# How can compounds be called reduced if nothing is known about the reaction they formed in?

I'm reading a research article about a model trying to explain the elemental compositions of meteorites.[1]

What confuses me, are statements of the following type (here in chapter 2.1, second paragraph about enstatite chondrites, a type of meteor):

[...] Enstatite condrites contain minerals that are remarkably reduced chemically, including abundant sulfides like oldhamite (CaS).

In my understanding reduction or oxidization only refers to a specific element in a specific reaction, but here they talk about a compound with unknown reaction path to form it.

• How can CaS be considered to be (remarkably) reduced, if it is not an element in a reaction?
• How can CaS be considered to be (remarkably) reduced, if we don't know the reaction path that it formed in?

1. Steven J. Desch, Anusha Kalyaan, Conel M. O'D. Alexander. The Effect of Jupiter's Formation on the Distribution of Refractory Elements and Inclusions in Meteorites. arXiv:1710.03809 [astro-ph.EP]
• CaS can be considered reduced compared to Earth conditions (where it is likely to get oxidized). The reaction path that created it is irrelevant. – Ivan Neretin Oct 24 '17 at 10:08
• @Ivan Neretin: You mean because both components, Ca and S would end up in compounds which would give them positive oxidization numbers? Else one would have to refer to either Ca or S as being reduced, but not CaS as molecule itself, no? – AtmosphericPrisonEscape Oct 24 '17 at 12:00
• @AtmosphericPrisonEscape You actually do not need to know the reaction details to consider that a given atom is in a more/less reduced form than another one. "Reduced" is also used to describe a state (i.e. the oxidation state) rather than a process or an action. – SteffX Oct 24 '17 at 13:08

While we don’t know exactly how minerals on meteorites were formed, we can make very educated guesses on the conditions they were formed in. Most notably, outer space only contains very low concentrations of the compound most associated with oxidation on Earth: gaseous oxygen.

But this is not the main reason why the label ‘remarkably reduced’ is accurate here. We will immediately recognise the mineral in question as an ionic compound and given that it is made up of a metal and a nonmetal, we can equally immediately assign oxidation states as shown below.

$$\ce{\overset{+II}{Ca}\overset{-II}{S}}$$

There is no surprise about calcium which generally is found in its $\mathrm{+II}$ oxidation state. The surprise here is that sulphide is the anion which is a rare occurance. On Earth, most mineralic sulphur is oxidised and mainly to sulphate ($\mathrm{+VI}$). This is basically the extreme opposite and caused among others by the high atmospheric oxygen concentration.

Of course, the label ‘remarkably reduced’ can only be applied to some of the elements of the compound. Language-wise, we are looking at the oxidation state and merely stating that a state this low is uncommon.

• The claim about no oxygen in space is highly debatable. Oxygen is one of the most abundant heavy elements floating around in the universe, but I guess it will be outcompeted by the reducing, still way more abundant hydrogen in planet-forming environments. Anyway, so by having this remarkable statement, we're still comparing with Earthly conditions. For chemists that might be ok, but I'm worried that this biases our view on minerals in space. Thoughts? – AtmosphericPrisonEscape Oct 24 '17 at 21:16
• @AtmosphericPrisonEscape First, I never said ‘no oxygen’. Second, just because oxygen atoms are very abundant doesn’t mean $\ce{O2}$ molecules are. Third, I specifically said concentration which — I hope you will agree — is extremely low in space and still very low on meteorites and other small rocky space objects with little gravity. – Jan Oct 25 '17 at 7:05
• On the topic of biased views, that could very well be the case; I don’t know what I think about it though. – Jan Oct 25 '17 at 7:07
• Sure, but the concentration of anything is low in space. The relative abundance still favors hydrogen reactions, just on longer timescales. Still minerals form, and reaction times can be rescaled with the proper physical parameters, so no point arguing about that. Thus the aim of my question was to view the space environment similar to a reducing environment, only on longer timescales for reactions. – AtmosphericPrisonEscape Oct 25 '17 at 11:21