Opal on Mars

Milliken et al. 2008 claim to have discovered opal on Mars. They used high resolution satellite-based IR spectra.
IR spectra show absorption that is caused by the vibrational frequency of molecular and crystallographic ionic bonds. In this case, it is the Si-OH, OH, and H₂O bands that were characteristic. Opal is not the only hydrated silica on Mars, but the exact energy of the various bonds is dependent on crystallographic and compositional constraints. Spectra of various silicates were taken, and opal was found to be a better match than sheet silicates or hydrated volcanic glass (e.g. obsidian). The details are in the data repository, to which I do not have access.
Various popular articles have suggested that this is evidence for surface water or hydrothermal alteration on Mars. Neither conclusion is sound. Opal generally forms in low temperature environments, and is easily dehydrated at modest temperatures. Here in Australia, it is generally precipitated by groundwater in areas that have been arid (more evaporation than precipitation) for millions to tens of millions of years. However, it is still significant.
Opal is not thermodynamically stable. As a hydrous amorphous phase, one might expect it to alter over a billion year timescale. This means that either it fails to devitrify for potentially interesting reasons, or it is young. These alternatives are generally testable. This is because opal often has high U contents and high U/Pb ratios, allowing for U/Pb geochronology to be performed (e.g. Amelin & Back 2006).
An aerial or satellite gamma ray survey should be able to identify any potential uranium enrichment, as long as the spacial resolution is as fine as the outcrop extent. If present, a sample return mission to an opalized paleochannel would potentially allow a minimum age to be determined, assuming that the opalization postdates the surface flow and predates the inversion of the terrain through differential erosion.
In Australia, opalized paleochannels can present as false positives in uranium exploration, due to the elevated but uneconomic uranium enrichment. I don’t have a literature reference for this though, as the information was obtained via a seminar series at the School of Hard Knocks.
Tuff cookie discussed this a few weeks ago.
Questions?

Full article:
R.E. Milliken, G.A. Swayze, R.E. Arvidson, J.L. Bishop, R.N. Clark, B.L. Ehlmann, R.O. Green, J.P. Grotzinger, R.V. Morris, S.L. Murchie, J.F. Mustard, C. Weitz (2008). Opaline silica in young deposits on Mars Geology, 36 (11) DOI: 10.1130/G24967A.1

References sited:
Yuri Amelin; Malcolm Back 2006 Opal as a U-Pb geochronometer : Search for a standard. Chemical Geology 2006, v. 232, n 1-2, p. 67-86

R.E. Milliken, G.A. Swayze, R.E. Arvidson, J.L. Bishop, R.N. Clark, B.L. Ehlmann, R.O. Green, J.P. Grotzinger, R.V. Morris, S.L. Murchie, J.F. Mustard, C. Weitz 2008 Opaline silica in young deposits on Mars. Geology, November 2008; v. 36; n. 11; p. 847–850.