My internet connection was on the fritz all day yesterday, so I missed the best timing for posting a mysterious green deskcrop of my own. I'm still going to post it, though, since I'm not even entirely sure what it is. There was plenty of this stuff around at Dana Point Harbor, where my mineralogy class went for a field trip on 1 March. Even the professor wasn't sure what it was, so we collected some of it (and I very nearly spoiled the sample with blood after slicing my thumb on my knife trying to extract some of the green stuff) for analysis. I ran XRD on it, and one of my classmates put it under SEM...and we came up with completely unrelated results. My guess is the problem might have been in the preparation of the thin section for SEM, since this stuff has a hardness of less than two and powders so easily that I'm sure getting a clean even slice would be really difficult. We're going to put the powder we prepared for XRD into SEM after spring break, in hopes of clearing this up. But in the spirit of geopuzzles, I am consulting you guys, too! I'll let you know if your guess is what I came up with in XRD, or what my classmate came up with in SEM, though neither one is confirmed. I don't have any prizes to give, though I guess I could draw a silly picture for someone if they get it, or have a compelling argument for something other than what we tested.
There were a bunch of places where the mysterious soft green stuff appeared to form puddles in larger rocks.
In other places, the mysterious green mineral clearly followed the foliation of the rock; this was further supported when I was drilling out the green for the XRD sample - the drill bit went very deep into a tiny surface spot of material, but was still churning out green.
In terms of environment, Dana Point, in Orange County, is dominated by the San Onofre Breccia, which consists mainly of blueschists, with some greenschists and amphibolites, in a red clay matrix. Many of the rocks there show signs of being kicked back and forth between brittle and ductile parts of the subduction zone. I'll put a bunch more photos in the next post.