Getting Inside an Erratic

This is by way of a mystery rock sort o’ thing. I took the rock hammer to a loose chunk of the maclargehuge erratic I found behind the house, got a few fresh surfaces, and (unsurprisingly) am still stumped. Identifying rocks from photos is extremely difficult, granted, but perhaps someone will have insights. We may at least be able to narrow things down.

Ready? Go:

Erratic Fresh Surface I
Erratic Fresh Surface II
Erratic Fresh Surface III
Erratic Fresh Surface IV
Erratic Fresh Surface V

These were all shot through my hand lens. The little sparkly flakes are tiny, and hard to see even under magnification. Some of them are pale and appear to be mica. Some are more gold and look for all the world like specks of pyrite, but I don’t know that they are. The rock itself is quite hard, but it can definitely be scratched with quartz. And, of course, I was able to break it with the rock hammer by giving it a few good smacks, using Lockwood’s techniques. In other words, I grabbed a handful of it and whacked it at an angle like I meant it. If I’d just set it on the ground and attempted to bang it, we would’ve gotten nowhere. Thank you, Lockwood!

It rang lustily, the sound echoing off the drumlin and the neighboring apartment buildings. I don’t know if the sound matters to identification in this case, as you couldn’t hear it, but my neighbors surely did. I’m hoping they’ll troop over one day and ask me why I’m hammering rocks, then photographing them through a lens.

I’m not sure what strength the lens is – George may know, as he’s the one who sent it to me. Along with the rock hammer. Both of which have been enthusiastically employed this season. Thank you, George!

Right, my darlings. There’s your fresh surface. Let me know what you think.

Non-magnified version.
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Getting Inside an Erratic
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5 thoughts on “Getting Inside an Erratic

  1. 2

    If it has mica in it, just rub your finger across the fresh surface and some of the mica should come off on your finger as glitter. If the gabbro diagnosis is correct (a reasonable identification though grain size is pretty fine), it should not have mica.

    Did your fresh surface pieces come from a darker portion or a greener portion of the maclargehuge erratic?

  2. 3

    Ooops, just realized I looked at the first erratic rather than the truly maclargehuge erratic serving as such a noble background for vous.

    Strike the question in my previous post.

  3. 4

    My first impression, not knowing the context or source, was that it is a chunk of peridotite or dunite, which is not that far removed from Callan’s guess of gabbro. A mafic plutonic rock of some sort.

  4. 5

    I’m inclined to agree with the mafic plutonic rock diagnosis, though I couldn’t say which mafic plutonic rock just from the photos.

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