Fe3O4是由离子构成的吗 如果是由分子构成那么为什么也是含有金属元素的化合物呢

Fe3O4的确是离子构成的，但是你看到的说法是完全错误的, 含金属元素的化合物不一定由离子构成
等轴晶系，a0=0.8396nm；Z=8。反尖晶石型结构。即1/2的Fe3 和全部的Fe2 占据八面体位置，另1/2的Fe3 占据四面体位置。晶格常数a0随Al3 、Cr3 、Mg2 替代量的增大而减小；随Ti4 、Mn2 的替代量增高而增大。
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Structure
Fe3O4 has a cubic inverse spinel structure which consists of a cubic close packed array of oxide ions where all of the Fe2+ ions occupy half of the octahedral sites and the Fe3+ are split evenly across the remaining octahedral sites and the tetrahedral sites.

Both FeO and γ-Fe2O3 have a similar cubic close packed array of oxide ions and this accounts for the ready interchangeability between the three compounds on oxidation and reduction as these reactions entail a relatively small change to the overall structure.[2] Fe3O4 samples can be non-stoichiometric.[2]

The ferrimagnetism of Fe3O4 arises because the electron spins of the FeII