Thursday, June 5, 2008

Rocks don't suffer deformation...

...they enjoy it.

Don't believe me? Then ask this muscovite, staurolite, and sillimanite, which have had the pleasure of growing in the space where an andalusite crystal has pulled apart:

Image with labels:

And a little explanation. Andalusite is one variety of aluminum silicate (Al2SiO5). There are three minerals with the same chemical composition: andalusite, sillimanite, and kyanite. They're stable at different temperature and pressures - andalusite is stable where it's hot at shallow depths (and is typically found in sediments that have been baked by the intrusion of magma); kyanite is found at higher pressures (that is, deeper in the crust); sillimanite is found at the highest temperatures (including in rocks that nearly melted). This rock tells a story: it was heated at shallow depths, and then was stretch while it was heated (and, I think, buried a little more).

(Elli, this is from Gallup Mills again. I'm working on probe data. The garnets have really cool zoning.)


Silver Fox said...

Ha! I think that's a neat way to look at it. At first I thought you might be telling us not to use "suffer" when talking about rocks, and so I was thinking, "the poor, suffering rocks." - It was fun (and a pleasure!) to read your little twist.

Elli said...

Random thought: don't the staurolite crystals in the muscovite look like two eyes, a nose and a mouth???

I'll look forward to seeing the garnet zoning patterns :)

Kim said...

Elli - yes, that's exactly what made me want to post the image. :D It looks like a smile... or at least a grimace.