at above ~ 420 K

Fig. 3 High-temperature behavior of polycrystalline bridgmanitewith different grain sizes. The data for the coarse-grainedpolycrystalline bridgmanite (> 10 μm) are based on a previous
study (Nishi et al. 2022). a Bridgmanite peak intensities as a functionof temperature. The error bars reflect the standard deviations (1σ)
derived taking into consideration the various diffraction peaks. b
Variation of relative volume (V/V0) as a function of temperature.
The dashed line shows the thermal expansivity of bridgmanite. cApparent pressure on bridgmanite grains. We did not determinepressure values at temperatures above 750 K due to the weak peakintensities observed

4 Discussion
Low n values in the Avrami rate equation indicatedthat the amorphization progress was hindered immediately after its occurrence. Further, these values differedfrom the general n values above 0.5, indicating continuous reaction progress over time (e.g., Kubo et al. 2004;
Starink 2001). Owing to the extremely low n value, thebridgmanite volume fraction almost stagnated with time
(Fig. 6b), even if the post-shock residual temperature ismaintained long-term after shock events (Nishi et al.
2022).

This study, as well as a previous study (Nishi et al. 2022),
has shown consistent results that bridgmanite amorphization induces static stress, which prevents further amorphization. The temperature required for amorphizationdiffers depending on grain size, as shown in Fig. 3. However, it appeared that the relationship between apparent
pressure and the bridgmanite fraction surviving after
amorphization remained unchanged irrespective of grain
size (Fig. 7). Therefore, it is reasonable to consider thatthe amorphization induced apparent pressure. Further,a recent experimental study on bridgmanite amorphization under pressure shows that the pressure increases theamorphization temperature (Kubo et al. 2022). Thus, thehindering of amorphization progress by apparent pressure is plausible. Without the effect of apparent pressureon amorphization kinetics, bridgmanite would disappear
during long post-shock annealing at above ~ 420 K.
We obtained apparent pressures of up to 1.1 GPa using
strong XRD peaks from bridgmanite at 750 K (Fig. 3c).
The XRD patterns observed at higher temperatures
showed the survival of a small amount of bridgmanite
at temperatures up to ~ 850 K, although the peak intensity was not strong enough to determine the cell parameters. Using the linear correlation between apparent
pressure and temperature (Fig. 3c), the pressure possibly
reached ~ 1.4 GPa at 850 K.
In many cases of polycrystalline materials, polymorphic transformation proceeds via interface-controlled
growth, following nucleation at the grain boundary.
Therefore, the overall reaction rate should be fasterNishi et al. Progress in Earth and Planetary Science(2023) 10:41

Fig. 5 High-temperature behavior of fine-grained polycrystalline
bridgmanite during heating and cooling processes (FBRD04).
The behavior during continuous heating (FBRD10) is shownfor comparison. a Bridgmanite peak intensities as a functionof temperature. The peak intensity was maintained during the coolingprocess. b Variation of relative unit cell volume (V/V0) as a functionof temperature. The dashed line shows the thermal expansivityof bridgmanite. c Apparent pressure on bridgmanite grains. The
amorphization-induced stress was maintained during the coolingprocess

for smaller grains, which have a larger grain boundary
area (Cahn 1956; Rubie and Ross 1994). In contrast, ourresults showed a faster amorphization rate for coarsegrained polycrystalline bridgmanite (Fig. 3). We consider that intracrystalline amorphization contributes
to the amorphization process for coarse-grained polycrystalline. Actually, a previous study indicated that
amorphization of coarse-grained bridgmanite occurs
inside the grains rather than at grain boundaries, possibly via the use of specific locations, such as twin planes
and subgrain boundaries (Nishi et al. 2022). Because
intracrystalline amorphization has not been observedin fine-grained natural bridgmanite, grain boundary

Page 6 of 9

amorphization may be the dominant mechanism forbridgmanite amorphization in shocked meteorites.
Figure 8 shows the relationship between temperatureand the amorphous molar fraction with various heatingtimescales. Owing to differences in grain size, the postshock residual temperatures estimated in this study
were higher than those reported in a previous study(Nishi et al. 2022). This difference may be due to intracrystalline amorphization in coarse grains, as discussed
above. Considering the small grains in natural bridgmanite (< 1 μm), the fine-grained data obtained in thisstudy were more suitable for estimating residual postshock temperature.
Shock conditions of bridgmanite formation within themelt vein in the Tenham meteorite were estimated to be
23 to 25 GPa and 2200 to 2400 K (Tschauner et al. 2014).
If such high temperatures were maintained after shockpressure release, bridgmanite amorphization would complete even if a short timescale of 1
­ 0–2 s (Fig. 8). Thus, it
is reasonable to consider that the shock vein was cooled
to a temperature roughly equivalent to that of the hostrock before the shock pressure was released. This result
is consistent with the previous estimation based on thethermal conductivity and transformation texture of constituent minerals (Ohtani et al. 2004; Xie et al. 2006).
If we assume that 10 mol% (8 vol. %) of the bridgmanite fraction survived after partial amorphization, thenthe estimated post-shock temperature would be ~ 700 K
at a constant annealing time of 1
­ 02 s, while this value
was 600 K in the previous underestimation (Nishi et al.
2022). Thus, the amorphization mechanisms and kinetics of fine-grained bridgmanite can facilitate bridgmanite preservation in shocked meteorites. The post-shock
temperature of meteorites increases with pre-impact
porosity. Our estimate of the post-shock temperature isin good agreement with previous theoretical estimate for
the Tenham meteorite with 7% porosity (~ 620 K) rather
than that for the zero-porosity case (~ 400 K) (~ 620 K)
rather than that assuming non-porous case (~ 400 K) (Xieet al. 2006).
Bridgmanite and its amorphous phase found inshocked meteorites have various chemical composition,
including Fe and Al, which may affect the amorphization
mechanisms and kinetics. Further experiments using Feand Al-bearing samples would contribute to constraining
the precise residual temperature.

5 Conclusions
The amorphization mechanisms and kinetics of finegrained polycrystalline bridgmanite were investigated
via time-resolved synchrotron XRD measurements.
Thus, we observed that irrespective of grain size,

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Fig. 6 a Plot of peak intensities as a function of time. Data were obtained from runs with the target temperatures varying in the range of 554–
613 K. The heating rate was 80 K/min. The temperatures were maintained from 0 s after the initial temperature increase (shaded area). b Plots of lnln(1/(1 − X)) as a function of ln (t) for the kinetic data at constant temperatures. The n values in the rate Eq. (1) correspond to the linear slope of theseplots. The solid lines shown in this figure are based on a fixed n value of 0.05. The back-transition kinetics from ­Mg2SiO4 wadsleyite to olivine
at 1273 K (n = 1.5) (Ming et al. 1991) is also provided for comparison (dashed line)

Table 2 Estimated values of kinetic parameters for bridgmanite
amorphization
Run NoTemperature (K) nln kln k for then value of
0.05

FBRD09554

0.03 (1)− 0.23 (7)− 0.33 (1)FBRD07576

0.06 (1)− 0.16 (5)− 0.10 (1)FBRD08613

0.00 (2)0.34 (8)0.16 (2)significant volume expansion due to amorphizationinduced static stress, which prevented further amorphization. Intracrystalline amorphization initiated fromtwin planes and/or subgrain boundaries in bridgmanite grains may contribute to the amorphization kinetics.
Thus, we observed that a higher temperature is required
to induce bridgmanite amorphization for finer grains
than for coarse grains. Considering the small grains innatural bridgmanite (< 1 μm), grain boundary amorphization may be the dominant amorphization mechanism
for bridgmanite in shocked meteorites. The survival of
10 mol% bridgmanite requires a residual temperatureof ~ 700 K, which is 100 K higher than the previously
estimated temperature using data corresponding to

Fig. 7 Plots of the apparent pressure of bridgmanite as a functionof the bridgmanite fraction that survived amorphizationcoarse grains. The preservation of bridgmanite in meteorites can be explained by the amorphization-induced
stress during post-shock annealing.

Nishi et al. Progress in Earth and Planetary Science(2023) 10:41

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Received: 27 March 2023 Accepted: 16 July 2023

Fig. 8 Survived bridgmanite fraction after amorphization
as a function of temperature. The solid curve shows the variation
between the bridgmanite and amorphous fractions with different
timescales of ­10–2–104 s based on the Avrami component n of 0.05.
The fraction change of coarse grains (Nishi et al. 2022) is also shownfor comparison (dashed curve). Assuming the fine-grainedpolycrystalline bridgmanite, the residual post-shock temperature
was estimated to be ~ 700 K when 10 mol% of bridgmanite
was preserved in the shocked meteorite at a timescale of ­102 s. Thistemperature was 100 K higher than the previously estimated value
assuming coarse grains

Abbreviation
XRD	X-ray diffraction
Acknowledgements
We thank Takumi Miura, Masakazu Ohno, Miyuu Soejima, and Ryo Tsuruoka
for their assistance with the experiments. We also thank Haruhiko Dekurafor valuable comments. Fruitful comments by Dr. Naotaka Tomioka and Dr.
Kishan Tiwari have greatly improved the quality of the manuscript. In situ
X-ray measurements were conducted at SPring-8 (Proposal No. 2022A1110).
The bridgmanite synthesis experiments were supported by the Joint Usage/
Research Center of PRIUS, Ehime University, Japan.
Author contributions
MN, TK, and TS proposed the topic and designed the study. MN, SJ, KK, SK, andYM performed experiments. MN, HK, and AY synthesized the polycrystalline
bridgmanite. MN wrote the manuscript. All authors have read and approved
the final version of the manuscript.
Funding
This work was supported by JSPS KAKENHI (grant number JP22H01322).
Availability of data and materials
The datasets supporting the conclusions of this article are included within thearticle.

Declarations
Competing interests
The authors declare that they have no competing interests.

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