CR Scientific: Helpful Articles: Mineralogy Experiments: Fluorides and Fluorine in minerals

WARNING: This procedure may involve generation of the extremely dangerous hydrofluoric acid (HF), though in very small amounts. 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 Fluorine & Fluorides
An Experiment in Mineral Chemistry

by Christian Thorsten

Introduction

Certain minerals, including the difficultly-soluble silicates, can contain fluorine with or without accompanying hydroxyl (OH) groups. While simple fluorides such as fluorite or villiaumite are easily solubilized with acid, the silicates require a different (but still fairly simple) method of attack.

The following is based roughly on a test in Charles Warren's Determinative Mineralogy (1921). It must be performed outside or in a fume hood, since hydrofluoric acid (HF) and other dangerous vapors may be liberated.

Safety
This is a micro-scale experiment and is not intended to be scaled up. The amounts of HF involved are very small and are not expected to be a life-threatening hazard, provided that appropriate safety equipment is used. Please be advised that scaling up this experiment could generate life-threatening amounts of HF.

Materials
Glass tubing or extra-small test tubes (no larger than 10x75 mm);
Safety goggles;
Heavy work gloves;
Small piece of broken glass;
Potassium bisulfate;
Hammer and steel plate¹;
Propane torch (or alcohol lamp and blowpipe);
Forceps;
Clean sheet of paper²;
Mineral sample (a "control" sample would be phlogopite, fluorite, or anything else known to contain fluorine)
.
Optional: stereo inspection microscope.

Procedure
If you are using the small test tubes, skip Step 1. However, please note that this test will require the sacrifice of a test tube (it will most likely be ruined if fluorine is present).

1.) Heat one end of a 2- to 2 1/2-inch length of glass tubing in a flame until the end softens and closes up. Borosilicate tubing is recommended, since crown glass melts too easily. Do not seal the other end. Put the hot tube aside on a fireproof surface and let it cool.

2.) Obtain a small piece of the mineral sample to be tested. Something about 3 x 3 millimeters should suffice. With the hammer and steel plate, carefully crush this into a fine powder. Being careful not to lose any of it, put this powdered mineral on the clean sheet of paper and set aside.

3.) Carefully crush the glass fragment into dust using the hammer and steel plate. Only a small amount is required- something about equal in size to the mineral fragment you crushed. Place the powdered glass on the paper with the mineral powder.

4.) Now crush a crystal or two of potassium bisulfate into powder with the hammer and steel plate. The amount required is no larger than the size of the mineral fragment used. Place the powdered potassium bisulfate on the paper with the mineral and glass powders.

5.) Being careful not to spill any, mix the three powders together completely. Take up a small amount of this mixture in the glass tube whose end you sealed in Step 1. If using a small test tube, a somewhat larger amount of powder will be necessary to get a good test. Either way, tap the tube lightly until the powder settles to the bottom of the tube. Just remember that the more powder used, the more HF could be generated.

6.) Outside or in a fume hood, hold the tube with forceps or a micro test-tube clamp and heat in a burner flame until the sample begins to glow dull orange-red. This should be hot enough to liberate any fluorides that might be present, even if it's a silicate mineral. If you are using soft glass tubing, the glass will begin to melt and droop by this time. Stop heating if the glass melts so far that it might be in danger of falling off or constricting the sample from the rest of the tube. Be sure you heat only the bottom portion of the tube; the upper section must remain relatively cool, for this is where we'll look for any sublimate that may form.

7.) When the tube cools for at least 15 minutes, look for a white ring around the upper portion of the tube. Warren (1921) also indicates that the glass may be etched near the mineral sample, but it's sometimes so faint that the absence of etched glass does not mean there's no fluorine in the mineral. The white ring is likely fluosilicic acid (1921). See below for further details.

8.) If there is a white ring, even a very faint one, fill the tube with distilled water and let stand for about 1/2 hour. Rinse this water out couple of times with a wash bottle of distilled water, pour the tube out, and let dry. If the tube is very narrow, the water will not pour out by itself (you may need to use a Pasteur pipette, the tip of which has been drawn out to extreme fineness in a flame). Do not get this liquid on your skin or in your eyes! Wear gloves and goggles during this procedure, and be sure to point the tube away from yourself. The water may have picked up some acidity.

9.) When the tube is dry, see if the white ring is still there. If so, it is almost certain the mineral sample contained fluorine. See discussion for why this occurs. Note: the white ring may be hard to see. Holding the tube against different backgrounds and trying different lighting conditions will help visualize it (if it's in fact there).

10.) If a white ring did form, try the test with a mineral you're sure contains no fluorine. Compare the two test results. You may even want to run a "blank" of powdered glass and potassium bisulfate and use that as a "control" sample.

Discussion

If a sample contains fluorine, especially if water of hydration or hydroxyl groups are also present, it will liberate HF when heated with the bisulfate flux.

The presence of powdered glass ensures that there's enough silicon present to form silicon tetrafluoride (SiF₄) and fluosilicic acid (H₂SiF₆), though some will of course form from HF's attack of the tube wall itself. The fluosilicic acid condenses in the upper (cooler) part of the tube during heating. It tends to decompose quickly into SiF₄ and HF, which then etches glass (see The Merck Index entry for "Fluosilicic Acid"). SiF₄ itself, while far too volatile to condense on test tube walls at normal temperatures, will decompose on contact with water or water vapors to give silicic acid (H₂SiO₃ or SiO₂H₂O)³ and more HF (see The Merck Index entry for "Silicon Tetrafluoride").

A faint but stubborn, white residue on the sides of the tube is likely silicic acid (precipitated silica); the ring itself appears to be etching from the deposit of H₂SiF₆ that likely deposited and decomposed there.

Any silica residue in the upper portion of the tube can be thought of as proof of fluorides in the sample. Potassium bisulfate and silica alone cannot produce any volatile silicon compounds; fluorine is also required to make these.

It is well-known in chemistry that one should not combine fluorides with sulfuric acid, because the inevitable result is the deadly HF, which then volatilizes out of the container (or, if there's enough of it, through the walls of the glass container that it just got done dissolving). The mineral test for fluorine and fluorides takes advantage of these chemical properties, but the test is done in a controlled fashion and on a very limited scale.

Notes:

¹ The steel surfaces should be clean. Hardened tool-steel is least likely to contaminate the sample, though for the fluorine test this isn't as much a concern as it is with tests for metal ions. The tool-steel mortar and pestle used in geology labs is very expensive; the experimenter can improvise the mortar part of it with a steel end-cap for a pipe. Just don't buy two of these at once, or you might attract the wrong kind of attention. Be advised that the galvanizing will contaminate your assay samples with zinc unless first removed thoroughly by chemical and / or abrasive means.
If one can obtain an old chisel, cut off the chisel end with an abrasive cutoff saw and smooth the rough edges, a steel pestle can be improvised as well.
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²This simply provides a clean place to put and mix powdered reagents and samples. Folding the paper in half allows powders to be poured into small containers, though another sheet of paper should be used beneath the container to catch what spills.
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³Every source available suggests this is only an approximate formula. It is similar or equal to the composition of opal, "hydrated silica". Apparently, the water content can vary; if it's low enough the material can be used as a dessicant to remove water and other vapors from the air. See Greenberg, S. Journal of Chemical Education. 36: 218-219 (1959).
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References:

Warren, Charles. Determinative Mineralogy. New York: McGraw Hill, 1921.

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