If yes, why are the current clastic sedimentary rocks not dominated by red beds?
Hello,
you have a very complicated question and the answer will not be clear, because the mineralogical nature of the coloring component is not sufficiently known. First of all, it should be noted that the conversion of goethite to hematite is not an easy transformation, but a total reconstruction, including dissolution, dehydration and crystallization. The crystal structures of both are incompatible. Ferrihydrite is considered to be a precursor of hematite. It is not always clear that the coloring component is really hematite. They may also be hematite-phyllosilicate complexes, such as macaulayite, perhaps. This is usually very difficult to prove, because if hematite is present, it is nanocrystalline, invisible even in SEM (mainly in oceanic red beds, in continental red beds hematite is sometimes present in (sub) micron particles). The XRD hematite contents in the red claystones, if at all detectable, are well below 1% wt.
Dalibor Matýsek Thank you for your answer. The red color in the red layer is caused by hematite, which is usually nano- or sub-micron. Many researchers working on the red beds agree with this view. Goethite is stable in the surface environment, but not necessarily deep underground. Some researchers believe that goethite may be dehydrated to hematite when subjected to higher temperature and pressure during diagenesis. and some researchers have observed the close connection between hematite and goethite under TEM. That's why I have this confusion. in addition, I'm more interested in what you said about macaulayite as a possible coloring component in red beds. could you send me a link to the relevant research content?
Dr. Wang,
I think the question whether hematite has been derived from dehydration of goethite sounds a bit like putting the cart before the horse because goethite is by no means the parent material of trivalent iron but biotite (dark mica) or Fe-bearing chlorite and mafic silicates such as olivine, pyroxene and amphibole arranged in order of decreasing susceptibility to chemical weathering. In S-enriched environment it is definitely not goethite by lepidocrocite. The transformation is first and foremost an intracrystalline processes corroborated by the common outward appearance of so-called „Rote Glaskopf “(red fibrous iron ore, kidney ore, wood hematite) and its brown analogue “Brauner Glaskopf” (brown fibrous iron ore…….,…). Moreover, in soil and laterite another isotype of Fe2O3 play a more widespread role; it is martite. The process takes place within the supergene alteration zone and as such does not belong to diagenesis. The situation is similar to that of anhydrite, bassanite and gypsum as far as the hydration is concerned and strongly dependent on the climatic conditions. In mid-latitude temperate/ humid zones hematite does not play a significant role but goethite. Red beds do not come into existence in that climate zones but in tropical wet and dry, semiarid to arid climate zones where the Fe2O3 modifications are the sort of oxides of choice.
HGD
Harald G. Dill thank you. There is no doubt that the iron in the red beds originally originated from iron-bearing silicate minerals or heavy minerals, but I am interested in the evolution of iron after it has undergone surface weathering and was buried. In fact, the vast majority of red beds are not weathered from laterite, and they are not necessarily all red when they are deposited.
there is a paper about the pseudomorphous transformation of goethite into hematite was observed as the temperature increased from 47 °C to > 105 °C with increasing burial depth (550 m → 2500 m) .
https://www.cambridge.org/core/journals/clay-minerals/article/abs/pseudomorphous-transformation-of-goethite-needles-into-hematite-in-sediments-of-the-triassic-skagerrak-formation-denmark/45F8EAC05F8C9D600C00AEB3F091CF1A#access-block
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