Monday, January 19, 2009

Monday Mineral: Serpentine

Serpentine is the result of hydration and alteration of ultramafic minerals, particularly high Mg olivine. So you can often find it in ophiolites. This sample is from highly broken (tectonized) upper mantle peridotite.
I picked this up 3 1/2 years ago when my sister and her family were visiting. We were drving up route 199 from the Redwoods (I would have never guessed there was a connection to Winnemucca ,Nevada), and commented that there was an interesting mineral spot coming up. My brother in law made it clear that this was something he was interested in, so we stopped.
(crop from the above photo; I suspect the black inclusions are chromite)
The samples in this outcrop are not immediately obvious; you need to poke along the fracture surfaces, brush off sediment that has washed down the cliff face, and pry apart seams to find the nicest pieces- but a little effort pays off nicely.
As with this sample, most of the material here is completely faced with slickensides. I think of serpentine as being somewhat plastic under any kind of shear, and it tends to be concentrated along fractures. This whole outcrop is little but fractures.
A crop from the above photo.
If you haven't played with Google Street View yet, it's fun. And while their coverage is far from complete, there are a number of nice rocky roads that have made it into their coverage. As it happens, this outcrop is one of them. The cliff to poke around in is just beyond that tree in the middle left of the photo. If you're coming from the coast, you'll pass this landmark just before you get to the creek and the road on the other side of the creek. (Photo from Google Earth)
And this is the Google Earth view. Pin coordinates are 41.875767 -123.843614

Now as it happens, I almost did this as my first Monday Mineral last week, but decided to go with the sunstones instead. I say, "as it happens," because serpentine should have been in the news last week. You probably heard all the fuss over methane on Mars.

(This is today's APOD)

That story actually broke shortly after the rovers landed five years ago, and at the time, it was generally reported that the only explanation was either igneous/volcanic activity or biological activity. Either of these would be revolutionary. Mars is too small and too cold to be a likely place for current igneous activity- though I suppose it's not completely beyond possibility. The discovery of life off of Earth, though, would IMHO be the most profound discovery in history- I can't imagine anything that would more dramatically change our view of our place in the cosmos.

So when it was announced a few months later that the process of serpentinization could create substantial amounts of methane, I was a little disappointed. Reassured by the self-correcting nature of science, but disappointed. Note that I am NOT saying serpentinization is creating the methane (nor methane hydrates created by this process), but all else being equal, it seems the simplest explanation- less revolutionary than either magmatism or life on that planet. Phil Plait has a nice piece today on the inabilty of science journalists to actually report science in the context of Martian methane. So the take-away message is not that methane on Mars is unimportant, nor is it that we have demonstrated life doesn't exist there. Rather it's that in this day and age, hype and showmanship- entertainment- take precedence over any sort of rational balance and respect for the facts. And that a little knowledge of a mundane (if quite pretty) mineral might- just might- offer a simpler explanation.

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