在自然界中,铁可以是金属($ \ ce {Fe ^ 0} $),黑色($ \ ce {Fe ^ 2 +} $)或铁($ \ ce {Fe ^ 3 +} $)。在赤铁矿,所有的铁是铁:$ \ ce {Fe ^ 3 + _2O3} $。在磁铁矿,两者的结合铁和亚铁:美元\ ce {Fe ^ 2 +铁^ 3 + _2O4} $。因此,无论是磁铁矿或赤铁矿稳定主要是由铁的氧化态。让我们来看一个eh ph图:[!(eh pH稳定性图铁氧化物和氢氧化物(雅伯et al ., 1989)][1]][1]美元$ x轴是pH -系统的酸度。低pH值意味着酸性、高pH值的基本手段。y轴是一个衡量美元伏可能不是直观的,但假设更高的价值意味着有更多的氧气和较低的值意味着有更少的氧气。正如你所看到的,图的左下角区域是由可溶性亚铁(你在那里看到的所有2 +)而右上方是由三价铁。您可能会注意到,这里没有赤铁矿和磁铁矿只存在在右下角,但那是因为这张图是对水饱和环境条件。 The important thing is where you get the ferric and ferrous iron. Igneous, metamorphic and some sedimentary rocks have an oxygen fugacity (another measure of how much oxygen is around) that results in most of the iron being ferrous, and some being ferric. This is why in these rocks you end up having ferrous iron bearing minerals (olivine, amphiboles, pyroxenes, biotite) and some ferrous+ferric iron bearing minerals (such magnetite). Once you expose those minerals to the surface, where there's loads of oxygen around from the atmosphere, you oxidise the ferrous to ferric iron, and you get hematite instead. Basically, the rocks begin to rust. Now, there are some processes where buried rocks can get highly oxidised, thus precipitating hematite, but that's not too common. It does happen though, and when it does it usually has something to do with groundwater (which are rather oxidised). Notice that according to the diagram, sometimes you don't need the system to be that much oxidised if it's basic enough (in terms of pH). There's also this open access paper that talks about conversion of magnetite to hematite: Jing Zhao, Joël Brugger, Allan Pring, **Mechanism and kinetics of hydrothermal replacement of magnetite by hematite**, Geoscience Frontiers, Volume 10, Issue 1, 2019, Pages 29-41, https://doi.org/10.1016/j.gsf.2018.05.015. [1]: http://i.stack.imgur.com/Evazz.gif