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Techniques for CollectorsNot All That Fizzes in Acid is Calcite

21st Apr 2012 18:20 UTCDana Morong

It has often been said (or implied), in various posts on the internet, that if a mineral effervesces (fizzes) in acid, then it is calcite. This statement is incomplete. Whereas it is true that calcite (unless it be too impure) does show the reaction in many acids, it is also true that several other carbonates also show this reaction – so not all that fizzes in acid is calcite. Other carbonates may also effervesce in varying degree, and there is even one mineral that will do it to the same degree, in the same acid and conditions, as calcite – another form of calcium carbonate: aragonite.

The evolution of gases from minerals in acids often indicates a carbonate, but a few other minerals may evolve a gas. The difference is that carbonates seem to be the only minerals which yield an odorless, colorless, gas – in this case carbon dioxide (CO₂). The others, including some sulfides, may yield a gas in acid, but such gases are usually odoriferous gases, and it usually takes only a bit to yield a lot of odor. So it is usually safe to assume that the effervescence of an odorless gas, in an acid from a mineral, may be from a carbonate (although Bolton admitted that some manganese oxides can release CO₂ from citric acid; this must be analogous to the release of chlorine from HCl by those same oxides).

Not all carbonates act the same in acids. Yedlin, in 1963, commented that “many collectors, in testing minerals, are not completely aware of what constitutes a proper reaction.” He told of a letter from Marge Sinkankas that mentioned one collector who thought a specimen “wasn’t smithsonite because it didn’t effervesce in acid. Actually it did, but not like calcite, malachite or aragonite, which respond like a TV alkaseltzer ad. Smithsonite, siderite and many others work slowly. Crush a bit to a powder. Then try them. Warm the acid. There’s bubbling. You can see it easily thru the ‘scope.” (note if you do examine such through a microscope, it is best to do so through a glass – a tiny test tube, or under a raised glass slide over the reaction – to keep the reaction from splattering miniscule drops of acid onto the objectives of the microscope. It may be easier to view a reaction with a hand lens – just make sure there is something between the reaction and your eyes – safety goggles if not already the glass of a test tube. And never enclose a reaction completely – you don’t want pressure to build up!).

Not all carbonates may act the same in the same acid, or even in the same strength of acid. The same carbonate may act differently in different types of acid, and in different strengths of acids; usually the difference is a matter of degree. To get a good reaction, some may require being powdered, others may require heating of the solution; others don’t require it but do get it better in such conditions. One carbonate, cerussite, does not yield a good reaction even in hydrochloric (HCl) or muriatic acid; it usually requires nitric acid to get that (if one suspects cerussite and has extra material, a different type of test, one to detect lead, may be more useful). Some carbonates, particularly the hydrated ones (such as found in very dry climates, many being easily water-soluble) yield a fast and quick reaction in almost any type of acid (just try some baking soda, the synthetic analog of nahcolite, with some vinegar, and see what happens!). For the most common carbonates that one is most likely to find in most areas, one can go by the qualitative testing guidelines as in the “Tests” entries in the carbonates of Pough’s Field Guide to Rocks and Minerals. The article by William S. Cordua (see in References) is also very useful.

Vinegar is a weaker acid and will have weaker responses, but can be useful. Vinegar is usually 5% acetic acid, and can be used as it is. Some say that calcite needs to be powdered before it will fizz in vinegar. However, I have found it active in vinegar. I think the difference may be one of degree. Certainly small chunks of calcite are not as active in vinegar as those samples of powdered calcite, and even then not as vigorous as in hydrochloric acid, but if one uses a hand lens to watch the reaction, one can see bubbles forming, although the action is slower than it would be in other conditions. Cold vinegar can be useful to distinguish between calcite and dolomite. T. S. Hunt discussed this in 1859, and Hillebrand & Lundell (1929) mentioned this. The best way to learn this is to actually try small known samples of each in equally-sized tiny chunks in the bottom of a test tube, in cold vinegar, and watch the reactions with a hand lens, taking some time to repeat the experiment.

There is one generality, however, which may be applied to calcite: If a small chunk with distinct rhombohedral cleavage, gives off bubbles of an odorless gas, in cold vinegar (one may use a hand lens to view it), then one can be reasonably certain that this, at least, may be calcite. It may take a few minutes to get the bubbles going, and the bubbles will be small, but it works.

One can use other acids, such as citric acid, with minerals; this was discussed by H. C. Bolton many years ago. This seems a possible area of research especially as citric acid can be obtained fairly easily, being a common ingredient in certain types of food preservatives.

To get equipment to test minerals in acids, small dropper bottles (with droppers built into the lids) can be bought in several places in cities (some ‘health-food’ stores, and possibly some drug stores, carry them); be sure to get ones that have wide enough bases so as not to tip over easily; one-ounce or two-ounce sizes are usually better. Label all containers as to what they contain, and what strength acid they are, and store them out of reach of children. Distilled water is useful for diluting the stronger acids, as well as for the final rinse in cleaning (if your water is ‘hard’ or contains traces of minerals); distilled water can be bought at the grocery store.

Test tubes have to be specially ordered; these should preferably be of Pyrex or of another heat-resistant borosilicate glass. Half a dozen should suffice (in case of breakage) as one is unlikely to use more than three at one time, but one may have to order a dozen. A test tube holder and stand are also useful, although one can make his own holder, and stand (space to hold at least three tubes is useful). One should also get and use splash-resistant chemical safety goggles. Communication with others can yield sources of equipment; sometimes even materials.

It is useful and instructive to make parallel tests: an unknown and one or two known samples tested under the same conditions. It is also helpful to write down and keep records, and read and follow all the safety rules such as those mentioned in the excellent article by William S. Cordua.

(Calcite and aragonite, being chemically similar, act similarly in acids. Calcite-aragonite tests are usually based on the differing rates that their different structures absorb colorants. I have not commented on the calcite-aragonite tests as that subject has been well and excellently covered – between the composing and the posting of this one – in a recent thread of postings in the Analytical Techniques section of Message Board on www.mindat.org ).

References:

- Bolton, H. Carrington (1881) Action of Organic Acids on Minerals. Mineralogical Magazine, #19, January 1881, in volume 4

- Cordua, William S. (2000) Doing Acid Tests on Rocks and Minerals. Rocks and Minerals , volume 75, #4 (July-August), p. 276-277.

- CR-Scientific (www.crscientific.com).

- Hillebrand, W.F. & Lundell, G.E.F. (1929) Applied Inorganic Analysis with Special Reference to the Analysis of Metals, Minerals, and Rocks. (in part “3: Behavior toward Different Reagents as a Means of Distinguishing Different Carbonates” on pages 824-825).

- Hunt, T. Sterry (1859) On some Reactions of the Salts of Lime and Magnesia, and on the Formation of Gypsums and Magnesian Rocks. American Journal of Science, v.28, 170-187 (especially pages 180-181 dealing with the differences in solubility in acetic acid). http://books.google.com >

- Knauff, Francis H. (1894) Distinction of Common Minerals of the Carbonate Group. The Mineral Collector, volume 1, #1 (March), pages 6-8
- Pough, Frederick H. (1953,1955,1960,1976,1996) A Field Guide to Rocks and Minerals

- Yedlin, Neal (1963) The Micro-Mounter. Rocks and Minerals, volume 38 (May-June 1963 issue), 267-269.

22nd Apr 2012 04:35 UTCRock Currier Expert

Dana,

Why don't you make that into a Mindat article? Its pretty good.

22nd Apr 2012 15:39 UTCOwen Lewis

Yes. Thanks; its a useful reference. If you do turn it into an article, you might add a a short paragraph on the moderating effect on an observed reaction according to the ambient temperature and pressure. E.g. For a given specimen, acid and degree of acid dilution in water, expect a faster (= more vigourous) reaction in a midday Summer sun in Arizona that with the same test done standing in Alaskan snow and at the same altitude. Hence, if possible, it's best to do all testing at close to NTP (normal temperature and pressure) and to be aware of the differences in reaction caused if one can't do this.

There are some problems with defining NTP (see http://en.wikipedia.org/wiki/Normal_temperature_and_pressure ) 'Definitions'. But for present purposes an approximation to atmospheric pressure near sea level and 68 deg F (20 deg C) is quite good enough for gauging the occurrence and comparative vigourousness of effervescence. Tease with extra heat (not pressure!) to stimulate an otherwise insignificant reaction. Atmospheric pressure becomes significant for present purposes only if tests are conducted in high mountainous areas. I come from a country the highest peak of which is less than 5,000 ft ASL ;-)

22nd Apr 2012 19:57 UTCRon Layton

Excellent article! For those on a budget, here's a site I buy most of my lab gear from at very reasonable prices:American Science & Surplus For example, they sell 8 borosilicate 13 x 100 MM test tubes for $3.50. Not a bad price considering and they have a lot of other useful stuff for the lab. I have no affiliation with these folks, I'm only passing on a resource.

22nd Apr 2012 23:11 UTCHenry Barwood

One suggestion for testing for release of gas: Powder some of the mineral, place it on a glass slide and put a cover slip on it. Touch a drop of acid to the edge of the cover slip and it will wick onto the powdered mineral. Any bubbles evolved will be trapped under the slip and easily observed. This will also protect your objective from F released from carbonate-fluorapatite, With a bit of experimentation you can even sort out minerals by reaction rates to different acids.

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