We investigated the back‐transformation mechanisms of ringwoodite and majorite occurring in a shock‐melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back‐transformed into olivine and enstatite, respectively. Ringwoodite (Fa_~18) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, fine‐grained polycrystalline olivine becomes dominant instead of ringwoodite. The back‐transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface‐controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs_16–17En_82–83Wo₁) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back‐transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.

中文翻译：

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普通球粒陨石中针铁矿和镁铁矿的逆转变机理

我们研究了在大气进入加热过程中，Yamato 75267 H6普通球粒陨石的冲击熔体脉（SMV）中发生的菱铁矿和多数铁矿的逆相变机理。熔融壳附近的激波熔体中的林木和菱铁矿分别逆转为橄榄石和顽辉石。林伍德（FA _〜18）在SMV发生作为细粒度多晶组合。接近融合壳时，细粒多晶橄榄石取代了林伍德石成为主导。从林木到橄榄石的逆转变是通过不连贯的成核和界面控制的生长机制进行的：成核发生在林木的晶界，随后橄榄石晶粒长大。多数（Fs _16-17 En _82-83Wo ₁）以细粒多晶组合的形式出现在SMV中。接近熔合壳时，主晶晶粒玻璃化。进一步接近熔融壳时，在玻璃化的主晶中会出现斜晶石/正钙钛矿晶粒。从玻璃体转变为顽辉石是由玻璃化作用引发的，随后在玻璃化的玻璃体中的成核作用使生长继续进行。