Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H₂ and reacts with SiO₂ to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr₂O₃ in a few R4-6 chondrites. Magnetite grains with high Cr₂O₃ behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr₂O₃ (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.

原始的CO3.00–3.1球粒陨石含有〜2-8％（体积）的磁铁矿，次要的三叶草以及辅助碳化物和铬铁矿。一些CO3.1球粒陨石具有丰富的铁橄榄石脉纹，球粒状边缘和真面体基体晶粒。所有的CO3.00–3.1球粒陨石都几乎不含金属Fe-Ni（0.4–1.2体积％）。CO3.2–3.7球粒陨石包含1-5％（体积）的金属铁镍，次要三叶形，副亚铬铁矿和0-0.6％（体积）的磁铁矿。通过母体水蚀作用，由金属铁在原始CO3球粒陨石中形成磁铁矿，导致金属铁镍含量降低，高镍金属晶粒比例增加。≥3.2亚型CO球粒陨石中缺乏或缺少磁铁矿，表明磁铁矿在热变质过程中被破坏；根据文献进行的热化学计算表明，磁铁矿被H ₂还原并与SiO ₂反应形成铁橄榄石和次生的金刚石。在其他球粒陨石群中也发生了类似的磁铁矿形成和破坏过程：（1）3型本征OC具有不透明的集合体，其中包含磁铁矿，碳化物，富Ni的金属和富Ni的硫化物，但是亚型> 3.4的OC几乎没有或没有磁铁矿。 。（2）原始的R3球粒陨石和碎屑（≲3.5型）含有最多6％（体积）的磁铁矿，但大多数R球粒陨石不含磁铁矿。主要例外是在一些R4-6球粒陨石中具有9-20 wt。％Cr ₂ O ₃的磁铁矿。Cr ₂ O ₃含量高的磁铁矿晶粒的行为类似于铬铁矿，并且在还原条件下更稳定。（3）CK球粒晶平均含铁量约为4％（体积）的磁铁矿₂ O ₃（最高〜15 wt。％）；这些磁铁矿晶粒在变质过程中也具有稳定的抗还原性。（4）CV3球粒陨石中磁铁矿的模态丰度随着变质程度而降低。（5）在那些被分类为CR6，CR7和CV7的稀有样品中，铬铁矿代替了磁铁矿。