CR Scientific: Helpful Articles: Mineralogy Experiments: Barium and Barite

CAUTION: Working with fire and/or molten salts can be dangerous and should be attempted only by competent persons. If you choose to attempt any of the experiments or procedures described on this site, you do so solely at your own risk.

Testing for Barium & Barite
An Experiment in Mineral Chemistry

by C. Thorsten

Introduction

Let us assume you're given a mineral sample that came from a nearby quarry. The piece was labeled "barite", but your instructor has asked you to prove if the identification is correct or not.

Suppose the sample has no distinct crystals and doesn't lend itself well to hardness testing. Assume the only test equipment available is that listed below (i.e., you do not have a specific gravity balance handy, and you don't happen to have an electron microscopy / EDS setup in your classroom.)

Barite or "heavy spar" has some distinct chemical properties that aid in its identification. Some of these properties are easy to detect using lab methods from the 19th and early 20th Centuries.

Materials
carbon block, safety goggles, heavy work gloves, potassium carbonate, potassium nitrate¹, potassium bisulfate, propane torch (or alcohol lamp and blowpipe), crucible tongs, test tubes, forceps, spot plate, distilled water, platinum wire, drill (approx. 1/4 to 3/8 inch diameter), droppers.
Optional: stereo inspection microscope, long-wave (LW) ultraviolet lamp ("blacklight"), ammonium sulfate (or potassium sulfate or dilute sulfuric acid).

Procedure

1.) Carbon blocks are provided with no holes or depressions. While they can be used just as they are, you're less likely to lose a micro-sample if you first drill a very shallow (1/8") depression toward one end of the block. This will hold the assay mixture from being blown away by the torch flame.

2.) Find a place on your "unknown" specimen where it won't be conspicuous to remove a tiny piece-- unless it's just a "test specimen" and you aren't concerned with its aesthetic qualities.

With the forceps (and the stereo inspection microscope if necessary), remove a couple of tiny fragments of the mineral you're going to test. If the specimen has any other minerals on it, make sure you don't get them along with the sample fragment. Magnification and a steady hand with the forceps will help you accomplish this.

3.) With the forceps, pick up a single, tiny shard of this mineral and hold it in the burner flame. Don't use up all of your sample - save some for the later steps.

If the sample is barite, you might get a green flame test at this point. However, it's very possible the sample is contaminated with sodium and other ions, obscuring the barium test. Therefore, a lack of a green coloration is not diagnostic at this early stage.

Furthermore, solid barium sulfate doesn't volatilize all that well for flame tests. A piece of barite that's been left standing in concentrated sulfuric acid is more likely to give a positive flame test, however.

4.) Place the remaining mineral fragments in the depression you've prepared in the charcoal block and set the block on a fireproof lab bench or outside on a dry, gravel or hard-pack dirt surface. Heat the sample with the blowpipe flame. If using a propane torch, take great care not to tilt the business end of the propane canister below the horizontal, or an inferno could result. It is best if the far end of the carbon block is elevated by some fireproof support so that the canister doesn't need to be tilted as much to heat the assay (see diagram, below).

Heat the small fragment for at least 30 seconds to 1 minute. Observe whether the fragment is melting or not (Barite melts at 1580 degrees Celsius; a propane torch flame is between about 1600 and 1900 °C, but that's the adiabatic temperature...). If so, turn off the flame and see whether any coating developed on the carbon block (2 or 3 cm away from the mineral fragment), and note the color of this coating while it's still hot. If the mineral fragment does not melt by the time the fragment begins to glow, turn off the flame and proceed to step 5. Do Not Touch the carbon block with your hands unless it has cooled for at least half an hour or more.

Because the carbon block acts as a heat sink, one can get maximum melting power if the suspected barite sample is held in forceps in the propane torch flame and the block is not used. Please keep in mind that those forceps will get hot. It is also important not to drop that glowing shard somewhere it could start a fire.

While direct heating of a specimen via forceps in a torch flame seems the obvious choice, it does not allow one to capture any coatings that might have formed if a block had been used. Such coatings can be diagnostic for certain minerals.

Optional: When the fragment cools, check for any fluorescence it may have in long-wave and short-wave ultraviolet light. Compare this with a known barite sample treated in the same manner.

5.) Let the block cool for about 10 minutes, and then cover the mineral sample in the depression with about 4 times its volume of powdered potassium carbonate.

Light the torch and heat in the reducing flame until the sample melts and just begins to glow. The reducing flame is at the tip of the blue cone; do not use "vortex" or turbulent-flow torch tips, as these eliminate the distinction between oxidizing and reducing flame.

Continue heating the bead for about 30 seconds. Then turn off the torch flame and note whether any coating has formed on the carbon block. If so, note the color while it's still hot.

Heating the mineral sample

A. Fireproof work surface.
B. Fireproof block or steel bar to elevate distal end of carbon block
C. Carbon block. Do not elevate too steeply or it could slide off the work surface.
D. Depression or shallow hole in block; put mineral sample here.
E. Propane torch

6.) Let the block cool for at least 20-30 minutes. If for some reason you must move it while it's still hot, use crucible tongs and wear thick gloves. If you had the block held at an angle by a non-flammable support, set it flat again when it's cool so the drop-tests won't roll off. It is best to use the tongs in case the block should still be hot.

A.) If possible², use a razor blade or the forceps to scrape off the solidified fusion now that the block is cool.

Try dampening a tiny piece of this fusion and leaving it in contact with a silver spoon overnight. If it blackens the silver, then sulfide is present. The original sample therefore contained sulfur, but only if all your reagents were sulfur-free (hence, only the carbonate fusion would work for this, not the bisulfate). If you're not certain, run a "blank" using just the reagents and no mineral sample.

B.) Put the fusion in a test tube and add about 1 mL of distilled water. Heat the test tube cautiously over a burner flame, moving the tube back and forth over the flame to prevent rapid boiling. Use an inert boiling chip if available. When the first bubbles appear, stop heating and set aside to cool.

C.) If a coating did form on the block, note its color when cool. If the coating is white or nearly-white, try a separate test using a "blank" composed of just the flux (no mineral sample). Do you still get a white coating? Why or why not? What might this coating be made of? (Hint: See if it dissolves in a few drops of water. Try testing this with a bit of pH paper. Does it give an alkaline reaction with turmeric paper? How about its flame-test coloration?)

7.) Watch the test tube from step 6B. Note any precipitate that forms. If you see some tiny, white crystals that settle to the bottom, try to determine their shape with a magnifier.

A.) If crystals did form, try to get one of them to stick to your platinum wire loop. Test it in the burner flame and note what color, if any, is imparted to the flame.

B.) Do the white crystals dissolve in an excess of hydrochloric acid? Why or why not? If they do dissolve, what happens when you add ammonium sulfate or dilute sulfuric acid to this solution?

Discussion

While sodium carbonate is usually used for fusions instead of potassium carbonate, the potassium salt has the advantage of not contaminating the flame test with yellow. The delicate mauve hue of the potassium flame does not obscure other colors as obnoxiously as the yellow of sodium.

Carbonate fusions work well in most cases, especially for silicates; roasting an alkali carbonate produces the caustic hydroxide, which attacks silica readily. However, if our unidentified sample happens to something which resists the carbonate flux, a fusion with 2 parts potassium bisulfate and one part potassium nitrate will most likely do the trick.

A sample the author tested, which was thought to be barite, failed to give the greenish flame test during step 3; in fact, it colored the flame yellow with a touch of red (yellow from sodium, red probably from calcium³). Uncertain if these colors were due to microscopic impurities⁴, the author then went through the procedure outlined above. However, he skipped the carbonate fusion and went right for the bisulfate-nitrate flux. The sample, when treated this way, gave white crystals at step 7 and a nearly pure-green flame test at step 7A.

The fusion of the sample and dissolution in water will carry barium, calcium, and sodium sulfates into solution. Of these three, barium sulfate (barite) is only "temporarily" soluble- that is, it went into solution because of the strong, hot sulfuric acid released during the fusion; putting this in water and diluting it caused barite to re-precipitate, leaving the sodium and calcium ions in solution. This re-precipitated barium sulfate then gave a decent flame test.

One might wonder why the original barite sample wasn't simply soaked in hydrochloric acid (to clean it) and then flame-tested. This can certainly be done, but the fusion is carried out in case further analysis is desired. The sample might also have embedded bits of other minerals such as Ca / Na silicates, some of which might not be soluble in hydrochloric acid. The fusion takes care of this.

The nitrate-bisulfate flux is worth commenting on in more detail. It provides an extremely harsh chemical environment when heat is applied to it. This fusion must be done in a very well-ventilated area, preferably outdoors. A face shield or goggles are essential in case the mixture spatters off the block. Molten bisulfate, combined with the water released when the crystals first melt, provides sulfuric acid; heating this with the potassium nitrate yields nitric acid and nitrogen oxides. Some sulfur trioxide undoubtedly forms as well. These make for a corrosive, reactive mixture that will attack some of the most recalcitrant minerals. These HNO3 / NOx / SO3 fumes are poisonous and extremely corrosive, but the procedure uses only tiny amounts of reagent and is, of course, performed in a well-ventilated area.

Mineral tests are best done with some idea of what the sample might be. If you've narrowed your unknown down to only two or three possibilities, it will be much easier to pick a procedure. As always, start with the easiest and least-destructive tests first. Try not to use up your whole mineral sample in one place, just in case you need to do additional tests.

¹ Try to obtain laboratory- or reagent-grade potassium nitrate (likewise for the potassium bisulfate). Technical- and commerce-grade potassium salts nearly always contain enough sodium to contaminate flame tests and obscure the delicate mauve color that potassium imparts. U.S.P. grade material contains enough sodium to give yellow flashes, but there's not usually enough Na to hide the potassium flame coloration entirely. Traditionally, a piece of cobalt glass was used to help visualize the mauve flame of potassium; all other flame colors would be nearly invisible.
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² In some cases the fusion is so difficult to break that it's better to get a sample of it by partially dissolving it in water. This is easily accomplished by putting a drop or two of distilled water on it, letting it sit for a moment, and taking up this liquid in a dropper. Also, the liquid may help loosen the bead from the carbon block. Don't try to force a stubborn fusion from the block by brute force; use water to soften it, collecting any runoff for further analysis.

When cleaning your carbon block, don't bother trying to scrape every bit of the material off the block or out of the hole; just dissolve it. Simply run it under hot water or leave it in a pot of boiling water to dissolve all the fusion. Give the clean block a final rinse in distilled water and then set it aside to dry. If it still has a white crust on it, use fine sandpaper to remove it.
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³ Lithium and strontium will cause a similar crimson to red flame coloration, but calcium is by far the most common in minerals. If you suspect lithium or strontium, further tests are necessary. It also helps to know what elements are abundant where your mineral sample was found. One might expect lithium from something found in lepidolite- and spodumene-rich pegmatites, for example.
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⁴ The yellow and red flame colorations could have come from tiny bits of calcite and miniscule droplets of sweat clinging to the mineral. An amount of sweat so tiny as to be invisible, clinging to the sample after handling with the fingers, will cause a persistent sodium flame.
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