Review of Go-To Iron Analysis Method Reveals Its Pros and Cons

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Source: Geochemistry, Geophysics, Geosystems

Iron is the most abundant transition metal in the earth’s crust – present in a wide variety of minerals and in multiple oxidation states, mainly ferrous, or +2, and ferric, or +3 – and its presence in different shapes in rocks can tell living stories. on the old environmental conditions of the planet, such as the past cycle of nutrients, geological activity and oxygen content.

Over the past few decades, scientists have probed the iron content and speciation in rock samples with a laboratory technique that uses different chemicals to sequentially dissolve or extract specific types of iron. First, acetate is used to dissolve iron in carbonates, then hydroxylamine hydrochloride is used for easily reducible oxyhydroxides, then dithionite for ferric iron oxides (oxyhydr) like goethite and finally, oxalate for magnetite.

In a new study, Slotznick et al. report on magnetic experiments and x-ray diffraction measurements of samples dating from 1.5 billion years ago in the Precambrian to the Holocene to verify the extent to which the mineral attribution associated with the sequential extraction process is actually precise. They found that for some steps, especially the one involving dithionite, the technique worked as expected; in other words, the dithionite effectively dissolved the target ferric iron (oxyhydroxy) oxides while leaving the other forms of iron intact. For the other steps, however, particularly the final step in which the oxalate is used to dissolve the magnetite, the researchers found that the process did not work as expected. They suggest that in this last step, the oxalate dissolved the iron bound in the clays rather than just the iron in the magnetite.

The researchers say their data indicates that the extraction technique is more complex than previously assumed. Overall, magnetic and X-ray diffraction analyzes suggested that the dissolution of the iron phases was more gradual than expected, with undissolved portions of minerals from the previous stages persisting and with slow dissolution of iron outside of the phases. planned targets. Part of the complication, scientists say, is that rock samples can be extremely heterogeneous, and variables like composition, grain size, and crystallinity can create differences that affect how iron dissolves.

The team’s analysis of a large compilation of data showed that the Precambrian sedimentary rocks contain more iron which is dissolved by the oxalate (and therefore that they potentially contain more of certain iron clays) than the Phanerozoic sedimentary rocks. The researchers say this observation suggests that a significant change in the iron cycle occurred between these two periods. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2019GC008666, 2020)

—David Shultz, science writer

March 27, 2020: This article has been updated to clarify the types of metals found in the crust.

Quote:

Shultz, D. (2020), Review of the benchmark iron analysis method reveals its advantages and disadvantages, Eos, 101, https://doi.org/10.1029/2020EO141919. Posted on March 27, 2020.

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