(2021) 8:59
Miyahara et al. Prog Earth Planet Sci
https://doi.org/10.1186/s40645-021-00451-6

Progress in Earth andPlanetary Science

Open Access

REVIEW

Natural and experimental high‑pressure,shock‑produced terrestrial and extraterrestrial
materials
Masaaki Miyahara1* , Naotaka Tomioka2 and Luca Bindi3

Abstract
Hypervelocity impacts are among the fundamental phenomena occurring during the evolution of the solar system
and are characterized by instantaneous ultrahigh pressure and temperature. Varied physicochemical changes haveoccurred in the building blocks of celestial bodies under such extreme conditions. The constituent material has transformed into a denser form, a high-pressure polymorph. The high-pressure polymorph is also thought to be the constituent of the deep Earth’s interior. Hence, experiments using a high-pressure and temperature generating apparatus
have been conducted to clarify its crystal structure, pressure–temperature stability range, and transformation mechanisms. A natural high-pressure polymorph (mineral) is found from terrestrial and extraterrestrial rocks that experienced
a hypervelocity impact. Mineralogists and planetary scientists have investigated high-pressure minerals in meteorites
and rocks near terrestrial craters over a half-century. Here, we report brief reviews about the experiments producing
high-pressure polymorphs and then summarize the research histories of high-pressure minerals occurring in shocked
meteorites and rocks near terrestrial craters. Finally, some implications of high-pressure minerals found in impactinduced shocked rocks are also mentioned.
Keywords: High-pressure mineral, Shock metamorphism, Meteorite, Impact crater
1 Introduction
High-pressure minerals have considered the major constituents of the Earth’s deep interior, and their physicochemical properties and transformation mechanisms are
essential parameters to understand the structure anddynamics of the Earth’s depths. Therefore, mineral physicists who work on high-pressure minerals have been
making long-standing efforts to clarify the structuralchanges and phase equilibria using a multianvil apparatus
and a laser-heated diamond anvil cell (LHDAC).
Mineralogists and planetary scientists have tried tofind high-pressure minerals in meteorites (Fig. 1) androcks near terrestrial impact craters because we cannot*Correspondence: miyahara@hiroshima-u.ac.jp
1
Graduate School of Advanced Science and Engineering, Hiroshima
University, Higashihiroshima 739‑8526, Japan
Full list of author information is available at the end of the article

get high-pressure minerals directly from the Earth’s deep
interior except for inclusions in diamond crystals (Moore
and Gurney 1985; Walter et al. 2011; Pearson et al. 2014)
and mantle xenoliths (Collerson et al. 2000). Indeed, thefirst high-pressure mineral was discovered in a terrestrial
impact crater; Chao et al. (1960) found a high-pressurepolymorph of S
­ iO2, then named coesite, from the Barringer crater in the USA, and Chao et al. (1962) then founda second S­ iO2 polymorph, named stishovite, from thesame crater (Fig. 2). A. El Goresy (1934–2019), who was aworld-leading mineral physicist/meteoriticist working on
shock metamorphism, discovered a new allotropic formof carbon, named chaoite in memory of the contributionsof E. C. T. Chao for his pioneer works, from the Rice crater in Germany (El Goresy and Donnay 1968).
The first discovery of a high-pressure mineral from a
meteorite is the (Mg,Fe)2SiO4 ringwoodite (Binns et al.
1969), and the second and third cases are the (Mg,Fe)© The Author(s) 2021. 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/.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

SiO3 majorite (Smith and Mason 1970) and wadsleyite (Price et al. 1983) (Fig. 2; Table 1). High-pressureminerals in shocked rocks are heterogeneously distributed on a micrometer scale, and their crystal sizes are
nano–micrometers across. In most cases, the occurrence of high-pressure minerals in shocked meteorites
is restricted in the vicinity of shock-induced melt veins(shock-melt veins) and melt pockets up to several millimeters in width (Fig. 1). Technical difficulties on thecharacterization of such minute crystals shrunk further
efforts to search for new high-pressure minerals andcaused a long stagnation since the 1970s (Fig. 2).
The breakthrough was brought by the introductionof a transmission electron microscope equipped with
an X-ray energy-dispersive spectrometer (TEM-EDS)(Fig. 2). TEM-EDS analysis enables textural observations,crystallographic scrutinies, and chemical composition
analyses at nano–microscales. A. El Goresy, M. Chen,
T. Sharp, and their colleagues showed a beautiful ringwoodite grain having dense stacking faults in shock-meltveins of the Sixiangkou L6 ordinary chondrite by TEM(Chen et al. 1996). Subsequently, Sharp et al. (1997) andTomioka and Fujino (1997) almost simultaneously foundnatural (Mg,Fe)SiO3 with the ilmenite structure (named
akimotoite later) from the Tenham L6 ordinary chondrite by using TEM-EDS analysis. In the same contribution, these authors also reported an electron diffraction
pattern of (Mg,Fe)SiO3 with the perovskite structure(named bridgmanite later by another set of authors)
along with akimotoite (Tomioka and Fujino 1997). A. El
Goresy and his colleagues also found a silica mineral withscrutinyite (α-PbO2)-type structure from the shergottite
Shergotty by TEM-EDS analysis (Sharp et al. 1999). We
cannot directly observe a high-pressure polymorph of(Mg,Fe)2SiO4, poirierite, without high-resolution TEM
analysis because of its complex and ultrafine crystals
within the host olivine (Tomioka et al. 2021).
The second breakthrough was caused by the introduction of in situ analysis techniques: (i) micro-fabricationand -pickup technology, a focused ion beam (FIB) system, (ii) focused synchrotron X-ray diffraction (sXRD)
techniques, and iii) electron backscattered diffraction(EBSD) analysis. An ultrathin film (thickness: ~ 100 nm)
had been prepared for TEM-EDS analysis and was prepared conventionally by Ar-ion-milling and ultramicrotomy. However, a target containing high-pressureminerals for TEM-EDS analysis is submicron-sized. We
must minimize the destruction of a valuable meteorite
sample during the pickup process of such a small portion.
The introduction of an FIB system overcame the difficulty(e.g., Miyahara et al. 2008; Wirth 2009). sXRD analysis,
which uses a high-power X-ray beam, is a powerful tool

Page 2 of 26

to identify the crystal structure of submicron-sized material. SEM-EBSD analysis can identify minerals in meteorites on the surface of a polished petrographic section
in a short time without sample destruction. The numberof new high-pressure minerals exponentially increased
since the introduction of these new analysis techniques(Fig. 2). Furthermore, the application of laser microRaman spectroscopy makes the characterization of highpressure minerals in shocked rocks very rapid and easy.
By the means of a 1-µm diameter laser beam, such a technique allows the identification of fine-grained minerals inpolished petrographic thin sections within several 10 s,thus enabling systematic investigations of high-pressureminerals (e.g., Ohtani et al. 2004; Miyahara et al. 2020).
However, meticulous interpretation of a Raman shift isnecessary for identification because some high-pressureminerals have similar Raman shifts (e.g., Baziotis et al.
2018). Cross-check by another analysis technique should
be carried out when necessary.
Here we review the syntheses of high-pressure minerals
and their discoveries in natural samples in the last six decades. In this review, a high-pressure mineral is termed asa mineral that is stable only at the Earth’s mantle pressure
conditions (above ~ 1 GPa) as listed in Table 1. Their formation mechanisms, representative phase diagrams, andtypical diagnostic Raman spectra are summarized in Gasparik (2003) and Tomioka and Miyahara (2017). Concise
but comprehensive introductions of shock-produced highpressure minerals are also available in Sharp and DeCarli(2006), Morrison and Hazen (2021) and on our databasewebsite (https://​sites.​google.​com/​site/​highp​miner​al).

2 High‑pressure minerals in meteorites
2.1 High‑pressure polymorphs of (Mg,Fe)2SiO4

Olivine [(Mg,Fe)2SiO4] is one of the major constituentsof meteorites, and its high-pressure polymorphs arefound from ordinary chondrites, lunar meteorites, Martian meteorites, and carbonaceous chondrites (Tomioka
and Miyahara 2017). High-pressure polymorphs for olivine (α-phase), spinelloid-type (Mg,Fe)2SiO4 (β-phase),spinel-type (Mg,Fe)2SiO4 (γ-phase), spinelloid-type­Mg2SiO4 (ε-phase), and orthosilicate ­Mg2SiO4 (ζ-olivine)
are reported. The fayalite components of olivine for H,
L, and LL group ordinary chondrites are 16–23, 23–26,
and 27–32 mol%, respectively (Weisberg et al. 2006). The
fayalite components of olivine in Martian meteorites andcarbonaceous chondrites are widespread and higher thanordinary chondrites and lunar meteorites. Hence, Fe-richhigh-pressure polymorphs of (Mg,Fe)2SiO4 dominate inMartian meteorites and carbonaceous chondrites compared with other meteorites.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Fig. 1 A heavily shocked meteorite (The Monze L6 ordinarychondrite). Black-colored veins (white-colored arrows) are shock-meltveinsPage 3 of 26

Major 1966a). The crystal structure of the distorted spinel phase was better studied four years later and regarded
as a new orthorhombic phase (β-Mg2SiO4) 8% denser
than forsterite (Ringwood and Major 1970).
Natural β-Mg2SiO4 with a quench texture was discovered in the black-colored shock-melt veins of the Tenham
and Peace River L6 ordinary chondrites by TEM observations (Putnis and Price 1979). The mineral was named
wadsleyite after A. D. Wadsley (Price et al. 1983). Latersystematic investigations revealed that, in most cases,
wadsleyites in ordinary chondrites have crystallized froma shock-produced chondritic melt simultaneously transformed by a solid-state reaction from olivine entrainedin the chondritic melt (Miyahara et al. 2020). The former always coexists with low-Ca pyroxene, and the latteroccurs at the grain boundaries or fractures in the olivine
grains (Fig. 3).
Although pure F­e2SiO4 with a spinelloid structure(β-Fe2SiO4) has not been synthesized, Finger et al. (1993)
obtained single wadsleyite crystals with 40% of fayalite
component at 15.2 GPa and 1973 K. A mineral with Fe
dominance over Mg in the octahedral sites was foundas inclusion in the shock-induced Fe–Ni droplets of the
Suizhou L6 ordinary chondrite and the Quebrada Chimborazo 001 CB carbonaceous chondrite and was named
asimowite after P. D. Asimow (Bindi et al. 2019).
2.1.2 γ‑(Mg,Fe)2SiO4: Ringwoodite–ahrensite

Fig. 2 The accumulated number of a new high-pressure mineralby year. The discovery of major minerals is shown in the figure. The
introduction years of instruments are also shown. Coe: coesite, Sti:stishovite, Rwd: ringwoodite, Wds: wadsleyite, Maj: majorite, Aki:akimotoite, Bdg: bridgmanite, Sei: seifertite. cXRD: conventional
X-ray diffraction, TEM: transmission electron microscopy, FIB: focusedion beam, sXRD: synchrotron X-ray diffraction, EBSD: electronbackscattered diffraction

2.1.1 β‑(Mg,Fe)2SiO4: Wadsleyite–asimowite

Ringwood and Major (1966a) conducted high-pressure
experiments in the M­ g2SiO4–Fe2SiO4 system using theearly-stage Bridgman anvil apparatus at 12–20 GPa and
900 °C to confirm an olivine-spinel transition. ­Mg2SiO4with a spinelloid structure was first reported as a distorted spinel phase in their experimental samples basedon X-ray diffraction (XRD) analysis (Ringwood andThe ­Mg2SiO4–Fe2SiO4 solid solution with a spinel
structure (γ-phase) in the compositional range of Fe/(Mg + Fe) = 0.15–0.75 was reported experimentally by
Ringwood and Major (1966a). As for the end-member
composition, γ-Fe2SiO4 was first synthesized at 3 GPa,
660 °C by a squeezer apparatus (Ringwood 1958a), while
the convincing evidence for γ-Mg2SiO4 was first reported
later via synthesis at 25 GPa and 1000 °C by a Kawai-type
multianvil apparatus (Ito et al. 1974). ­Mg2SiO4 with a spinel structure (γ-Mg2SiO4) was also obtained first experimentally by Ringwood and Major (1966a). Binns et al.
(1969) found purple/blue isotropic grains in the shockmelt vein of the Tenham L6 ordinary chondrite by optical
microscopic observations and obtained a powder XRD
pattern from the grains. The latter could be assigned
to γ-Mg2SiO4 and the mineral was named ringwoodite
after A. E. Ringwood (Binns et al. 1969). Jeanloz (1979)
claimed that ringwoodite is merely a complex mineralaggregate, misidentified as a spinel phase. Against thecriticism, Putnis and Price (1979) presented conclusive
evidence for the existence of ringwoodite, as they provide
a TEM image of a single ringwoodite crystal in the shockmelt vein of the Tenham chondrite.
Most ringwoodite in shocked meteorites is a polycrystalline assemblage (across < 1–2 μm) (Xie and Sharp 2004;

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Page 4 of 26

Table 1 Inventory of high-pressure minerals
Year#

Mineral name

Structure

Chemistry

References
Prior to 1960–

Diamond

DiamondC

1962

CoesiteUnique monoclinic structure

SiO2

Chao et al. (1960)

1966

Lonsdaleite

WurtziteC

Frondel and Marvin (1967)

1967

Stishovite

RutileSiO2

Chao et al. (1962)

1968

ChaoiteCarbyneC

El Goresy and Donnay (1968)

1968

Ringwoodite

Spinel

Mg2SiO4

Binns et al. (1969)

1969

Majorite

GarnetMgSiO3Smith and Mason (1970)

1982

Wadsleyite

β-spinelloid

Mg2SiO4

Price et al. (1983)

1997

Akimotoite

Ilmenite

MgSiO3

Tomioka and Fujino (1999)

2000

Allabogdanite

Co2Si(Fe,Ni)2P

Britvin et al. (2002)

2001

Tuite

Ba3(PO4)2

Ca3(PO4)2

Xie et al. (2004)

2001

ReiditeScheelite

ZrSiO4

Glass et al. (2002)

2004

Lingunite

HollanditeNaAlSi3O8

Liu and El Gorsey (2007)

2004

Seifertite

Scrutinyite

SiO2

El Goresy et al. (2008)

2006

Dmitryivanovite

CaGa2O4-II

CaAl2O4

Mikouchi et al. (2009)

2007

XieiteCaTi2O4

FeCr2O4

Chen et al. (2008)

2007

Akaogiite

Baddeleyite

TiO2

El Goresy et al. (2010)

2010

Icosahedrite

Icosahedral quasicrystal

Al63Cu24Fe13

Bindi et al. (2011)2013

Ahrensite

SpinelFe2SiO4

Ma et al. (2016)2013

Tissintite

Clinopyroxene(Ca,Na,v)AlSi2O6

Ma et al. (2015)2013

Liebermannite

Hollandite

KAlSi3O8

Ma et al. (2018)

2014

Bridgmanite

Perovskite

MgSiO3Tschauner et al. (2014)2014

Jeffbenite*

Unique tetragonal structure

Mg3Al2Si3O12

Nestola et al. (2016)2015

Decagonite

Decagonal quasicrystal

Al71Ni24Fe5

Bindi et al. (2015a)

2015

RiesiteSrilankite-like structure

TiO2

Tschauner et al. (2020b)

2015

Zagamiite

Hexagonal barium ferrite

CaAl2Si3.5O11

Ma and Tschauner (2017)2016

Hemleyite

Ilmenite

FeSiO3

Bindi et al. (2017)

2016

Wangdaodeite

LiNbO3FeTiO3Xie et al. (2020)

2017

Stöfflerite

Hollandite

CaAl2Si2O8

Tschauner et al. (2021)

2017

Vestaite

Schreyerite(Ti,Fe)Ti4O9

Pang et al. (2018)

2017

Ice-VII*

Cuprite-like structure

H2O

Tschauner et al. (2018)2017

Chenmingite

Harmunite

FeCr2O4

Ma et al. (2019b)

2017

FeiiteSr2Tl2O5

Fe2+2(Fe2+Ti4+)O5

Ma et al. (2021b)

2017

LiuiteGdFeO3FeTiO3Ma et al. (2021b)

2017

Maohokite

Harmunite

MgFe2O4

Chen et al. (2019)2017

Tschaunerite

CaTi2O4(Fe2+)(Fe2+Ti4+)O4

Ma et al. (2021a)

2018

Asimowite

β-spinelloid

Fe2SiO4

Bindi et al. (2019)

2018

Proxidecagonite

Decagonite approximant

Al34Ni9Fe2

Bindi et al. (2018b)

2018

Donwilhelmsite

Unique hexagonal structure

CaAl4Si2O11

Fritz et al. (2020)2018

Poirierite

ε-spinelloid

Mg2SiO4

Tomioka et al. (2021)

2018

Breyite*Walstromite

CaSiO3

Brenker et al. (2021)

2019

Hiroseite

Perovskite

FeSiO3

Bindi et al. (2020c)

2020

Davemaoite*

Perovskite

CaSiO3Tschauner et al. (2020a)

2020

Elgoresyite

Unique monoclinic structure(Mg5Si2)O9

Bindi et al. (2021)#

Year for new mineral-proposal (IMA No.) to the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical
Association (IMA)*

High-pressure minerals occur only as inclusions in diamond but are not produced by shock metamorphismMiyahara et al. Prog Earth Planet Sci(2021) 8:59

Xie et al. 2006; Chen et al. 2007; Miyahara et al. 2010;
Sharp et al. 2015) (Fig. 4a). Some olivine-ringwoodite (orahrensite) assemblage composed of a ringwoodite (Rwd)(or ahrensite (Ahr)) sheet with a specific crystallographic
orientation to preserve the close-packed oxygen layersof both structures—(100)Ol//{111}Rwd/Ahr—is also foundin an olivine (Ol) grain in contact with a shock-melt veinof the Yamato (Y)-791384 L6 ordinary chondrite (Miyahara et al. 2010) (Fig. 4b) or in the shock region of a grainof the Khatyrka CV3 carbonaceous chondrite (Hollister
et al. 2014).
γ-Ni2GeO4 as an analog of ­Fe2SiO4 with a spinel structure (γ-Fe2SiO4) was synthesized by Ringwood (1958b).
Natural γ-Fe2SiO4 was discovered in the shock-melt veinof the Umbarger L6 ordinary chondrite by TEM observations (Xie et al. 2002b). Later, it was also found from themelt-pocket of the olivine-phyric shergottite Tissint by a
combined powder XRD and EBSD analysis and named
ahrensite after T. Ahrens (Ma et al. 2016). Like ringwoodite, most ahrensite grains are also polycrystalline assemblages, and some olivine-ahrensite assemblages consistof an ahrensite sheet with a specific crystallographic
orientation, namely (001)Ol//{111}Ahr (Miyahara et al.
2016). Unlike wadsleyite, most ringwoodite–ahrensite
have formed through a solid–solid-state transformationin ordinary chondrites and Martian meteorites (Walton
et al. 2014; Ma et al. 2016; Miyahara et al. 2016, 2020;
Tomioka and Miyahara 2017). In contrast, some ringwoodites in ordinary chondrites have formed from olivine melt or chondritic melt (Xie et al. 2002b; Miyahara
et al. 2008, 2009, 2020). Ringwoodite crystallized from achondritic melt has a dendritic texture (Fig. 4c).
An interesting point is that ringwoodite–ahrensite
exhibits the normal spinel structure, with Mg(Fe) in
the octahedral A site and Si in the tetrahedral B site.
Through A and B site-disorder, the inverse spinel has
four-coordinated A cations and the six-coordinated site
hosts a mixture of A and B cations. This process affects
the density and impedance contrast across the boundaries in the transition zone and seismic-wave velocities inthis portion of the Earth. Bindi et al. (2018a) reported thefirst synthesis at high pressure (20 GPa) and temperature(1600 °C) of a Cr-bearing ringwoodite with a completely
inverse-spinel structure. The chemical, structural, andcomputational analysis confirmed the stability of inverse
ringwoodite. The degree of inversion in these minerals is
crucial, as discussed by Ma et al. (2019c), because it could
influence the inferred conditions of the meteorite during
shock-melt cooling.
When an oceanic plate subducts along a trench
into the Earth’s deep interior, hydrous minerals suchas clay minerals and hydroxides are transported into
the Earth’s depths, resulting in the formation of dense

Page 5 of 26hydrous minerals. Many kinds of hydrous high-pressurepolymorphs such as δ-AlOOH have been synthesized
through high-pressure and -temperature experiments(e.g., Ohira et al. 2014; Bindi et al. 2020d). High-pressure
experiments prove that nominal anhydrous ringwoodite
and wadsleyite can also contain a small amount of waterin their structures (< 2–3 wt.% ­H2O) (e.g., Kohlstedt et al.
1996; Inoue et al. 1998).
A natural hydrous ringwoodite (> 1.4 wt.% of ­H2O) was
found as an inclusion in terrestrial diamond (Pearson
et al. 2014). A hydrous ringwoodite (< 1132 ppm ­H2O) is
also found near the melt-pocket of the shergottite Tissint(Hallis et al. 2017). Mars explorations by rovers andlanders indicate that water has existed extensively on thesubsurface of Mars (e.g., Bandfield 2007). Some impact
events might occur under hydrous conditions. The shergottites NWA 7397 and 10416 have evidence for bothshock metamorphism and aqueous alteration (Herd et al.
2016; Yoshida et al. 2021). Hydrous high-pressure minerals may have been overlooked in shocked meteorites.
2.1.3 ε‑Mg2SiO4: Poirierite

Based on the topological analyses of the crystal structures of the olivine polymorphs in shocked meteorites,
shear mechanisms without long-range ionic diffusion
are posited to promote their polymorphic transformations. The transformation model also predicted the possible occurrence of an intermediate spinelloid structure(ε-phase) (Madon and Poirier 1983). After the prediction over three decades, the ε-phase was found as metastable intergrowth within ringwoodite grains in theshock-melt vein of the Tenham L6 ordinary chondriteby meticulous high-resolution (HR)-TEM observations (Tomioka and Okuchi 2017). The ε-phase was also
found within ringwoodite and wadsleyite grains from theSuizhou L6 and Miami H5 ordinary chondrites, respectively. Single-crystal XRD analysis of the ε-phase in the
Suizhou L6 ordinary chondrite, as well as results of firstprinciples’ calculations, confirmed that the ε-phase hasan orthorhombic unit cell with the smallest dimensions
among the preexisting spinelloid structures, and thisphase was named poirierite after J.-P. Poirier (Tomioka
et al. 2021) (Fig. 5). The topotaxial intergrowths of poirierite with wadsleyite/ringwoodite in shocked chondrites
support the idea proposed by Madon and Poirier (1983)
where poirierite is a way point in the shear transformations between olivine and wadsleyite/ringwoodite (e.g.,olivine becomes ringwoodite via poirierite).
2.1.4 ζ‑olivine

ζ-olivine was found in the black-colored shocked-olivine in the chassignite Northwest Africa (NWA) 2737
and lherzolitic shergottite NWA 1950 (Beck et al. 2006;

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

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Fig. 3 Backscattered electron (BSE) images of wadsleyite. a A chondritic fragment in the shock-melt vein of Yamato (Y) 000973 L5 ordinarychondrite. b A high-magnification image of a white-colored box in a. A part of olivine in the fragment has been replaced with polycrystalline
wadsleyite. The matrix of the shock-melt vein consists of wadsleyite and low-Ca pyroxene. Ol: olivine, Pyx: Low-Ca pyroxene, Wds: wadsleyite, Jd:
jadeitevan de Moortèle et al. 2007). Based on HRTEM observations and first-principles’ calculations, the ζ-olivine
consists of quasi ideally close-packed oxygen sublatticewith disordered cation sites and is regarded as a metastable high-pressure form of α-(Mg,Fe)2SiO4 produced
at high-pressure and relatively low-temperature conditions (Guyot and Reynard 1992; Beck et al. 2006; van deMoortèle et al. 2007). A mineral name has not been given
to the ζ-olivine. Takenouchi et al. (2019) reproduced theblack-colored shocked-olivine that potentially contains
the ζ-phase by shock recovery experiments.

Fe-rich silicate perovskite (hiroseite) as the dissociation
products of olivine in the Suizhou chondrite L6 ordinarychondrite. High-pressure experiments suggest that olivine melts incongruently into magnesiowüstite and liquid
above 8 GPa and 2100 °C (Kato et al. 1998; Ohtani et al.
1998). Natural dissociation of olivine by incongruent
melting is found from the Kamargaon L6 ordinary chondrite and Apollo 15 regolith breccia (Tiwari et al. 2021;
Satta 2021).

2.1.5 High‑pressure dissociation of (Mg,Fe)2SiO4

The pyroxene is the second major constituent of chondritic meteorites and the Earth’s upper mantle. Becauseof the four main end-member components of pyroxenes (enstatite: ­MgSiO3, ferrosilite: F­eSiO3, diopside:­CaMgSi2O6, and hedenbergite: ­CaFeSi2O6), the pressure–temperature phase equilibria of pyroxene are more
complicated than that of olivine. A natural high-pressurepolymorph of the (Mg,Fe)SiO3 pyroxene was first discovered in ordinary chondrites. Mason et al. (1968) discovered aggregates with an olivine composition in the
Coorara L6 ordinary chondrite using powder XRD and
identified their crystal structure as a garnet structure. The
garnet composition was initially misidentified becauseof overlaps of their neighboring ringwoodite grains.
A more careful examination showed that their actual
chemical composition is close to (Mg,Fe)SiO3 (Smith and
Mason 1970). Two years before the discovery, the mineral was also synthesized from glass with an Al-bearing­MgSiO3 composition at 20 GPa and ~ 900 °C (Ringwood

Phaseequilibrium
diagraminthesystem­Mg2SiO4-Fe2SiO4 predicts that olivine dissociates into
bridgmanite + magnesiowüstite (+ stishovite) around
23–25 GPa (Ito and Takahashi 1989), although the dissociation reaction depends on temperature and chemical composition. Miyahara et al. (2011b) found evidence
for the dissociation reaction in olivine grains in contact
with the shock-melt vein of the olivine-phyric shergottite
Dar al Gani (DaG) 735. Granular bridgmanite + magnesiowüstite (Fig. 6a) and lamellar bridgmanite + magnesiowüstite (Fig. 6b) are found in the dissociated olivine
grains (with bridgmanite almost vitrified). In most cases,
Martian meteorites are heavily shocked compared withordinary chondrites (e.g., El Goresy et al. 2013; Miyahara et al. 2020). Hence, the high-pressure dissociationof olivine is found mainly in shocked Martian meteorites(Fritz and Greshake 2009; Walton et al. 2014; Miyahara
et al. 2019). On the other hand, Bindi et al. (2020c) found

2.2 High‑pressure polymorphs of (Mg,Fe,Ca)SiO3
2.2.1 (Mg,Fe)SiO3‑Mg3Al2Si3O12: Majorite‑pyrope

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

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Fig. 4 Bright-field (BF)-TEM images of ringwoodite. a A ringwoodite
crystal with a dense stacking fault in the Y-790729 L6 ordinarychondrite. b An olivine grain replaced with ringwoodite in the
Y-791384 L6 ordinary chondrite. A part of the olivine grain in contact
with the shock-melt vein is replaced with polycrystalline ringwoodite.
Also, thin layers of ringwoodite occur in the olivine grain. The (100)of olivine is parallel with the {111} of ringwoodite. c A ringwoodite
crystal with a dendritic texture in the Y-74445 L6 ordinary chondrite.
Ol: olivine, Rwd: ringwoodite

and Major 1966b). Generally, garnet has only aluminumin its oxygen 6-coordinated sites, while the newly foundgarnet has a unique characteristic when the structure has
Si > 3 atoms per formula unit. Indeed, by the Tschermak
substitution ­(2Al3+ ⇔ ­Mg2+ + ­Si4+) in the octahedral
sites on the ­MgSiO3-Mg3Al2Si3O12 (pyrope) join, we canfind octahedral silicon in this structure, which is symptomatic of a very high-pressure regime. The new highpressure mineral found in Coorara was named majorite(Smith and Mason 1970) after A. Major. Al-rich majorite
is one of the dominant phases in the matrices of shockmelt veins (e.g., Chen et al. 1996; Miyahara et al. 2020).
According to the phase diagram of the Allende CV3 carbonaceous chondrite, Al-rich majorite is thought to havecrystallized as a liquidus phase from chondritic melt in
the pressure range of 14–26 GPa (Agee et al. 1995; Chenet al. 1996).
Al-rich majorite has a cubic symmetry as usual for silicate garnets, while almost Al-poor/free majorite in thesystem ­MgSiO3–FeSiO3 has a tetragonal distorted symmetry caused by the ordering of Mg and Si in the octahedral sites (e.g., Kato and Kumazawa 1985; Angel et al.
1989; Heinemann et al. 1997; Nakatsuka et al. 1999;
Tomioka et al. 2002). Natural Ca-rich tetragonal majorite as a dissociation product of subcalsic pyroxene wasfound in the host-rock fragments in shock-melt veins in
the Tenham L6 ordinary chondrite (Xie and Sharp 2007;
Tomioka et al. 2016). Polycrystalline low-Ca tetragonal
majorite was also found in the same chondrite (Fig. 7)(Tomioka et al. 2016). Following a comparison between
the degrees of cation ordering in synthetic and natural
majorites, the cooling rate of shock-melt veins during
shock metamorphism was constrained to be ­103 °C/s(Tomioka et al. 2002, 2016).
Majorite, which was formed by melt crystallizationand solid-state transformation, has been reported inmany chondrites and Martian meteorites (shergottite)(Tomioka and Miyahara 2017). As for terrestrial impact

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

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Fig. 5 TEM images of poirierite in the Miami H5 ordinary chondrite. a BF-TEM image of a wadsleyite grain partly replaced by poirierite. b
High-resolution (HR) TEM image of poirierite. Lattice fringes with 0.8 nm spacings correspond to d001. Wds: wadsleyite, Poi: poirierite

structure, the Ries crater also yielded Al-rich majorites
with various amounts of Na, Fe, K, Ca, and Fe in shocked
amphibolite (Stähle et al. 2010, 2017). Majorite wascrystallized from amphibolite melt or chlorite-rich melt
induced by an impact.
2.2.2 MgSiO3–FeSiO3: Akimotoite‑hemleyite

Trigonal ­MgSiO3 phase with the ilmenite structure phase
was synthesized at 17.5–27.5 GPa and 1000–1400 °C by
an LHDAC (Liu 1976) following the finding of hexagonal ­MgSiO3 by Kawai et al. (1974). About 20 years after
the syntheses, the natural occurrence of the phase wasreported in the shock-melt veins of the Acfer 040 L5–6
and the Tenham L6 ordinary chondrites based on TEM
observations (Sharp et al. 1997; Tomioka and Fujino
1997). The matrix of the shock-melt veins in Acfer
040 consists mainly of M
­ gSiO3-rich amorphous grains(~ 2 µm in size), ringwoodite, and M­ gSiO3-ilmenite. The­MgSiO3-ilmenite occurs as plate-like grains and contains considerable amounts of the ­Na2O, ­Al2O3, and FeO
components (Sharp et al. 1997), while the ­MgSiO3-rich
amorphous material contains significant amounts of­Na2O, ­Al2O3, CaO, and FeO. The assemblages and chemical compositions of high-pressure minerals suggest thatthey have crystallized from shock-induced melt over
26 GPa (Sharp et al. 1997). In Tenham, clinoenstatite[(Mg,Fe)SiO3], fragments of the host rock entrained in
the shock-melt veins, is associated with aggregates ofsubmicron-sized grains and no other minerals in theirinterstices. Their chemical composition is identical tothat of the host clinoenstatite (Tomioka and Fujino 1997)(Fig. 8). The ilmenite phase is thought to have formed
directly from the host clinoenstatite without melting. The­MgSiO3-ilmenite was named akimotoite after S. Akimoto(Tomioka and Fujino 1999). Akimotoite has been foundin chondrites and shergottites (Xie and Sharp 2004; Imaeand Ikeda 2010; Tomioka and Miyahara 2017), whereas
they have not been reported in impact craters.
Recently, the Fe-rich ilmenite with Fe/(Mg + Fe) = 0.56
was also discovered in an unmelted portion of the heavily shocked L6 ordinary chondrite Suizhou associatedwith olivine, clinoenstatite, and Fe-bearing pyroxene(Bindi et al. 2017). The occurrence suggests that the newphase formed by a Fe-rich clinoenstatite—formed by thethermal metamorphism in its parent body—was subsequently shock-transformed to Fe-rich akimotoite. The
mineral was named hemleyite after R. J. Hemley (Bindi
et al. 2017). So far, the F­ eSiO3 solubility into akimotoite
increases up to ~ 10 mol% with increasing pressure from
20 to 25 GPa at 1100 °C based on a phase equilibrium
study (Ito and Yamada 1982), whereas at the ­FeSiO3 content above 10 mol%, bridgmanite and majorite are stable
rather than akimotoite at a higher pressure and lower
temperature, respectively (Ohtani et al. 1991). Therefore,
hemleyite would be metastably formed during a shortpressure and temperature excursion in the shock event.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Page 9 of 26

Fig. 6 BF-TEM images of dissociated olivine in the shergottite DaG 735. a Granular bridgmanite + magnesiowüstite, b lamellar
bridgmanite + magnesiowüstite. Most bridgmanite have been vitrified. Bdg: bridgmanite, Mw: magnesiowüstite

2.2.3 MgSiO3–FeSiO3: bridgmanite‑hiroseite

Silicate perovskite has been first synthesized from pyrope
at 30 GPa and above 800 °C in the Al-bearing system
by an LHDAC (Liu 1974). Subsequently, pure ­MgSiO3
perovskite was synthesized at 30 GPa and ~ 1000 °C by
an LHDAC (Liu 1975) and 28 GPa and ~ 1000 °C by a
Kawai-type multianvil apparatus (Ito and Matsui 1978).
These phases have an orthorhombic symmetry with thesame structure as perovskite ­(CaTiO3). The mineral isof particular interest to geophysicists because the phase
occupies ~ 77 vol% of the Earth’s lower mantle (Irifune
1993).
In nature, (Mg,Fe)SiO3 grains (~ 0.3 µm in size) adjacent to a clinoenstatite grain were identified in the
Tenham L6 ordinary chondrite to be crystalline as a perovskite-structure phase using TEM (Tomioka and Fujino
1997). The perovskite phase has the same chemical composition as clinoenstatite in the host rock, which is considered to have formed in a solid phase transition as well
as akimotoite in the Tenham chondrite. Fine-scale intergrowth of (Mg,Fe)SiO3-perovskite and magnesiowüstite,
as post-ringwoodite dissociation products of olivine,
were also found in the shergottite DaG 735 (Miyahara
et al. 2011b). The presented TEM works did not give fullcrystallographic data on the perovskite phase because of
its large susceptibility to electron beam radiation. However, synchrotron X-ray micro-diffraction finally gavedata to determine the unit cell of (Mg,Fe)SiO3-perovskite
in the Tenham chondrite. The mineral was named bridgmanite after P. W. Bridgman (Tschauner et al. 2014).
Recently, a Fe-rich silicate perovskite phase with Fe/(Mg + Fe) = 0.59 was discovered in a shock-melt veinin the Suizhou chondrite L6 ordinary chondrite basedon XRD, TEM, and micro-Raman spectroscopy (Bindi

Fig. 7 BF-TEM image of a monomineralic aggregate of tetragonal
majorite in the Tenham L6 ordinary chondrite. The majorite grains
formed in a solid-state transformation of low-Ca pyroxene entrapped
into a shock-melt vein

et al. 2020c). Interestingly, Fe nanoparticles less than
100 nm in size were found embedded in the host perovskite phase that has both ferric and ferrous iron. Thatis the first evidence for a subsolidus charge disproportionation reaction in natural high-pressure minerals:­3Fe2+ → ­Fe0 + ­2Fe3+. The mineral was named hiroseite
after K. Hirose (Bindi et al. 2020c). So far, bridgmanite
has not been found in terrestrial impact craters.
It is noteworthy that a bridgmanite-like mineral was
discovered in the Acfer 094 carbonaceous chondrite(ungrouped C2). The mineral is ~ 300 nm in size and hasan oxygen isotope anomaly (Vollmer et al. 2007). ElectronMiyahara et al. Prog Earth Planet Sci(2021) 8:59

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2.2.4 Other high‑pressure minerals with the pyroxene
compositions

Fig. 8 BF-TEM image of akimotoite (Aki) in the Tenham L6
ordinary chondrite. The akimotoite grains formed in a solid-statetransformation from low-Ca pyroxene (clinoenstatite: Cen) with (100)twin lamellae in the host rock entrapped into a shock-melt veindiffraction patterns of the mineral indicate a superstructure of the orthorhombic perovskite structure. A possible
explanation of the formation process of the novel mineralis that presolar ­MgSiO3-pyroxene particles floating ininterstellar space have transformed into the perovskitelike phase by interstellar shock waves before incorporation into its parent body.
CaSiO3 with the perovskite structure was also confirmed at 16 GPa and 1500 °C by an LHDAC (Liu andRingwood 1975). It is not an atmospheric pressure
quenchable phase but vitrified during decompression.
As for shocked meteorites, ­CaSiO3-rich amorphous
phase coexisting with Ca-rich majorite was reported
in a shock-melt vein of the Y-75100 H6 ordinary chondrite (Tomioka and Kimura 2003). The diopside in thehost rock would have initially dissociated into ­CaSiO3
with the perovskite structure plus Ca-rich majorite during prograde shock metamorphism, and then, the perovskite phase became amorphous during the subsequent
retrograde stage. Crystalline ­CaSiO3 perovskite with an
orthorhombic cell was found in a terrestrial environment
as inclusion in a super-deep diamond from South African
Cullinan kimberlite (Nestola et al. 2018). More recently,
cubic ­CaSiO3 perovskite was also reported in a diamondfrom Botswanan Orapa kimberlite, and then named davemaoite after H.-K. Mao (Tschauner et al. 2020a).

The ­MgSiO3 component has various pyroxene-structure
polymorphs with different crystal symmetries caused byconfigurational changes of the ­SiO4 tetrahedral chains athigh pressures and temperatures: low-temperature clinoenstatite (space group P21/c), orthoenstatite (Pbca),
protoenstatite (Pbcn), high-temperature clinoenstatite(C2/c), and high-pressure clinoenstatite (C2/c) (Gasparik1990). High-pressure clinoenstatite was found by in situ
XRD study at pressures above ~ 7 GPa (Angel et al. 1992;
Shinmei et al. 1999); however, it cannot be recovered tothe ambient condition as it inverts to low-temperatureclinoenstatite during decompression. Recently, evidence
for high-pressure clinoenstatite was found in the shockmelt vein of the Grove Mountains 022,115 L6 ordinarychondrite by HRTEM (Guo et al. 2020). The occurrenceof high-pressure clinoenstatite embedded in amorphoussilicate and associated with majorite and ringwoodite
suggests that the assemblage was initially crystallized as
majorite at 20–23 GPa and over 1800 °C and was subsequently back-transformed to high-pressure clinoenstatite
during retrograde shock metamorphism. High-pressure clinoenstatite would have been metastably frozen
because of extremely rapid cooling (Guo et al. 2020).
A novel Fe, Al-bearing ­MgSiO3-rich phase wasreported in the silicate glass matrix of shock-melt veinsin Tenham L6 ordinary chondrite by TEM (Xie et al.
2011). The mineral has an acicular morphology (≤ 0.5 µm
in length) and its electron diffraction pattern indicates anolivine structure. A possible explanation to reconcile thediscrepancy between the olivine structure and the pyroxene stoichiometry of the phase is the presence of cation
vacancies at the M site in the olivine structure. The estimated density of 3.32 g/cm3 is slightly larger than thoseof olivine and pyroxenes; therefore, the mineral would
have metastably crystallized from chondritic melt underhigh pressure during rapid cooling (Xie et al. 2011).
2.3 High‑pressure polymorphs of feldspar
2.3.1 Jadeite + silica

Albitic plagioclase is one of the major constituents ofequilibrated ordinary chondrites. Albite (­NaAlSi3O8)
dissociates into jadeite (­NaAlSi2O6) + quartz ­(SiO2),
jadeite + coesite ­(SiO2), and jadeite + stishovite withincreasing pressure and temperature (Liu 1978). However, these dissociation products do not occur in theshock-melt veins of equilibrated ordinary chondrites.
In most cases, jadeite solely occurs in originally plagioclase grains (e.g., Ohtani et al. 2004; Ozawa et al. 2009;Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Miyahara et al. 2013b). Jadeite shows several kinds ofcrystal habits, namely, dendritic, granular, stringlike, andpolycrystalline (Ozawa et al. 2009; Miyahara et al. 2020),
which depend on the formation processes. Even in thelarge-scale shock metamorphism, pressure duration is
not long enough (< several seconds) to form coesite orstishovite along with jadeite because of its sluggish nucleation (Kubo et al. 2010). However, the jadeite + coesite
assemblage has replaced an albitic plagioclase grain insome ordinary chondrites (Miyahara et al. 2017, 2020).
On the other hand, Ma et al. (2020) propose that highly
defective jadeite (Na,Ca,v)(Al,Si)Si2O6 (v = vacancy) withexcess silicon has formed in an albitic plagioclase grain ofsome ordinary chondrites.
2.3.2 Tissintite

Plagioclase in Martian and lunar meteorites is richer inanorthite components compared with equilibrated ordinary chondrites (Papike 2018). Tissintite is clinopyroxene-type (Ca,Na,v)AlSi2O6 and found in an anorthitegrain in contact with the melt-pocket of the shergottite
Tissint (Ma et al. 2015). Tissintite was synthesized from
amorphous plagioclase under 6–8 GPa and 1273–1623 K(Rucks et al. 2018). Most tissintite grains in shocked
meteorites show a dendritic or spherulitic texture, suggesting rapid crystallization from anorthitic melt (Fig. 9).
2.3.3 Lingunite–Stöfflerite–Liebermannite

Hollandite-type ­KAlSi3O8 was synthesized first at 12 GPaand 900 °C by Ringwood et al. (1967). Also, Liu (1978)
synthesized hollandite-type N­ aAlSi3O8 above 20 GPa
at ~ 1000 °C using an LHDAC. In contrast, subsequent
experimental works in the system ­NaAlSi3O8-KAlSi3O8
reported that the hollandite phase is stable only in K-richcomposition (Na/(Na + K): <  ~ 0.4), while calcium ferrite­(CaFe2O4)-type ­NaAlSiO4 + stishovite appears at Na-richcomposition (Na/(Na + K): >  ~ 0.4) above ~ 20 GPa and1000 °C (Yagi et al. 1994; Liu 2006). This discrepancy wasreconciled by a recent transformation kinetics study thatclarified that hollandite-type ­NaAlSi3O8 is a metastable phase crystallized from amorphous plagioclase glass(Kubo et al. 2017).
Hollandite-type ­NaAlSi3O8 replacing albitic feldspar was found in the shock-melt veins of the Sixiangkou and Tenham L6 ordinary chondrites (Gillet et al.
2000; Tomioka et al. 2000; Xie and Sharp 2004). A small
amount of K-feldspar has also been found in some Martian and lunar meteorites. Hollandite-type K­ AlSi3O8
was found in the melt-pocket of the basaltic shergottite
Zagami (Langenhorst and Poirier 2000). Hollandite-type­NaAlSi3O8 and ­KAlSi3O8 were named lingunite after L.

Page 11 of 26

G. Liu (Liu and El Gorsey 2007) and libermannite after
R. C. Liebermann (Ma et al. 2018), respectively. Lingunite was also found from the doleritic rocks of Lockne
impact structure (Agarwal et al. 2016). Hollandite-type­CaAl2Si2O8 has yet to be synthesized but was recently
found in a melt-pocket of the basaltic shergottite NWA
856 and named stöfflerite after D. Stöffler (Tschauneret al. 2021).
2.3.4 Donwilhelmsite–zagamiite

A calcium aluminum silicate phase (­CaAl4Si2O11) was
synthesized by Irifune et al. (1994) and tentatively named
the Ca–Al–Si-rich (CAS) phase. Its crystal structurewith the ­CaAl4Si2O11 composition was determined to beof the hexagonal ­BaFe2O4-type based on single-crystal
XRD (Gautron et al. 1999). A mineral with (­Cax,Na1–x)
Al3+xSi3-xO11 composition was discovered in some shergottites such as Zagami and NWA 856 by micro-Raman
spectroscopy (Beck et al. 2004; El Goresy et al. 2013).
The mineral is intergrown with acicular stishovite grains
in melt pockets. A natural CAS phase was also foundfrom the feldspathic lunar meteorite Oued Awlitis 001
and was well characterized by three-dimensional electron diffraction pattern analysis (Fritz et al. 2020). This
mineral was named donwilhelmsite after D. E. Wilhelms(Fritz et al. 2020). Ma et al. (2019a) also found a hexagonal ­BaFe2O4-type ­CaAl2Si3.5O11 in the melt-pocketof Zagami and NWA 856 and named it zagamiite. Zagamiite is expected to be a crystallization product from aplagioclase-rich melt with some clinopyroxene components because it has small amounts of Fe and Mg and asubstantial deficit of Na (Ma et al. 2019a).
2.3.5 Dense plagioclase glass

Experimentally shocked plagioclase becomes an amorphous phase with a high refractive index without lossof short-range ordering in its structure (Milton and deCarli 1963; Kitamura et al. 1977). The first identificationof such a dense plagioclase glass is from the shergottite
Shergotty and has been called maskelynite after N. S.
Maskelyne (Tschermak 1872). Similar dense plagioclase
glass has been then commonly found in strongly shockedchondrites and achondrites (e.g., Stöffler et al. 1986,
1991). The glass almost retains the morphology of theoriginal plagioclase but loses its crystallinity. Therefore,
it is easy to be identified by its optical isotropy under apolarizing microscope.
One of the big debates about maskelynite is its formation process. Some maskelynite shows a flowing texture
or mixing texture with other melts (schlieren), suggesting that the maskelynite has formed by the quenching ofplagioclase shock-induced melt (El Goresy et al. 2013).
By contrast, some maskelynite has retained original

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Fig. 9 BF-TEM image of tissintite in the shergottite Tissint. Spherulitic
tissintites have occurred in an original plagioclase grain in contact
with the melt-pocket of Tissint. The interstices are filled withamorphized plagioclase (probably because of melting). Tis: Tissintite,
Mas: maskelynite

compositional zoning, suggesting that the maskelynite
has formed through a solid–solid-state phase transformation (Jaret et al. 2015).
Tschermak (1883) originally described that maskelynite
has a non-vesicular flow texture resulting from melting
and liquid migration followed by solidification. Based on
the historical background, only material with evidence
for melting like schlieren should be called maskelynite,whereas material without evidence of melting should
be called diaplectic plagioclase glass. Normal glass isquenched from a melt which is still liquid after pressure
release (Stöffler 1984). In case that the formation process (solid-state amorphization or melting) is not clear,but its high density can be assumed by its higher refractive index than normal plagioclase glass under an optical microscope (Stöffler et al. 1986, 1991), we suggest theterm “dense plagioclase glass” instead of “maskelynite”
and “diaplectic plagioclase glass” to avoid confusion.
2.4 High‑pressure polymorphs of silica

The synthesis and naming histories of high-pressure polymorphs of silica, except for seifertite, are mentioned ina later section because their first discoveries are from aterrestrial impact crater. Coesite, stishovite, and seifertite are high-pressure polymorphs of silica. Quartz is rare
but cristobalite and tridymite are abundant in meteorites(Papike 2018). Hence, most high-pressure polymorphsof silica have occurred as replacements of cristobalite or
tridymite.

Page 12 of 26

As a discovery from a meteorite, Weisberg and Kimura(2010) found coesite in the Gujba CB carbonaceouschondrite for the first time by Raman spectroscopy.
Coesite is mainly found in basaltic lunar meteorites andeucrites (Miyahara et al. 2014; Tomioka and Miyahara
2017; Kayama et al. 2018) but very rarely from enstatitechondrite and Martian meteorite (Kimura et al. 2017; Huet al. 2020). TEM observations revealed that these coesite
crystals are granular and always coexist with silica glassor quartz (Miyahara et al. 2013a, 2014) (Fig. 10). Coesite
is also found as one of the decomposition products ofalbitic plagioclase in shocked ordinary chondrites (Miyahara et al. 2017, 2020).
As for stishovite replacing silica minerals, it was discovered from the lunar meteorite Asuka-881757 for the
first time (Ohtani et al. 2011). Stishovite crystals are
acicular and always coexist with silica glass, the same ascoesite (Ohtani et al. 2011; Miyahara et al. 2013a, 2014;
Kayama et al. 2018). Stishovite is mainly found in basaltic lunar meteorites, eucrites, and shergottites (Tomioka
and Miyahara 2017) and very rarely from carbonaceouschondrite (Bindi et al. 2012) and iron meteorite (Holtstam et al. 2003). Some stishovites have formed throughthe high-pressure decomposition of plagioclase in someshocked shergottites (Langenhorst and Poirier 2000;
Beck et al. 2004; El Goresy et al. 2013). Kaneko et al.
(2015) discovered stishovite from regolith breccia Apollo
15,299. This is the first case of a high-pressure mineral being discovered in extraterrestrial rocks by sample
return missions.
Scrutinyite (α-PbO2)-type silica was synthesized inshock recovery experiments of quartz up to 90 GPa (German et al. 1973). Its natural analog was found from thebasaltic shergottite Shergotty (Sharp et al. 1999; Deraet al. 2002) and named seifertite after F. Seifert by El Goresy et al. (2008). Seifertite is also found from other shergottites and basaltic lunar meteorites (El Goresy et al.
2013; Miyahara et al. 2013a). In situ XRD studies showthat seifertite is stable in the pressure range of 120–260
GPa (Murakami et al. 2003; Kuwayama et al. 2005), while
Kubo et al. (2015) clarified that seifertite has metastably
formed from cristobalite even above ~ 11 GPa. Therefore, seifertite is unlikely an indicator of megabar shockpressures in shergottites and lunar meteorites. Seifertite
in backscattered electron images always shows a tweedlike texture, which is one of the diagnostic features for its
identification (El Goresy et al. 2008). Like stishovite, seifertite crystals exhibit an acicular morphology and always
coexist with silica glass (Miyahara et al. 2013a) (Fig. 11).
El Goresy et al. (2000) also found an unnamed post-stishovite phase with the baddeleyite ­(ZrO2)-like structurefrom one shergottite.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Page 13 of 26

2.5 Others
2.5.1 High‑pressure polymorphs of phosphate minerals

A ­Ca3(PO4)2 mineral with a ­Ba3(PO4)2-structure[γ-Ca3(PO4)2] was synthesized by Murayama et al.
(1986). Xie et al. (2002a) reported that merrillite­[Ca9NaMg(PO4)7] has transformed into the γ-Ca3(PO4)2
phase in the shock-melt vein of the Suizhou L6 ordinarychondrite. The γ-Ca3(PO4)2 phase was named tuite after
G. Tu (Xie et al. 2004).
2.5.2 High‑pressure polymorphs of chromite

CaTi2O4-type ­FeCr2O4 phase was synthesized byChen et al. (2003a), and a natural one was found in the
Suizhou L6 ordinary chondrite (Chen et al. 2003b). The­CaTi2O4-type ­FeCr2O4 phase was named xieite after
X. Xie (Chen et al. 2008). Harmunite ­(CaFe2O4)-type­FeCr2O4 phase was synthesized by Chen et al. (2003a),
and a natural one was found in the Suizhou L6 ordinarychondrite (Chen et al. 2003b). The mineral was named
chenmingite after M. Chen (Ma et al. 2019b).
2.5.3 High‑pressure polymorphs of other oxides

LiNbO3-type ­FeTiO3 phase was first reported in the Ries
crater (Dubrovinsky et al. 2009). The phase was also
found in the Suizhou L6 ordinary chondrite and named
wangdaodeite after D. Wang (Xie et al. 2020). Schreyerite-type ­(Ti4+Fe2+)Ti34+O9 phase was found in the meltpocket of the eucrite NWA 8003 and named vestaite after
the name of its parental asteroid Vesta (Pang et al. 2018).
­Sr2Tl2O5-type ­Fe2+2(Fe2+Ti4+)O5 phase and ­GdFeO3-type­FeTiO3 have been recently reported from the Martian
meteorite Shergotty and named feiite after Y. Fei andliuite after L.-G. Liu, respectively (Ma et al. 2021b). In thesame meteorite, ­CaTi2O4-type ­(Fe2+)(Fe2+Ti4+)O4 (simply ­Fe2TiO4) with a post-spinel structure was also discovered (Ma et al. 2021a) and was named tschaunerite after
O. Tschauner. ­CaGa2O4-type ­CaAl2O4 was discovered ina Ca–Al-rich inclusion (CAI) of the NWA 470 CH3 carbonaceous chondrite and named domitryivanovite after
D. A. Ivanov (Mikouchi et al. 2009). The mineral was
also reported in an unusual Ca–Al–Fe rich inclusion in
the Vigarano CV3 carbonaceous chondrite (Maruyama
and Tomioka 2011). Previous high-pressure experimentssuggest that domitryivanovite is stable above ~ 2 GPa (Itoet al. 1980).
2.5.4 High‑pressure polymorphs of other high‑pressure(Mg,Fe)‑silicatesInterestingly, a new phase found as a unique grain in a
shock-melt vein of the Suizhou L6 ordinary chondrite
has the composition of (Mg,Fe)5Si2O9 and was named

Fig. 10 BF-TEM image of coesite in the eucrite Béréba. Sphericalcoesite crystals have formed in cristobalite or tridymite grains incontact with the shock-melt veins of Béréba. A part of the original
silica has become amorphous. Coe: coesite, Si-Gla: silica-glass

elgoresyite after A. El Goresy (Bindi et al. 2021). The crystal structure of this new iron–magnesium silicate, neverobtained in laboratory experiments so far, is the same as
the iron oxide F­ e7O9, strongly suggesting that silicatesalso form the ((Mg,Fe)O)m+n(SiO2)n series that are isostructural to iron oxides via ­(Mg2+,Fe2+) + ­Si4+ = ­2Fe3+
substitution. The newly found iron–magnesium silicate isa potential constituent mineral in rocky planets with relatively high MgO + FeO content.
2.5.5 High‑pressure polymorph of phosphide

Co2Si-type (Fe,Ni)2P was discovered in the Onello ironmeteorite and named allabogdanite after A. N. Bogdanova (Britvin et al. 2002). The mineral was regarded to bea high-pressure polymorph of barringerite that is stable
above 6 GPa and 900 °C based on previous high-pressure
experiments (Dera et al. 2008) and natural occurrences
in several other iron meteorites (Britvin et al. 2019).
However, further detailed experimental and theoreticalstudies clarified that the mineral is also stable at ambient pressure below 500 °C (Litasov et al. 2020). Hence,
the results suggest a possibility that the formation ofallabogranite in iron meteorites does not require shock
metamorphism.
2.5.6 High‑pressure polymorphs of carbon

The first natural occurrence of diamond in shocked materials was reported, as in the Novo-Urei ureilite, in thelate nineteenth century (Kunz 1888) and have currently
been found in more than 10 ureilites (e.g., Goodrich et al.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Fig. 11 BF-TEM image of seifertite in the basaltic lunar meteorite
Northwest Africa (NWA) 4734. Lath-like seifertite crystals have formedin cristobalite grains in contact with the shock-melt veins of NWA
4734. Most parts of the original cristobalite grains have become
amorphous. Sei: seifertite, Si-Gla: silica-glass

2004; Miyahara et al. 2015; Nestola et al. 2020; Barbaro
et al. 2021). Experimental works on the synthesis of diamond for industrial purposes succeeded by static compression of graphite over 3 GPa by Bundy et al. (1955)
and shock-synthesized at 30 GPa by DeCarli and Jamieson (1961). Diamonds in ureilites are also expected to beformed under transient high-pressure condition shock
metamorphism (e.g., Berkley et al. 1976; Nakamuta andAoki 2000; Nakamuta and Toh 2013; Nestola et al. 2020).
On the other hand, some diamonds might form under
static high-pressure conditions in the deep interior ofan ureilite parent body (Urey 1956; Miyahara et al. 2015;
Nabiei et al. 2018) or at low pressure through chemical
vapor deposition (Fukunaga et al. 1987).
2.6 Natural quasicrystals

One of the most remarkable findings of exotic extraterrestrial materials has been the discovery of natural
quasicrystals (QCs) (Bindi et al. 2009, 2012). They havebeen detected as rare materials in the Khatyrka meteorite, a peculiar CV3 carbonaceous chondrite (MacPherson et al. 2013) containing Cu, Al-metallic compounds.
QCs, short for quasiperiodic crystals, are solids that areable to violate the conventional rules of crystallography
because their structure is quasiperiodic rather than periodic (Bindi et al. 2020b). The first quasicrystalline phasefound in nature, icosahedrite ­Al63Cu24Fe13 (Fig. 12) (Bindi
et al. 2009, 2011), displayed a five-fold symmetry in two
dimensions and icosahedral symmetry in three dimensions. Then, a second QC, decagonite ­Al71Ni24Fe5 (Bindi

Page 14 of 26

et al. 2015a, 2015b), was found in the same meteorite andit was the first mineral to exhibit crystallographically forbidden decagonal symmetry. Both icosahedrite and decagonite, however, showed compositions matching those ofsynthetic quasicrystalline phases found earlier (Tsai et al.
1987, 1989) in the laboratory at standard pressure.
The origin of these unusual metallic alloys is enigmaticas they contain metallic aluminum that forms underhighly reducing conditions not normally found in nature.
Furthermore, the puzzling combination of metallic aluminum, a refractory lithophile element, and copper, amoderately volatile siderophile or chalcophile element,
makes these alloys even more mysterious. When firstreported, a plausible explanation was that the samples
were byproducts of some laboratory or industrial process. Previous and successive studies, however, provided
compelling evidence that the quasicrystals are naturaland from a common meteoritic source: (i) 14C-dating ofmaterials from undisturbed clay layers where some of thesamples were collected yielded 6.7–8.0 ka BP, (ii) the Cu–
Al metallic alloys were found to be intimately intermixed
with oxides/silicates with nonterrestrial oxygen isotope
composition (Hollister et al. 2014; Lin et al. 2017), (iii)clear evidence was found of high-pressure-induced phasetransitions requiring transient conditions consistent withan asteroidal collision event, at least 5 GPa and 1200 °C,
sufficient to melt and rapidly quench the Al–Cu bearing
alloys (Asimow et al. 2016), (iv) noble gas measurements
confirmed that the shock event reached pressure above
5 GPa and occurred at least hundreds of million years
ago (Meier et al. 2018), and (v) robust petrographic andchemical evidence established that some metallic alloy
grains (including QCs) found in the samples predated
the shocks. Moreover, the recent discovery of a COtype chondritic spherule from the Nubian desert, Sudan(Suttle et al. 2019), containing the same assemblage ofaluminum, iron, and copper and with a morphology
remarkably similar to Khatyrka provided further support and independent evidence that these samples wereformed in outer space.
By means of laser ablation inductively coupledplasma mass spectrometry measurements of the
trace element contents of the two naturally occurring
quasicrystalline minerals, ­Al63Cu24Fe13 icosahedriteand ­Al71Ni24Fe5 decagonite, Tommasini et al. (2021)
hypothesized a three-stage model for the formationof the quasicrystals and their inclusions: a high-temperature condensation stage and a low-temperature
vapor-fractionation stage to make nanoparticles, followed by a third stage that leads to the formation ofquasicrystals incorporating the two different typesof nanoparticles and their incorporation into the CV
chondrite parent body of the Khatyrka meteorite.

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

In a recent speculative essay, Bindi et al. (2020a)
reported that although the only known examples of natural quasicrystals are from the Khatyrka meteorite, thisdoes not necessarily mean that quasicrystals must be
extremely rare in the universe. The authors present several reasons why quasicrystals may prove to be among
the most ubiquitous minerals found in the universe.
2.7 Significance of high‑pressure minerals from meteorites

Ringwoodite can be identified easily by an optical microscope because it shows purple/blue color in transmitted
light. Hence, in the classic shock classification criterion(S1–S6), the identification of ringwoodite in ordinarychondrites became a criterion for very strongly shocked(S6) (Stöffler et al. 1991). Many kinds of high-pressureminerals were found from ordinary chondrites after theshock classification criterion was proposed. The criterion
was revised, Stöffler et al. (2018) defining that not only
ringwoodite but also wadsleyite, ahrensite, majorite, akimotoite, bridgmanite, and lingunite may be present in
ordinary chondrite that satisfies shock stages S5–S6.
High-pressure melting experiments of the Allende CV3
carbonaceous chondrite and peridotite were conducted,
and their phase equilibrium diagrams were obtainedusing their run products (Zhang and Herzberg 1994;
Agee et al. 1995). There are similarities in the species,
assemblages, chemical compositions, and grain sizes ofhigh-pressure minerals between the recovered samples
and constituents in shock-melt veins. Based on the similarities, Chen et al. (1996) estimated the shock pressure
and temperature recorded in a shock-melt vein using thephase equilibria diagrams. This scheme was applied toseveral ordinary chondrites, eucrites, Martian, and lunar
meteorites to estimate the shock pressure and temperature (e.g., Xie et al. 2003, 2006; Ohtani et al. 2004; El Goresy et al. 2013; Pang et al. 2016; Zhang et al. 2021).
The major disruption that occurred on the early parent body of ordinary chondrites was deduced by thesystematic investigations of high-pressure minerals(Miyahara et al. 2020). Most ordinary chondrites haveexperienced thermal metamorphism after accumulation into the early parent body. Based on the maximum
metamorphic temperature and cooling rate recorded ineach ordinary chondrite, the early parent body of ordinary chondrites had an onion shell structure before its
major disruption (e.g., Trieloff et al. 2003). Assuming theonion shell model, shock pressure should be the highest

Page 15 of 26

at the surface (least thermally metamorphosed one: petrographic type 3) of the parent body. However, unexpectedly, the shock pressure estimated by high-pressureminerals increases with increasing petrographic typefrom 3 to 6 (petrographic type 6 is the highest thermally metamorphosed one corresponding to the deepest
interior of the parent body) (Miyahara et al. 2020). The
results suggest that the apparent lower shock pressure on
the surface would be explained by spallation at the opposite side of the parent body to the impact point.
Recent studies point out that shock metamorphism
recorded in shocked meteorites is not simple. Some ordinary chondrites seem to record a dual impact because (i)a shock-melt vein is penetrated by another shock-meltvein (Hu and Sharp 2016) and (ii) a high-pressure polymorph includes another high-pressure polymorph indicating different shock pressures (Miyahara et al. 2011a).
The dual impact (or dual heating event) record is also
deduced from radioisotope ages in some shocked meteorites (e.g., Treiman et al. 2007).
The formation of a high-pressure mineral and resettingof a radioisotope’s age depend on the thermal history.
However, the thermal history recorded in a shock-meltvein or melt-pocket is also complex. Pressure decreases
drastically (several microseconds’ order) but temperature
decreases gradually (several seconds’ order) during an
adiabatic decompression stage, resulting in low-pressureand high-temperature conditions. If the low-pressureand high-temperature conditions are prolonged, the backtransformation of a high-pressure mineral occurs, which
underestimates the shock pressure (Kimura et al. 2004;
Hu and Sharp 2017; Miyahara et al. 2019). However,a back-transformation mechanism of a high-pressuremineral and its kinetics have not been deeply investigated except for a few works (Ming et al. 1991; Fukimoto et al. 2020; Miyahara et al. 2021). We must work onthese issues to trace a pressure–temperature-time path
recorded in a shock-melt vein or melt-pocket as future
work.
Phase transformation is kinetically controlled. Ohtani
et al. (2004) suggested estimating the magnitude of an
impact event recorded in an ordinary chondrite using
the grain-growth kinetics of ringwoodite. The grain sizesof ringwoodite crystals in shocked ordinary chondrites
are similar to each other (across < 1–2 μm) (Ohtani et al.
2004; Miyahara et al. 2010; Kato et al. 2017). Hence, theestimated duration of shock pressure is almost the same

(See figure on next page.)
Fig. 12 BSE images of icosahedrite in the Khatyrka CV3 carbonaceous chondrite. a Grain 126A of Khatyrka CV3 carbonaceous chondrite. A
red dashed box indicates the region to be enlarged in b. b The area where there is the metal assemblage containing the icosahedral phase(icosahedrite). c The enlarged image of the region indicated by a red dashed box in the panel b. d Electron backscatter diffraction pattern of the
icosahedral phase obtained from the region marked with a cross in the inset. The pattern matches that predicted for a face-centered icosahedral
quasicrystal

Miyahara et al. Prog Earth Planet Sci

Fig. 12 (See legend on previous page.)(2021) 8:59

Page 16 of 26

Miyahara et al. Prog Earth Planet Sci(2021) 8:59(< several seconds) (Ohtani et al. 2004; Xie et al. 2006;
Sharp et al. 2015). Also, the phase transition rates of plagioclase and silica are applied to estimate the duration ofshock pressure (< ~ 5 s) in some ordinary chondrites andlunar meteorites (Kubo et al. 2010; Miyahara et al. 2013a,
2017; Ozawa et al. 2014). On the other hand, recent timeresolved femtosecond diffractometry using an X-ray freeelectron laser revealed that ringwoodite can form fromlaser shock compressed olivine through ultrafast lattice
shearing in nanoseconds (Okuchi et al. 2021).

3 High‑pressure minerals in impact craters
3.1 High‑pressure polymorphs of carbon

In terrestrial impact structures, diamond was foundin the Barringer (Ksanda and Henderson 1939), Ries
(Hough et al. 1995), Zapadnaya (Gurov et al. 1995), andPopigai (Koeberl et al. 1997) craters. A previous highpressure experiment succeeded in synthesizing nanopolycrystalline diamond (NPD) with grain sizes less than
100 nm by direct conversion from pure polycrystalline
graphite at 12–25 GPa and 2300–2500 °C (Irifune et al.
2003). The NPD was brought to material scientists’ attention because it has a larger stiffness than a single-crystal
diamond. Its natural analog was also discovered from the
Popigai impact crater (Ohfuji et al. 2015).
A modified diamond structure with a hexagonal cell
was synthesized by a high-pressure experiment (Bundy
and Kasper 1967). The natural hexagonal diamond was
first found in the Barringer crater and was named lonsdaleite after K. Lonsdale (Frondel and Marvin 1967). It
is a very enigmatic mineral. It was also discussed that itshould not exist as a discrete mineral because it could
correspond to a faulted and twinned cubic diamond(Németh et al. 2014). In the Popigai crater, both diamond and lonsdaleite were found in impact melt rockand breccia. These high-pressure phases of carbon havebeen formed from graphite in the target gneissic rocks bylattice-shear (martensitic) transformation mechanismsof the graphite structure by shock metamorphism (Koeberl et al. 1997; Nakamuta and Toh 2013). The formationof diamond and lonsdaleite was estimated at 35–60 GPa
based on shock effects on the host-rock minerals.
A novel carbon crystal was reported in the Ries crater. The crystal shows a diffraction pattern different
from those of preexisting crystalline carbon polymorphs(graphite, diamond, or lonsdaleite) and named chaoite,
as mentioned previously (El Goresy and Donnay 1968).
A similar allotropic form of carbon was synthesized during the sublimation of pyrolytic graphite (Whittaker andKintner 1969). The mineral is thought to have a crystal
structure comprising chains with alternating single andtriple bonds (–C≡C–), which is called the carbyne structure. Five variations of carbynes were also found fromPage 17 of 26

the Allende and Murchison carbonaceous chondrites(Whittaker et al. 1980). The finding was once questioned
and claimed as misidentified phases by powder XRD asa mixture of quartz and Fe-smectite (Smith and Buseck1982). However, subsequent experimental studies ensure
the existence of carbynes (e.g., Chalifoux and Tykwinski 2010; Pan et al. 2015). First-principles’ calculations
clarified that carbyne is about twice as stiff as the stiffest
known materials under tension (Liu et al. 2013); therefore, chaoite is also a potentially important mineral formaterial science.
Another novel allotropic carbon was discovered in a
shocked gneiss from the Popigai impact crater (El Goresy et al. 2003). Carbon platelets, embedded in quartz
and cristobalite, consist of graphite and lonsdaleite
and an unknown carbon phase. The unknown phasehas considerably high polishing hardness overcoming
diamond powder. Synchrotron XRD clarified that thephase has a cubic cell with the space group Pm-3m,
although its density and the detailed crystal structure
have not yet been obtained. Its origin and formation
conditions are also unknown because of a lack of experimental and theoretical studies.
Finally, Németh and Garvie (2020) have recently
reported the discovery of cage-like nanostructured carbonaceous materials, including carbon nano-onions
and bucky-diamonds, formed through extraterrestrial
impacts in the Gujba C­ Ba carbonaceous chondrite.
3.2 High‑pressure polymorphs of silica

Dense ­SiO2 crystals are the first synthesized minerals among silicates and oxides related to the Earth andplanetary materials. Coesite and stishovite were first
synthesized by static compression by Coes (1953) andStishov and Popova (1961), respectively, and stishovite was also shock-synthesized by DeCarli and Milton(1965). Phase equilibria studies based on static highpressure experiments clarified that coesite and stishovite are stable above ~ 2 GPa and ~ 8 GPa, respectively.
Stishovite was shock-synthesized at pressure over 12
GPa, while there is no convincing report that coesite
can be formed by shock recovery experiments. Smaller
shock pressure duration ­(10–6 s) in laboratory shock
experiments than that in terrestrial impact cratering(> ­10–3 s) (Kieffer 1971) and sluggish kinetics in thequartz to coesite transformation would cause difficultyin the artificial production of the mineral (e.g., Nagai
et al. 1997; Perrillat et al. 2003).
The first finding of natural dense silica was performedin the early 1960s. E. C. T. Chao and his colleagues
applied hydrofluoric acid leaching to a large amount of
the Coconino sandstone of the Barringer crater, which

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

was produced by the impact of the Canyon Diablo ironmeteorite ~ 49,000 years ago. Coesite and stishovite,
the high-pressure polymorphs of ­SiO2, were identified in the residues by powder XRD (Chao et al. 1960,
1962). In particular, coesite is one of robust evidence
for impact origin and found in over 20 impact craters.
Stishovite has been found in three more impact craters,
namely Popigai (Vishnevsky et al. 1975), Ries (Stöffler
1971; Stähle et al. 2007), and Vredefort (Martini 1978,
1991; Spray and Boonsue 2018), in addition to the Barringer crater.
3.3 High‑pressure polymorphs of ­TiO2

Rutile transforms into scrutinyite (α-PbO2, ­TiO2-II), baddeleyite ­(ZrO2), orthorhombic-I, and cotunnite structures with increasing pressure (Nishio-Hamane et al.
2010). Scrutinyite-type ­TiO2 with an orthorhombic cell
was first synthesized by a static compression experiment (Bendeliany et al. 1966). In nature, almost pure
scrutinyite-type ­TiO2 was first discovered in shocked
garnet gneisses from the Ries crater (El Goresy et al.
2001). The phase was also found from suevite in a drilling core of granitic basement rock from the Chicxulub
crater (Kring et al. 2020), which was estimated to havebeen shocked at ~ 20 GPa in the Cretaceous–Paleogene
boundary (Timms et al. 2019). The T­ iO2-rich scrutinyitephase had been known as mineral srilankite [(Ti,Zr)O2],
where Ti and Zr ions are disordered at the cation sites.
The Ries crater also yielded almost pure T­ iO2 with the
scrutinyite-like structure that was named riesite after
the name of the location (Tschauner et al. 2020b). Riesite
occurs in shock-melt veins within xenoliths of bedrock insuevite. Srilankite has one cation site for Ti and Zr ionsand one oxygen site, while there are two cation and oxygen sites each in the riesite structure, causing the monoclinic distortion. Riesite has not been synthesized, but its
higher density (4.37 g/cm3) than rutile (4.25 g/cm3) suggests riesite is a high-pressure mineral. Thus, riesite hasformed through the back transformation from the baddeleyite phase upon pressure release.
Baddeleyite-type ­TiO2, which has 7-coordinated Ti ions,
was first observed at 20 GPa, 770 °C by in situ XRD usinga Kawai-type high-pressure apparatus (Sato et al. 1991)
but could not be recovered at ambient condition. In contrast, in nature, baddeleyite-type ­TiO2 was first found in
the suevite breccia from the Ries crater and named akaogiite after M. Akaogi (El Goresy et al. 2001, 2010).
3.4 High‑pressure polymorphs of ­MgFe2O4

A high-pressure phase of magnesioferrite (­ MgFe2O4) was
discovered in a drill core of shocked diamond-bearing
gneiss from the Xiuyan crater (Chen et al. 2019). Powder

Page 18 of 26

XRD and TEM clarified that the mineral has the harmunite ­(CaFe2O4) structure as same as chenmingite­(FeCr2O4) discovered in the Suizhou L6 ordinary chondrite (Chen et al. 2003b) and the shergottite Tissint (Chenet al. 2019). The mineral was named maohokite after
H.-K. Mao (Chen et al. 2019). Maohokite coexists withmagnesioferrite embedded in calcite and is thought tobe a decomposed product of ankerite. The high-pressure
phase equilibria of M
­ gFe2O4 have not been establishedby experimental studies. However, based on pressure andtemperature conditions for the post-spinel transformation (e.g., Andrault et al. 2001), maohokite is thought tohave formed at 25–45 GPa and 800–900 °C (Chen et al.
2019).
3.5 High‑pressure polymorphs of ­ZrSiO4

The high-pressure phase of zircon is an important key
mineral to understanding the shock metamorphism in
impact craters because of its robustness against weathering. The scheelite (­CaWO4)–type ­ZrSiO4 was discovered in the upper Eocene impact ejecta layer in marine
sediments by powder XRD and named reidite after A.
F. Reid who first synthesized this phase (Glass and Liu
2001; Glass et al. 2002). Subsequently, reidite was foundin five impact craters: Chesapeake Bay (Malone et al.
2010), Xiuyan (Chen et al. 2013), Rock Elm (Cavosie et al.
2015), Ries (Erickson et al. 2017), and Woodleigh (Coxet al. 2018) craters. Shock recovery experiments of zircon
indicate that the transition to reidite is above ~ 30 GPa(Kusaba et al. 1985), while static high-pressure experiments demonstrated that the stability field of the phaseis in the pressure range of ~ 8–25 GPa (Ono et al. 2004;
Tange and Takahashi 2004). The discrepancy between
two shock pressure estimations would be caused bydifferent high-pressure timescale and/or differential
stress between the static and dynamic high-pressure
experiments.
3.6 Significance of high‑pressure minerals from impact
crater rocks

High-pressure minerals do not occur as commonly interrestrial impact craters when compared with thosein shocked meteorites. One of the possible reasonsis the different cooling histories after shock pressure
release between the surface materials of asteroids andthe Earth’s crust. Shocked surface rocks in the parent bodies of meteorites are excavated and released tospace, resulting in rapid cooling. In terrestrial impact
craters, part of the surface and near-surface materials
are ejected and also rapidly cooled to preserve highpressure minerals, which are mostly inclusions in lithic
impact breccia (suevite). Nevertheless, the materials
in larger burial depths of the craters such as those inMiyahara et al. Prog Earth Planet Sci(2021) 8:59

thick sediments and basement rocks should experience
longer residual shock heating because the temperature
decreased mainly by thermal conduction within therocks. In such a case, high-pressure minerals would
have been back-transformed to low-pressure minerals
even if they were once formed during shock metamorphism. Another reason for the weak preservation ofhigh-pressure minerals is aqueous alterations after thecrater formation. Abundant shock-induced fractures
play a role in fluid pathways, and fine-grain size andglassy materials containing high-pressure minerals are
susceptible to alterations by weathering and impactinduced hydrothermal activities (Grieve et al. 1996;
Kring et al. 2020).
Nevertheless, technical developments on microanalyses and constant efforts by impact petrologists enabled the findings of many high-pressure minerals from
impact craters. Since the discovery of coesite in the Barringer crater, over 10 new high-pressure minerals havebeen found from terrestrial impact craters (Table 1).
These dense minerals are direct and robust evidence
for the impact origin (but not a volcanic origin) of thecraters. Thus far, shock microstructures of quartz and,in particular, crystallographic orientations of planar
deformation features are commonly used to evaluate
shock pressures in the pressure range of ~ 10–35 GPa of
sedimentary and granitic rocks in impact craters (Langenhorst 2002; Stöffler et al. 2018). Also, phase transformations of quartz such as formations of diaplectic(dense) glass, lechatelierite (normal glass), coesite, andstishovite further provide pressure constraints in a
wider pressure range. A recently found superstructureof quartz from the basement rock of the Chicxulub crater (Tani et al. 2018) is another possible shock pressure
indicator. The previously described high-pressure phasesof carbon, oxides, and silicates would also contribute toelucidate spatial shock pressure distribution in impact
structures considering the results of static and dynamic
high-pressure experiments.
The attenuation of shock pressure in impact cratering is expressed by the following equation, P = P0(R/Lp)–b, where pressure P at the distance R from the isobaric core is a function of initial peak pressure P0, R,
the radius of impactor Lp, and attenuation coefficientof shock wave b (Mizutani et al. 1990). When we canobtain a shock pressure gradient based on pressures ofdifferent locations within an impact crater, the size ofthe impactor can be constrained. For this purpose, theoccurrences of high-pressure minerals can be usefulanchor points of shock pressures.
Shock pressure distribution during impact cratering is one of the most fundamental parameters toPage 19 of 26

understand shock temperature distribution, rock deformation, and impactor’s size and velocities. The information is closely related to impact-induced phenomena
such as the size and morphologies of the impact craters (Morgan et al. 2016; Riller et al. 2018), post-impacthydrothermal activities (Kring et al. 2020), atmosphericconditional changes (Artemieva et al. 2017), and biological mass extinctions (Lowery et al. 2018). Therefore, future research of high-pressure minerals will play
important roles to elucidate how the impacts of extraterrestrial materials affect Earth’s environments.

4 Summary
Pioneer works in the 1960s by R. A. Binns, E. C. T. Chao,
A. El Goresy, and their colleagues clarified the presenceof high-pressure minerals in shocked meteorites androcks around terrestrial craters. After their discoveries,
mineralogists and planetary scientists have continued tofind high-pressure minerals from natural shocked materials. The identification of new high-pressure minerals has increased drastically in the twenty-first century
because of the introduction of nanoanalysis technologies. The search for high-pressure minerals in shockedmaterials was initiated by finding the analog materials of
the deep Earth’s interior. This initial purpose is almost
achieved at present. However, the crystal structures andformation mechanisms of high-pressure minerals innaturally shocked materials are more complicated thanthose expected based on experiments and theoreticalsimulations. As the next step, we aim for better quantification of P–T histories of the shocked metamorphism
based on the high-pressure and high-temperature transformation kinetics and of impact histories in the solar
system by combining shock petrography and radioisotope ages.
Acknowledgements
We dedicate this paper to Prof. Ahmed El Goresy (1934–2019) who devoted
his life to science. In 2020, a new high-pressure mineral was named elgoresyite
after Ahmed El Goresy in honor of his contributions. We appreciate the helpfulcomments and suggestions by reviewers Profs. M. Akaogi and S. Xie as well as
an associate editor Prof. M. Kimura. We also thank Profs. E. Ohtani, A. Yamaguchi, T. Kubo, Drs. S. Ozawa, and Y. Kodama for their constructive comments andtechnical assistance.
Authors’ contributions
MM, NT, and LB conceived this review and collected and arranged theprevious studies. MM, NT, and LB wrote the manuscript. All authors read andapproved the final manuscript.
Funding
This study was supported by Grants-in-Aid for Scientific Research No.
18H01269 and 20H01965 by MEXT to M. M and N. T., respectively. This study
was also supported by NIPR, Japan, through General Collaboration ProjectNo. 26–31, No. 31–25, and Project Research KP-307. This work was in part supported by Hiroshima Institute of Plate Convergence Region Research (HiPeR)(the MEXT program for promoting the enhancement of research universities,
Japan).

Miyahara et al. Prog Earth Planet Sci(2021) 8:59

Availability of data and materials
Representative occurrences, transformation mechanisms, and Raman spectraof high-pressure polymorphs are in Tomioka and Miyahara (2017). The
inventory of high-pressure polymorphs in shocked meteorites is included in
Tomioka and Miyahara (2017) and Miyahara et al. (2020). The latest information about high-pressure minerals is available at https://​sites.​google.​com/​site/​
highp​miner​al.

Declarations
Competing interests
The authors declare that we have no competing interests.
Author details
1
Graduate School of Advanced Science and Engineering, Hiroshima University,
Higashihiroshima 739‑8526, Japan. 2 Kochi Institute for Core Sample Research,
Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku,
Kochi 783‑8502, Japan. 3 Dipartimento Di Scienze Della Terra, Università Degli
Studi Di Firenze, Via G. La Pira 4, 50121 Florence, Italy.
Received: 17 May 2021 Accepted: 29 September 2021

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