All mesosiderites previously reported were subjected to thermal metamorphism and/or partial melting on the parent body. Therefore, their primordial features have been mostly lost. Here, we report detailed petrological and mineralogical features on a mesosiderite, Northwest Africa (NWA) 1878. This meteorite comprises silicate lithology and aggregates of small spheroidal Fe‐Ni metal grains. Silicate lithology typically shows igneous texture without recrystallization features, and mainly consists of low‐Ca pyroxene and plagioclase. Pyroxenes often show normal zoning. Exsolution lamella of augite is rarely noticed and very thin in width, compared with other mesosiderites. A few magnesian olivine grains are encountered without typical corona texture around them. They are not equilibrated with pyroxene on a large scale. Plagioclase shows a wide compositional range. These results show that NWA 1878 hardly experienced thermal metamorphism, distinguished from mesosiderites of subgroups 1–4. Therefore, we propose that this is classified as subgroup 0 mesosiderite. Nevertheless, NWA 1878 was locally subjected to secondary reactions, such as weak reduction of pyroxene and Fe‐Mg diffusion between olivine and pyroxene, on the parent body.

中文翻译：

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最原始的中观菱铁矿1878年西北非洲，亚组0

先前报道的所有中观菱铁矿都在母体上发生了热变质和/或部分熔化。因此，它们的原始特征大部分已丢失。在这里，我们报告了中西部菱铁矿（1878年西北部）的详细岩石学和矿物学特征。这种陨石包括硅酸盐岩性和小的球形Fe-Ni金属晶粒的聚集体。硅酸盐岩性通常表现为火成岩质地，没有重结晶特征，主要由低钙辉石和斜长石组成。辉石经常显示出正常的分区。与其他中观菱铁矿相比，极少见到闪锌矿的析出薄片，其宽度非常薄。遇到一些镁质橄榄石晶粒，周围没有典型的电晕纹理。它们不能与辉石大规模地平衡。斜长石蜡显示宽范围的组成。这些结果表明，NWA 1878几乎没有经历过热变质作用，与亚组1-4的中观菱铁矿不同。因此，我们建议将其分类为0类中铁菱铁矿。尽管如此，NWA 1878仍会在局部发生二次反应，例如在母体上辉石的还原力减弱以及橄榄石和辉石之间的Fe-Mg扩散。