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Testing carbonates in weak acids

Posted by George Balogh  
Testing carbonates in weak acids
July 04, 2012 06:00PM
Hello,

Calcite freely produces bubbles when placed in household vinegar. I usually place a tiny fragment of the mineral
to be tested in a small shot glass and look for bubbles on the surface of the sample under low power microscope.

My question is will other carbonates also test positive in household vinegar? Specifically interested in distinguishing
calcite from rhodochrosite.

Thanks,

George Balogh

Portage, MI
Re: Testing carbonates in weak acids
July 04, 2012 06:33PM
In vinegar, aragonite will bubble as much as calcite. Dolomite will bubble but very slowly. I haven't seen any reaction with rhodochrosite or siderite. Siderite was the most resistant of the group, in my limited experience - You need a strong mineral acid, and even then you have to heat it. Even with vinegar, heating makes a big difference to speed of reaction.

(At this point on Mindat, somebody usually steps into the discussion to remind us how dangerous this is and say we're all gonna die, so I'd like to state up front that I use only tiny grains of mineral, in a few drops of acid, and point the test tube away from my eyes while warming over the flame, so death is far more likely to come via my skis, bike or french fries than from my chemistry set.)



Edited 1 time(s). Last edit at 07/04/2012 06:34PM by Alfredo Petrov.
avatar Re: Testing carbonates in weak acids
July 04, 2012 08:02PM
This is very dangerous and you're all going to die!!!!!! Sorry, I couldn't resist.... grinning smiley

I don't ever remember rhodochrosite bubbling in vinegar, but in heated HCL it will bubble vigorously. Same with dolomite, magnesite, and a fresh surface of siderite. If it's what I'm thinking you're trying to ID George (UP specimens?), the rhodo will have a very slight pink tint to it where the calcite is white, in most cases of course.
avatar Re: Testing carbonates in weak acids
July 04, 2012 11:23PM
Yes, Alfredo, vinegar will kill you. No more Italian dressing or balsamic reductions for you.
Re: Testing carbonates in weak acids
July 05, 2012 12:03AM
ca    
If you have a known sample it is a good idea to put a small piece of the known in with the unknown and compare the reaction to decide what you have.
Re: Testing carbonates in weak acids
July 05, 2012 05:29AM
A standard field geologist's test is to try applying dilute acid to a carbonate before and after powdering. Calcite will fizz without the need to powder the sample. Dolomite will fizz but only after the rock sample is powdered, usually by vigourously scratching the surface of the mineral with a tungsten carbide scriber.
There are a whole series of simple mineral tests given in Donald Peck's book, Mineral Identification. You can get this from the Mineralogical Record website. A very useful book for anyone interested in identifying their own specimens. Both amateurs and professionals use it.

Regards,

Ben.
avatar Re: Testing carbonates in weak acids
July 05, 2012 03:51PM
Thank you, Ben.

It is important to remember that the rate of a reaction is related to the size of the particles, the temperature, the strength of the acid (degree of ionization), and the concentration (dilution). So one has to be careful in generalizing about the different species of carbonates and how much or how little they bubble with acid. For careful work, I prefer using a small amount of finely powdered mineral in the well of a depression microscope slide. The acid can be placed above the well on the slide until it is under the microscope and then drawn into the well with a probe. If heating is required, the slide can be heated by passing it through an alcohol lamp flame or setting it on a lamp bulb. I have not tried to quantify the difference in effervescence among mineral species, except for calcite and dolomite. The detection of iron or manganese is extremely easy and can be done on the drop or two of solution remaining in the well.

Another thing, vinegar is easily available, but its concentration is not always the same. In most of the US it is 5%, but in some states it is higher. I think in the UK it may be 20%. All this affects the rate of reaction.
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