If we had access to a totally new kind of rock (maybe a new meteorite), is it possible to tell what the rock is composed of without touching it / drilling into it?

I'm thinking of something like firing sound waves or some kind of electromagnetic radiation at it and seeing what emerges from the other side.

Are there any techniques like this in geology?

  • $\begingroup$ Can you take it to a lab, or do you have to do the tests at the place where it is found? $\endgroup$
    – Gimelist
    May 12 '20 at 23:17
  • $\begingroup$ You can take it wherever you like I guess. Just wondered if there was a way to see what it's made of without cutting into it. $\endgroup$ May 12 '20 at 23:34

You are looking for a portable XRF.

enter image description here

It does (almost) exactly what you said. It sends electromagnetic radiation to the rock (X-rays), which excites electrons in the atoms and when they bounce back they return X-ray of different energies which are detected by the instrument. It looks like this, for example:

enter image description here

Using the different peaks you can figure out what the rock is made of.

This is not as good as lab-based XRF, but it's so easy to use it's now standard practice for exploration and mining geologists to use it.

Note that this will only give you surface information. It analyses what it can see. If you want to know what is inside the rock, you have to drill or break it.

If you want to know what is inside the rock without breaking it, you can use a CT scanner, but it only gives information on density. Inferring what the material is based on its density can be a challenging task and it is made easier if you have pre-existing information on what it could be. There are no portable CT scanners however (at least not hand-held). They can me mounted on trucks though.

There are other methods, such as infrared, Raman, LIBS, etc etc.

The Mars Curiosity Rover has quite a lot of those instruments on board. It is essentially a semi-automated geologist exploring another planet.


To piggyback on Gimelist's answer:

Another important tool X-ray diffraction. XRD is used to determine crystalline structures. Specifically, XRD sees the dimensions of different crystals (and most "rocks" are crystalline). When used with the information that XRF or other elemental analyses produce, skilled operators can identify what specific materials are present in the rock by comparing the XRD diffratograms to those of lab-grown pure crystals.

Some applications of these analyses include identifying rock origins. XRF will tell you how much iron the rock has. But XRD will tell you if it's got Olivine or Pyrite in it; the implication being that Olivine implies volcanic origin and Pyrite suggests biological origin (and maybe there's coal or oil in the area).

  • $\begingroup$ Nice addition! The chemical composition is one thing, but knowing which mineral phases are present can be precious: if your chemical analysis says that there is pure carbon in the rock, you might wanna know if it's graphite or diamond! :) $\endgroup$ May 13 '20 at 15:48
  • $\begingroup$ Yes, although unfortunately, unlike XRF, there are no field-deployed XRD machines that I am aware of. The small modern one I used in grad school was ~4x4x6 ft. $\endgroup$
    – CPO
    May 13 '20 at 19:20
  • 1
    $\begingroup$ XRD is a destructive method. You need a bit of powder sample from your rock. It's also possible to do XRD on a surface, but these are new methods even in a lab-based settings, and the data quality is greatly inferior to what you get from powder, so for practical reasons I would say it doesn't give you too much information. $\endgroup$
    – Gimelist
    May 13 '20 at 23:09
  • $\begingroup$ That's a good point. I came to XRD through clay mineralogy in soil, and only turned towards rocks in academic contexts. For confirming we had similar minerals (a basic rock->soil mineral comparison), the non-destructive method was fine. In any case, I realize I did read the OP's point wrong: they don't want to even touch it, and at a minimum for non-destructive XRD you'd still have to transport it to the lab. $\endgroup$
    – CPO
    May 14 '20 at 11:55

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