Wednesday, January 13, 2010

Amorphous silicate question for astronomers

To a first approximation, Earth is a giant ball of magnesium silicate. This Mg-silicate is crystalline, with a low pressure composition that is a mix between fosterite (Mg2SiO4) and enstatite (MgSiO3). These phases (particularly olivine) are notoriously hard to quench to a glass. I have read* that when astronomers observe interstellar dust clouds, they mostly see amprphous magnesium silicates, not crystalline ones.

Given that interstellar space is cold (compared to terrestrial labs), and poorly shielded from radiation, I was wondering how easily crystalline Mg-silicates can be amorphized by radiation damage. Trouble is, most papers I find are done at room temerature, where self-annealing may occur. And they mostly focus on spectral reddening of Fe-bearing olivine, which is a different issue.

Does anyone know if a cryogenic amorphization experiment has been done? Anyone? Bueller?

* but not been able to chase the reference paper-trail to the primary source.

6 comments:

Anonymous said...

Naive question: presumably any single crystal grains in space dust will be far too small to resolve individually and also very numerous and randomly oriented. Wouldn't that look amorphous even if the grains are crystalline? Or do they somehow do powder diffraction??

Chuck said...

Different IR adsorption, I think.

Chris Phoenix said...

Adsorption of photons? Do you mean absorption, or is this a spectroscopy distinction?

Chuck said...

absorption, of course. Brain is not working correctly at present.

Carolyn vdB said...

You might have a look at some comet literature (and annealing of "amorphous smokes"), since comets are thought to be composed of ISM. The going ideas are that the amorphous silicates are annealed by radiation, flash heating, or by close proximity to a star (see for example, Wooden, 2002, Earth, Moon and Planets 89, 247-287). (There's lots of info on annealing of silicates in Section 4.)

Carolyn vdB said...

I should add that the midplane of an early solar nebula isn't as cold as you might think! ~1000K in the inner region of the early solar nebula (for ~300,000 years). (See section 4.1.2 of the Wooden reference.)