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Xenon Oxide, XeO2, a mineral?

Posted by Jeff Weissman  
Jeff Weissman March 04, 2011 08:05PM
According to a recent article in J. Am. Chem. Soc., "Synthesis of the Missing Oxide of Xenon, XeO2, and Its Implications for Earth’s Missing Xenon", DOI: 10.1021/ja110618g (Feb 22, 2011), XeO2 can occur in high pressure, high temperature enviroments in SiO2, could there be enough of a concentration to have discrete grains of XeO2?

abstract: The missing Xe(IV) oxide, XeO2, has been synthesized at 0 °C by hydrolysis of XeF4 in water and 2.00 M H2SO4(aq). Raman spectroscopy and 16/18O isotopic enrichment studies indicate that XeO2 possesses an extended structure in which Xe(IV) is oxygen bridged to four neighboring oxygen atoms to give a local square-planar XeO4 geometry based on an AX4E2 valence shell electron pair repulsion (VSEPR) arrangement. The vibrational spectra of Xe16O2 and Xe18O2 amend prior vibrational assignments of xenon doped SiO2 and are in accordance with prior speculation that xenon depletion from the Earth’s atmosphere may occur by xenon insertion at high temperatures and high pressures into SiO2 in the Earth’s crust.
Albert Mura March 04, 2011 10:02PM
Have they found it occurring naturally yet?
Jeff Weissman March 05, 2011 12:50AM
Seems theoretical to me...
Andrew G. Christy March 05, 2011 02:26AM
Hi, Jeff - thanks for the link to an interesting article; unfortunately, it is not going where you think that it is going!

First, a recommendation re. references, namely, to include authors and article title, which in this case were:

David S. Brock and Gary J. Schrobilgen (2011) "Synthesis of the Missing Oxide of Xenon, XeO2, and Its Implications for Earth’s Missing Xenon."
article is only in press/published online at the moment, so journal volume# and page## will appear down the track.

The main news is that the authors have actually made and unambiguously characterised a new compound, XeO2, for the first time,
They are aware of the "missing Xe" problem, and try to link the two. Various plusses and minuses emerge.

1. Evidently, from the citations in the text, there is a small industry out there of people doing calculations trying to put Xe into SiO2 crystal structures. A more geologically/solid-state oriented paper would take pains to state pressure, temperature, structure state of SiO2 and Xe content. All that I can glean without looking up a lot of refs is that "high P and T" are relevant to the Earth's crust (i.e, neither is actually very high by geological standards), the SiO2 phase being considered is therefore quartz, and that one remarkable high Xe content of 2.2% is quoted.

2. The authors point out the implausibility of Xe8+ replacing Si4+ in SiO2, and plug the idea of Xe4+. I agree with them on bond valence and charge balance grounds, and also agree that Xe4+ will not go on the Si tetrahedral site but will find a square-planar interstitial position instead. This can be charge balanced by a missing Si at time of growth, and does not imply that any specific redox reaction occurs in which Xe gas reduces SiO2 silicon to the element. The latter reaction ever becoming energetically favourable is highly unlikely, and the authors' speculations on it are a little bizarre.

3. OK, so quartz can accommodate a fair amount of Xe in solid solution under high P and T from calculations. Realising this in practice would presumably need forcing conditions of Xe partial pressure and redox (highly oxidising conditions to drive Xe0 up to Xe4+). The deep crust is not normally very oxidising, and even if 90% of the Xe were not missing, it is still rare, so I suspect that the equilibrium content of Xe in deep quartzes is actually much, much lower than 2.2%. Nobel gas geochemists must have some data already.

4. I have not attempted to quantify this, but it is more likely that Xe is distributed at very low concentrations, in a wide range of silicates (not just quartz) through the much greater volume of crust PLUS mantle. Note that quartz itself is not stable through most of this volume.

5. Even if Xe were common, it is vanishly unlikely that a phase as reactive and endothermic as XeO2 would ever occur as a mineral. Much more stable compounds of common elements, like sodium hydroxide or potassium permanganate, do not. But when a rarer element goes readily into solid solution with a commoner element, it reduces the chances still further! Note that gallium and germanium are not massively uncommon elements, but make very few minerals of their own: they are mainly dispersed at low concentrations, replacing Al and Si, and there are few geological situations in which they become separated/concentrated up sufficiently to make their own compounds.
Fred A. Schuster March 05, 2011 03:04AM

It's a Gas!
Jeff Weissman March 06, 2011 03:30PM
Andrew - thank you for the detailed response - I was thinking more along the lines of microcrystal inclusion in beta-quartz or other high temperature minerals. The question is has any one found gas bubbles, in mantle-derived material, with high Xe content? Fun to speculate...
Andrew G. Christy March 16, 2011 08:58PM
David - at low pressures, certainly. At higher pressures, the amount of Xe in interstitial sites in crystal structures would tend to increase. In either case, the Xe remains non-bonded Xe0. Still much less exotic than the scenario considered about in the paper that is referenced, where they speculate about cationic Xe in solid solution. I'd suspect that redox conditions in the deep Earth would never favour that, so the synthetic and modelling work in the area are not relevant to natural environments.
spera April 06, 2011 08:31PM
what needs to be done here straight away is a DFT calculation or possibly FPMD simulations of this. DFT should be able to handle this.
Dr. David Brock April 03, 2012 03:08AM
Hi All,
I was the primary author of the article in question. I happened to come across this discussion and it caught my attention because it is one of the few discussions that look at the article scientifically. To answer Mr. Weissman's question, no, we are not proposing XeO2 as a mineral. It is extremely unstable and decomposes even at -78 oC over a few days. In fact, I must admit, initially I was skeptical that it could be incorporated into a quartz lattice at all as was proposed in a previous article reported in Science and was only convinced of the "plausibility" (I stress that I am not saying it DOES substitute but that it is possible) based on quantum chemical calculations. However, unlike spera suggested, DFT calculations are not suitable for these systems. DFT gives unreliable energies when large atoms like Xe are involved and require ab initio calculations instead. You can look at many of my (and my previous supervisor Dr. Schrobilgen's) work relating to noble-gas systems.
We were lucky enough to avoid having to carry out such calculations because they have already been done. See M. I. J. Probert J. Phys. Condens. Matter 2010, 22, 1. which is referenced in our paper. As stated in our paper, this computational paper, and as Andrew has pointed above, Xe insertion into interstitial spaces is favourable under all conditions and does not alter its oxidation state. However, this computational paper also revealed that under the temperatures and pressures suggested in the Science paper Xe substitution for Si are energetically neutral. This would suggest that Xe is indeed oxidized and Si reduced. However, this is not to say that the silicon itself oxidizes the Xe. What the author suggested is that the process occurs in a two-step process where Si is removed from the lattice leaving behind peroxide linkages followed by Xe insertion. Xe is presumably oxidized by the highly oxidative peroxide linkages. Looking at the thermodynamics of this process, also considered in our paper, it must be that the pressure/volume work (entropy) force overcomes the enthalpy of the reaction. In any case, the square planar local environment puts a strain on the lattice and you cannot incorporate two Xe atoms anywhere near each other in the lattice. So no, you won't find XeO2 mineral anywhere.
While our research is primarily of a fundamental nature and we were very excited and more focused on the novelty of the missing oxide of Xe (XeO3 and XeO4 are known and XeO is shown computationally to be unstable), we found the prospect of Xe incorporation into quartz puzzling. I had to do a lot of background research into the geochemistry, since admittedly it is not my strong suit, but from a chemical standpoint, although i cannot say with 100% confidence they DID substitute Xe for Si in quartz, the evidence they reported is not in contrast with what we observed for pure XeO2.

Anyways, thanks for the interest in the article and keep up with the great science questions and drive.

Dr. Brock
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