[Ask a Geologist] Of Meteorites and Jars 2

The lovely and cupcake-alicious E. Catherine Tobler had a couple of geology-related questions, which I have simplified because sooper sekrit reasons:

1) Pretend it’s before 1900–how do you test if a piece of jewelry is made from a meteorite?

After trawling around on the internet a bit, the most likely thing I could come up with is just checking if the jewelry is magnetic and then doing a double check with a streak plate… basically, if the piece is attracted to a magnet, that at least indicates it involves naturally magnetic minerals. And then if you scratch it on a ceramic streak plate and it leaves a metallic gray streak, that’d be a pretty good indicator that it’s at least not one of the usual suspects. Magnetite leaves a black streak and hematite leaves a red streak.

The streak test is a pretty old school one for mineral identification, so it would at least indicate that something weird is going on if you have this magnetic thing that’s not leaving an expected streak color. I know there are also chemical tests you can do to see if something contains nickel, which would be a big hint since all metallic meteorites are nickel/iron.

I don’t know precisely when the streak test came into wide use or when they really started cataloging streak color for minerals. But Mohs hardness scale was invented in 1812, and that kind of testing has been in use since basically the Greeks, just not standardized. Streak testing is an outgrowth of hardness testing, since when you rub something on a substance that is harder than it, you leave a streak of powdered mineral behind.

That said, I didn’t quite trust my own answer on this, so I e-mail my planetologist buddy John Dee since he knows space rocks so much better than I do. Here’s what he had to suggest:

Easy-peasy – just cut it in two, etch it with nitric acid and look for the Widmanstätten pattern. The pattern is formed when the iron core of a planetismal slowly cools and creates interlocking crystals. When the planetismal is later broken into pieces, it forms the stoney and iron meteorites.

Magnetism won’t be much use, as the planetismal probably wasn’t large enough to have an intrinsic magnetic field. And the streak pattern won’t be diagnostic because you’ll get the same result for native iron. But only a meteorite will give you the Widmanstätten pattern!

I don’t know if cutting the jewelry in half is actually an option, but even just etching the outside of it should reveal the Widmanstätten pattern. And so long as the jewelry was made without completely melting down the meteorite–if you just heated it enough to get it to bend instead, for example–that would be the best indicator for certain.

Question number two was a little less out of this world:

2) You’re in ancient Egypt. What kind of rock would you use to make a container for a liquid?

I found this awesome site that listed Ancient Egyptian quarries and mines and what each one produced. Which made answering this a lot easier. Basically, the material would need to be workable (well, presumably anything in the above quarries were things the Egyptians knew how to work), would need to be durable, would need to not react with what you put in it, and would need to be non-porous (to prevent seeping or desiccation).

So this eliminated things like schist (flaky), sandstone (potentially porous), and gypsum (too dang soft).

That leaves a lot of good options still:

  • Quartzite would definitely work. A good quartzite will be completely cemented with quartz, so that would take care of the pore space issue. Quartz is also pretty resistant to chemical weathering, so wouldn’t interact with much you’d put in it to the best of my knowledge… it’s a tough mineral. And we know that the Egyptians had access to a quarry with quartzite in it.
  • Travertine and regular limestone (which would potentially be cool looking and fossiliferous) also might work all right since as far as I’ve been able to find, and unless you filled it with acid wouldn’t interact with the liquid. (And as long as you keep them in a dry climate, both of those will last forever and ever.) The big thing again would be to make certain it was non-porous limestone–it just can’t have vugs in it, or a lot of dissolution molds. I think Travertine would likely be good since it’s hydrothermal… though the other thing to keep in mind is that hydrothermal sourced rocks might have some other nasty impurities in them since a lot of metallic and heavy elements tend to get kicked up in hydrothermal systems.
  • Granite or diorite could potentially work too. Since all the crystals are interlocking, it’d make for a water-tight or even air-tight vessel, but it could be a bitch to work and polish up I imagine.
  • The one option I like the best is serpentinite. There’s a quarry for that, and it would make a darn cool looking green or black jar. I also found a reference that said serpentinite was used for small decorative containers, so there you go.

Looking up all that information about quarries was really fun!

2 thoughts on “[Ask a Geologist] Of Meteorites and Jars

  1. Reply Chelsea Conlin Nov 26,2013 17:15

    Wow, the Widmanstätten pattern is beautiful!

    • Reply Rachael Nov 27,2013 13:56

      There is also this thing called spinifex texture you see in rocky meteorites and komatiites that’s super pretty too.

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