Does a “T” in a formula subscript indicate all possible numbers of atoms? $\ce{FeO_T}$ is the specific one I came across (in the "bulk chemistry" table here: https://sciences.ucf.edu/class/simulant_lunarmare), it seems to make sense in context but I've not done chemistry since school (tried googling already) so thought I'd better check.

  • $\begingroup$ Never saw this one and wouldn't believe it even if I did. Little "x" would do, though. $\endgroup$ Aug 23 '19 at 13:36
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    $\begingroup$ geokem.com/glossary.html. See the 10th entry. $\endgroup$ Aug 23 '19 at 14:08
  • $\begingroup$ @IvanNeretin extremely common in geochemistry. For a visible example, see the Earth Wikipedia page, in the chemical composition table. $\endgroup$
    – Gimelist
    Aug 24 '19 at 10:55

Interesting question. It looks like this is a rather common notation in geology and refers to the total iron(II) and iron(III) oxide (e.g. ferrous and ferric) content. For a chemist this is probably somewhat confusing as “total” is normally denoted via subscripted abbreviation “tot”. Flipping through numerous sources, it appears there is no standardized notation and alongside with $\ce{FeO_T},$ $\ce{FeO_t},$ $\ce{FeOt}$ and $\ce{FeO^\mathrm{t}}$ are being used.

An introductory textbook by Shikazono [1, p. 21] explicitly defines $\ce{FeO^\mathrm{t}}$:

$\ce{FeO^\mathrm{t}}$: Total iron $(\ce{FeO} + \ce{Fe2O3})$


  1. Shikazono, N. Introduction to Earth and Planetary System Science: New View of Earth, Planets and Humans; Springer: Tokyo; New York, 2012. ISBN 978-4-431-54058-8.

I will add to the correct answer by andselisk that the reason for this notation is that in most geochemical analytical methods, oxygen is not measured. Instead, all metals are measured and oxygen is calculated by stoichiometry assuming (often very reasonably) that all metals are as oxides.

As the vast majority of major elements in geological materials have only one oxidation state throughout all geologically reasonable conditions, this works out very well.

Iron is an exception: it is a major element that commonly exists both as divalent and trivalent. Because the oxidation state is much harder to analyse than the iron contents, iron is commonly reported as either all 2+ as FeO(t) or all 3+ as Fe2O3(t).

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    $\begingroup$ This seems similar to, for example, how a fertiliser can have its phosphorus content measured in "percent weight as $\ce{P2O5}$" or "percent weight as $\ce{PO_4^{3-}}$. I've always found it confusing and opaque; why perform the completely unnecessary assumption that the element is found as a particular substance, especially in cases which clearly do not reflect reality, such as $\ce{P2O5}$? Such easily avoidable problems... $\endgroup$ Aug 24 '19 at 3:35
  • $\begingroup$ @NicolauSakerNeto that's mostly a matter of opinion and convention. To me, as a geochemist, using $\ce{PO4^3-}$ is misleading, because it adds oxygens that do not "belong" to the phosphorus. It is also not a neutral thermodynamic component, which personally makes me cringe. On the other hand, $\ce{P2O5}$, or $\ce{PO_{2.5}}$, is perfectly fine as it tells you exactly how much phosphorus (and associated oxygen) there is in the compound, without any assumptions. $\endgroup$
    – Gimelist
    Aug 24 '19 at 10:36

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