Because I’m getting close to finishing, my ability to find things to procrastinate about is increasing…
Today there were painters in one of the admin offices, and they had moved a bunch of stuff out of the office. Including this (slice of) rock.
Now, I looked at this a bunch of times during the day (it’s between my office and the coffee machine), but I never really thought about it – it’s a rock, and I’m an atmosphere guy. But near the end of the day, one of the geologists pointed out this feature:
It kind of looks like a growth within the crystal. For a few moments we were perplexed – a growth like that couldn’t really happen, could it? Then we realized what was actually going on – take a close look at the layers of precipitated crystal.
———————-^^^—— Here is the intersection between the crystals precipitating off the walls and the crystals precipitating off the intrusion (unless the font messes up the position of the carats 😉 )
You can see how the precipitated layers around the “growth” are mirror images of the layers growing out from the walls of the cavity. What must have happened is that there was an intrusion of rock from the inner wall into the cavity that ended just outside the plane of this cut. Then, when the layers of crystal formed, they also formed around the intrusion, creating this interesting feature.
Here’s the plane from the back: