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Assay Lab Resources

Mineral Assay

The pages linked above provide a general survey of the equipment, reagents, and procedures which may be used to determine the major elements present in a given specimen, and aid in mineral identification.

For texts which provide a more rigorous description of each procedure, browse our Reference List here.

 


Dana's Manual of Mineralogy

Edward S. Dana

This is a Classic text!

ASSAYING EQUIPMENT

Blowpipe:   A standard brass blowpipe 10 to 12 inches long and equipped with a removable tip that is perforated at the end with a tiny round hole is essential. The blowpipe should have a small air chamber at the crux of the pipe which eliminates the problem of moisture condensing from the breath of the user. Proper and efficient use of the blowpipe requires some practice. It is important that the air blown into the blowpipe is introduced as gently as possible. The blast should not be delivered directly from the lungs but introduced gradually from air captured in the fully distended cheeks of the user. This air should be gently allowed to escape from the mouth into the blowpipe. Again, care should be taken not to forcefully blow into the pipe during use. The blowpipe flame consists of three concentric cones, each one a function of fuel content and oxygen content. The inner cone is blue in color and consists of unburned fuel mixed with air from the blowpipe. The second cone is pale violet with a fringe of yellow and consists of carbon monoxide and water, both products of incomplete combustion of fuel. Carbon monoxide is a strong reducing agent; hence this portion of the blowpipe flame is called the reducing flame. Ideally, the blowpipe is held back a bit from the gas flame to produce the best reducing flame. The outer cone of the blowpipe flame is invisible and consists of hot carbon dioxide and water (in this case, products of complete combustion of fuel) and oxygen from the atmosphere. The presence of oxygen in this portion of the flame makes it the oxidizing flame and also makes it the hottest part of the flame. To produce the best oxidizing flame, the blowpipe tip is held just within the gas flame. Ideally, the best oxidizing flame is entirely nonluminous.

Lamp:   Several types of lamps can be employed including the simple oil, tallow, paraffin, and sterno-burning varieties, but the most convenient form of lamp is the standard Bunsen gas burner, especially one equipped with a shorter, lower gas tube. This shorter version of the burner is ideal for blowpiping when a specialized flattened tube is inserted over the top of the burner. This tube is flattened to a hairline slit through which gas is forced and is generally equipped with a projecting metal point for the blowpipe to rest upon during use. If none of the above mentioned lamps are available, a simple candle will work surprisingly well.

Forceps:   A variety of stainless steel forceps and tweezers are required for pyrognostic tests employed by the assayer. Nickel-plated forceps work just as well as the stainless steel varieties while those with platinum tips are, in most cases, superior to all. (As it turns out, fusion of arsenic or antimony-bearing metallic minerals can damage the platinum tip on the forceps.) Fusion or ignition of minerals is accomplished by use of the forceps and a lamp, candle, or Bunsen gas burner. The relative degree of fusibility of a mineral is determined by using the following scale:

  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)

Platinum wire:   A strand of 26 gage (approximately 0.4 mm in diameter) platinum wire equipped with a glass handle and formed into a small loop (about 3 mm in diameter) at its end is essential for various flame and bead tests. As it turns out, some minerals impart a distinctive color to the flame upon ignition. The following table summarizes some of the more common substances and their flame tests:

  • 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)

A somewhat stouter length of platinum wire is used for bead and flux tests. The most common fluxes used by the assayer include borax, sodium carbonate, and sodium metaphosphate. Less commonly used fluxes include Von Kobell’s Flux (a 1:2:1 mixture of potassium iodide, sulfur, and potassium bisulphate) and Turner’s Flux ( a 1:3 mixture of powdered fluorite and potassium bisulphate).

Charcoal:   Good quality pine or willow charcoal slabs (about 4 inches long, 1 inch wide, and ¾ inch thick) are essential in conducting various reduction tests with the blowpipe. The charcoal should be clean and pure, yielding only minor amounts of ash when burned.

Glass tubes:   Both open and closed glass tubes are used in assay work. Open glass tubes should be wider (7–8 mm in diameter) than those employed for closed tube tests (3-5 mm in diameter) and both types can be easily constructed using so-called “hard” or “combustion” glass tubing. The open tube is used for mineral ignitions where an oxidizing environment is required while the closed tube is employed where reducing, oxygen-poor conditions are needed. When conducting open tube tests always use finely powdered mineral dust; when performing closed tube tests always use small mineral fragments rather than powder.

Hammers:   A small, wire-handled assayer’s hammer is required for more fragile specimens while a larger Estwing rock hammer may be needed for heavier work.

Anvil:   A small anvil is best but any smooth, flat iron surface (including the side of a sledgehammer or prospectors pick) will suffice.

Magnet:   A permanent magnet is a useful gross test for minerals such as magnetite and pyrrhotite. In general; it can also be used for testing the presence of iron in fused mineral specimens.

Test tubes:   Ideal sizes are 4 to 5 inches in length.

Test tube holders:   Individual brass or stainless steel test tube holders are extremely useful as are the larger wooden stands that can hold and store multiple test tubes.

Streak plate:   The best varieties are slabs of clean, white, unglazed porcelain.

Watch glasses:   Slightly concave pyrex lens used as a multipurpose cover or receptacle.

Agate mortar and pestle:   It is important to use agate mortars rather than those made of metal to limit contamination of the sample during grinding.

Diamond mortar and pestle:   Actually composed of extremely hard steel, this type of mortar and pestle is used to grind minerals of hardness 6.5 or greater.

Glass funnel.

Filter paper.

pH paper:   Both blue and red litmus paper and yellow turmeric paper are extremely useful to the assayer.

Platinum crucible:   Although a platinum crucible is extremely expensive and is a nonessential piece of equipment, it is extremely useful in certain tests and in all high-temperature firings.

Porcelain crucible.

Glass beakers:   A variety of sizes (including 50 ml, 100 ml, and 250 ml) are essential for laboratory work

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