Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite is predicted to condense via a reaction between pre-condensed forsterite and gaseous SiO. However, previous studies have shown that some enstatites in chondrite matrices and AOAs do not have a genetic relationship with forsterite, arguing against formation by the predicted forsterite-gas reaction. Here we report the occurrence of enstatite in a unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 in the Efremovka CV3 chondrite. Enstatite in this FGI is present as an outer layer on spinel-anorthite-diopside nodules and separates the FGI from an amoeboid olivine aggregate (AOA) -like material. Enstatite shows elevated CaO and Al₂O₃ contents (up to a few weight percent). Four FIB sections were extracted from this FGI to investigate the microstructures of enstatite and its relationship with other phases using TEM techniques. The TEM observations show that the enstatite is dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, and is sometimes associated with thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly have a crystallographic orientation relationship with adjacent diopside, but do not exhibit any replacement relationship with forsterite in the AOA-like material surrounding the FGI. Investigations of several other fine-grained CAIs from the Efremovka and Leoville CV3 chondrites show that enstatite is more common in these inclusions than previously thought and typically forms discontinuous layers or islands on the diopside layers.

Based on SEM and TEM observations, we suggest that the LCLEN-OREN intergrowths in Ef1014-01 formed by transformation from a protoenstatite (PEN) precursor, which may be a product of direct condensation or reheating in the solar nebula. The crystallographic orientation relationship between enstatite and diopside suggests that epitaxial growth of enstatite occurred, lowering the activation energy for nucleation and facilitating direction condensation of enstatite from the gas phase, rather than by reaction of the gas with forsteritic olivine. The microstructures of enstatite are indicative of an extremely rapid cooling rate (∼10⁴ K/h) that is within the range of chondrule cooling rates. Such a rapid cooling rate may imply that the cooling rates of FGIs are indeed much higher than other types of refractory inclusions. Alternatively, the rapid cooling rate may not reflect the primary cooling of the FGIs, but is the result of rapid cooling after a short-lived secondary reheating event in the solar nebula.

A fractionated gas with a lower Mg/Si ratio than the solar value is required to condense enstatite. Such a gas could be produced by isolation of pre-condensed forsterite or repeated evaporation-recondensation processes. The presence of both enstatite-bearing and enstatite-free CAIs in CV3 chondrites suggests that at least two gaseous reservoirs with different Mg/Si ratios were present in the CAI-forming regions.

顽辉石是球状陨石，行星际尘埃颗粒和彗星样品中的普遍相。在平衡缩合模型中，预计顽辉石会通过预缩合的镁橄榄石与气态SiO之间的反应而凝结。但是，先前的研究表明，球粒陨石基质和AOA中的某些顽辉石与镁橄榄石没有遗传关系，这是由于预计的镁橄榄石-气体反应会阻止其形成。在这里，我们报告了Efremovka CV3球粒陨石中独特的，细晶粒的，尖晶石丰富的内含物（FGI）Ef1014-01中发生的顽辉石。此FGI中的顽辉石以尖晶石-钙长石-透辉石小球的外层存在，并将FGI与类阿片橄榄石聚集体（AOA）物质分开。顽辉石显示CaO和Al ₂ O ₃升高含量（不超过几个重量百分比）。从该FGI中提取了四个FIB切片，以使用TEM技术研究顽辉石的微观结构及其与其他相的关系。TEM观察表明，顽辉石主要为低温斜铁辉石（LCLEN），表现出丰富的孪晶，有时还与薄正钙钛矿（OREN）薄层相关。斜辉长石晶粒通常与相邻的透辉石具有晶体取向关系，但在FGI周围的AOA类材料中不表现出与镁橄榄石的替代关系。

根据SEM和TEM观察，我们认为Ef1014-01中的LCLEN-OREN共生物是由原钙钛矿（PEN）前体转化形成的，这可能是太阳星云中直接凝结或再加热的产物。顽辉石与透辉石之间的晶体取向关系表明，发生了顽辉石的外延生长，降低了成核的活化能并促进了顽辉石从气相的方向凝结，而不是通过气体与镁橄榄石的反应来实现。顽辉石的微观结构表明冷却速度极快（〜10 ⁴K / h）处于软骨冷却速率范围内。如此快的冷却速度可能意味着FGI的冷却速度确实比其他类型的耐火夹杂物高得多。可替代地，快速冷却速率可能不能反映FGI的主要冷却，而是太阳星云中短暂的二次再加热事件之后快速冷却的结果。

Mg / Si比低于太阳值的分馏气体需要凝结顽辉石。可以通过分离预凝结的镁橄榄石或重复进行蒸发-再凝结过程来产生这种气体。CV3球粒陨石中同时存在含顽辉石的CAI和不含顽石的CAI，这表明在CAI形成区域中至少存在两个具有不同Mg / Si比的气态储层。