theGeoZone

General Assaying Procedures

A number of assaying procedures are employed in the study of minerals; these include various blowpipe and chemical techniques such as:

1. fusibility test
   
2. flame test
   
3. roasting on charcoal
   (heating in an oxidizing environment)
   
4. fusion or ignition on charcoal (melting or burning the sample in either an oxidizing or reducing environment)
   
5. reduction of metals by fusion with a flux
   
6. ignition in open or closed tubes
   
7. precipitates from solution
   
8. color of solution
   
9. solubility test

 

 

 

 

 

Fusibility test: The test for fusibility is performed using a lamp, candle, or Bunsen gas burner and a forceps to hold the mineral fragment. In most cases, a mineral fragment about 1.5 mm in diameter is sufficient to conduct the test, but when dealing with refractory minerals that fuse with difficulty, a much smaller fragment with very thin edges may be required to accurately determine fusibility. During ignition, a number of other phenomena may be noted including a distinct coloration of the outer blowpipe flame, swelling of the mineral fragment, exfoliation or peeling of the mineral surface, glowing of the mineral fragment, decrepitation, and intumescence. In addition, the final state of the fused mass should also be noted.

1. Fragments larger than 1.5 mm fuse easily in a luminous flame or in a closed tube below red heat (ex: stibnite, realgar, orpiment, and sulfur)
   
2. Fragments 1.5 mm in size fuse easily in a luminous flame while a small fragment fuses in a closed tube at red heat (ex: chalcopyrite, galena, arsenopyrite, and apophyllite)
   
3. Fragments 1.5 mm in size fuse readily to a globule using a blowpipe but only the thinnest edges are rounded in a luminous flame (ex: almandine, malachite, and stilbite)
   
4. Edges of a fragment 1.5 mm in size are easily rounded and fine splinters of the mineral fuse easily to a globule (ex: actinolite, tremolite, wollastonite, and barite)
   
5. Edges of a fragment 1.5 mm in size are rounded only with difficulty and only the finest splinters fuse to a globule (ex: orthoclase, sphalerite, biotite, and scheelite)
   
6. Only the finest splinters and the thinnest edges can be rounded at all (ex: bronzite, enstatite, and serpentine)
   
7. Absolutely infusible in the blowpipe flame (ex: quartz)

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Flame test: The flame test is best performed using a clean, dry platinum wire formed into a small loop at one end and a lamp, candle, or Bunsen gas burner for a flame source. The wetted end of the platinum wire (moistened with distilled water, HCl, or for some minerals concentrated H₂SO₄) is lightly dipped into the powdered mineral sample and then exposed to the flame.

• lithium (carmine-red)
• strontium (purplish-red)
• calcium (reddish-orange)
• sodium (intense yellow)
• barium (yellowish-green)
• boron (green)
• oxides of copper (emerald green to bluish-green)
• phosphates (bluish-green)
• antimony (greenish-blue)
• arsenic (whitish-blue)
• selenium (azure-blue)
• chlorides of copper (azure-blue)
• potassium (violet)
• molybdenum (yellowish-green)
• sulfides (yellowish-green)
• zinc (bluish-green in streaks)
• tellurium (pale blue)
• lead (blue)

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Roasting on Charcoal: This procedure is employed using a charcoal block, a lamp, candle, or Bunsen gas burner, and the blowpipe. The best results are attained when a fine layer of powdered mineral is spread out along the charcoal block and the oxidizing portion of the flame is passed along its length. The mineral powder should be heated no more than a dull red. The volatile components of the sample are thus driven off, a few of which betray their presence by a distinctive odor. These include the solfateric smell of sulfur, the garlic smell of arsenic, the noxious smell of the sulfur dioxides, and the decayed horseradish smell of selenium. The nature of the burned residue should also be noted.

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Fusion or Ignition on Charcoal: This procedure requires the same equipment as those listed above, but differs in the following ways:

1. two or three pinhead-sized fragments are used rather than powder
   
2. the sample is placed in a small depression near one end of the charcoal block rather than spread out along its length
   
3. the sample is immersed in the flame (both oxidizing and reducing) rather than at some distance from it
   
4. the sample is fused (if possible) rather than just heated

Occasionally, the reducing portion of the flame is capable of separating a metallic element from a compound when the ignition is performed on a charcoal block. Many times, a flux is used to enhance the procedure of metal reduction.

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Reduction of Metals by Fusion with a Flux: Pyrognostic testing of mineral samples is facilitated by the use of fluxes which in many cases promote melting and also help to reduce the constituent metals present in the sample. The powdered mineral sample is mixed with the flux (usually by dipping a red hot bead of flux into the mineral powder) and then reignited, first in the oxidizing and then in the reducing portion of the flame. As it turns out, only a small amount of the mineral powder is needed when fluxes are used. These bead tests are performed using platinum wire and occasionally charcoal blocks. During ignition, a number of distinctive characteristics may be noted including the color of the bead when exposed to both the oxidizing and reducing portions of the flame, the presence or absence of a metallic globule, and the nature of the residue.

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Ignition in Open or Closed Tubes: This procedure is usually performed in the assay lab on minerals that contain some volatile component. The closed tube is employed to simulate a reducing environment during heating while the open tube is used to insure a steady stream of atmospheric oxygen and hence an oxidizing environment during heating. Heating the mineral sample in a closed tube may produce a volatile product which usually sublimates on the surface of the tube above the assay. During heating, other phenomena like decrepitation, phosphorescence, changes in color, and glowing may occur. Heating the mineral sample in an open tube may also produce a sublimate accompanied by the odor of escaping gases. The most common gas encountered in assay work occurs when sulfide minerals are roasted in an oxidizing flame. The irritating smell of SO2 gas that is produced during this reaction is very distinctive. It is important to remember that finely powdered mineral is used in open tube experiments while small mineral fragments are used in closed tube tests.

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Precipitates from Solution: Depending on the chemical conditions of a given solution, some compounds may be insoluble and therefore form a precipitate near the bottom of a test tube or beaker. A few important generalizations may be noted:

1. adding excess ammonia to an acid solution containing aluminum, beryllium, bismuth, chromium (III), iron, lead, or titanium will precipitate hydroxides of those metals
   
2. adding BaCl₂ to a sulfuric acid solution containing barium will precipitate BaSO₄
   
3. adding HCl to a nitric acid solution containing silver, lead, and mercury will precipitate chlorides of those metals
   
4. adding dilute H₂SO₄ to a neutral or slightly acidic solution containing lead, barium, and strontium will precipitate sulfates of those metals

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Color of Solution: On occasion, complete or partial dissolution of a mineral will yield a colored solution. In some cases, the color is diagnostic. When dissolved in acidic solutions, copper-bearing compounds generally yield a blue or greenish-blue color while iron-bearing compounds usually give a yellowish tint to the solution. Cobalt-bearing compounds impart a rosy pink color while tellurium-bearing compounds give a beautiful reddish-violet color to the solution. Compounds that contain nickel will generally yield an apple-green solution when dissolved in nitric acid.

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Solubility Test: This procedure is employed using both dilute and concentrated forms of HCl, HNO₃, H₂SO₄, and aqua regia. In most cases, HCl is the preferred acid, although HNO₃ is used to dissolve many metallic minerals containing lead and silver. Many minerals are completely soluble in acid, dissolving with little or no reaction. But some minerals effervesce as they dissolve, giving off a gaseous by-product. The carbonates produce CO₂ gas when dissolved in HCl while some sulfide minerals give off H₂S gas when exposed to the same acid. Chlorine gas is produced when manganese oxides are immersed in HCl, while many metallic minerals produce NO₂ gas when treated with HNO₃. Minerals that are only partially soluble will generally leave an insoluble residue at the bottom of the test tube or a gel floating about in the solution. Silica and sulfur are common residues left over from the partial solution of a mineral. Finally, many minerals are not attacked by acids in any way. They are for all intents and purposes insoluble.

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