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"Reason for why some marble or chalk does not bubble (fizz) when HCl is applied?"



October 2, 2015

Q. I am a Geologist and I teach at a university in Toronto. When we did our rock ID labs the last couple of weeks I was a bit amazed because multiple of our chalk samples as well a marble samples do not fizz with HCl. The samples for each rock type came form the same sources.
Is it possible that it is the same as with dolomite, that they won't fizz due to bearing some Magnesium as well? We tried to make a powder in one small section but even that did not fizz.
I am puzzled, as were my students and the lab tech.
Thanks for your help in advance
Heidi

Heidi Daxberger
- Toronto, Canada
^


October 6, 2015

A. Only two things I can think of:

1) Blackboard chalk is calcium sulfate, not carbonate. It won't fizz.

2) There's a kind of "marble" that isn't metamorphized limestone at all, but is some form of serpentine. That's a silicate mineral and it won't fizz either.

Real limestone, calcite, marble, etc. are all CaCO3 and ought to release CO2 when exposed to HCl. Is the acid strong enough? Try 10%.

dave wichern
Dave Wichern
Consultant - The Bronx, New York
^


October 27, 2015

Q. Sorry for the delay.

We had some marble as well as some chalk samples in our collection (which we use for the rock ID classes), and these are supposedly form the same source as the others (and the look the same) and they do not fizz.

The acid in our lab is 10% HCl, so it should be fine.

I will try a bit stronger acid next time. And I also will try it with a powdered version of each.

We had some marble on a field trip this weekend, that did only fizz in powdered state. Maybe these samples are more dolomite rich and the Mg interfered with the reaction? Would that make any sense?

Thanks for your comment Dave! Any help with this is much appreciated.

Heidi

Heidi Daxberger [returning]
- Toronto, Canada
^

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