2
$\begingroup$

I have a rock tumbler - you take rubber barrel, put some water, silicon carbide grit and random rocks inside and spin it for weeks to make rocks shiny.

It always goes like this - for the first several days there is danger that gas release would pop barrel open so I have to check on it and release excess gas if necessary.

Washing rocks does not help, so it is not about dirt, and it is always only first few days so I guess it has something to do with weathered surface.

Usually a set of random rocks is tumbled, but I saw the same behavior with only quartz and only chert loads.

BUT after that I see that barrel's rubber bottom is sucked in. It is like now air is absorbed by some new reaction.

So what exactly is going on?

I did a quick search - 'r/RockTumbling' has a FAQ and there is some info on 'rocktumbler com / questions.shtml' but nobody talks about specific reactions.

Improve this question
$\endgroup$
6
  • 3
    $\begingroup$ What rocks$\ce{ }$? $\endgroup$
    – Mithoron
    Commented Oct 9 at 19:56
  • 3
    $\begingroup$ To exapnd on @Mithoron comment: some minerals, such as olivine (nrdc.org/stories/olivine-carbon-eater), absorb CO2 or other gases. Others, such as calcite, might outgas CO2 (osti.gov/biblio/6459505) or water vapor (water of hydration). Plus, there might be temperature changes causing volume changes. $\endgroup$
    – DrMoishe Pippik
    Commented Oct 9 at 21:30
  • $\begingroup$ @Mithoron I personally tried quartz only load, and chert only load - same behavior for both cases. Also sets of random rocks. $\endgroup$
    – Vashu
    Commented Oct 9 at 22:41
  • 1
    $\begingroup$ Is the quartz pure? Note that this information belongs to question rather than comment. $\endgroup$
    – Paul Kolk
    Commented Oct 10 at 0:23
  • 1
    $\begingroup$ @PaulKolk Added this info to question. Well, there are always some impurities in rocks unless some rare specimens are used. But I don't really have a way to measure how many impurities are there or what are they. $\endgroup$
    – Vashu
    Commented Oct 11 at 1:33

1 Answer 1

Reset to default
1
$\begingroup$

Based on comments and my own experiences, the explanation is simple. The water you use is acidic and the rocks consist of calcite (chalk) inclusions inside quartz matrix. There may be olivine in there as well (which is supposedly adsorbing carbon dioxide as it gets powdered). As the quartz is ground away, the acid reacts with calcite to release $\ce{CO2}$ gas. After a few days the acid is fully consumed by that reaction and $\ce{CO2}$ gets consumed by another, slower, adsorption reaction onto powdered mineral surfaces.

OP wrote:

I guess it has something to do with weathered surface.

No. It has nothing to do with weathered surfaces. You could test my explanation by fully replacing the water after the gas release has ended. (make sure this water comes from the same source used previously) Even without replacement of the stones, more gas should be produced over a few days.

Improve this answer
$\endgroup$
2
  • $\begingroup$ I guess that's it. People recommend adding soda to water to counter that. But I got lots of gas after that. Maybe I need to wait longer before closing barrel. Or use less/more soda (I just dump teaspoon on about 400 g of water) $\endgroup$
    – Vashu
    Commented Oct 11 at 1:36
  • $\begingroup$ Of course, add incrementally more soda until it no longer bubbles. Mix while adding it to see when the reaction stops. Then wait before closing the barrel. $\endgroup$
    – Paul Kolk
    Commented Oct 11 at 8:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.