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BEAD TESTS Bead tests are fairly easy to perform and are colorful, but there are points to observe with them. This discussion is not meant as final word but only to supplement the books, some of which were discussed in an article in the October 1994 issue of Mineral News. However, the books don’t always include all hints, so some are included here. Bead tests generally work best with the oxides of the first row of the transition elements (these are found in the middle of the Periodic Table of the Elements), such as Ti, V, Cr, Mn, Fe, Co, Ni, Cu. Also Mo, U and sometimes W. The support is usually platinum wire; the best size is 26 gauge (about 16 thousandths of an inch, or 40% of a mm), although 28 gauge (o.0126" or o.32 mm) will also work well. Platinum is susceptible (when hot) to semi-metals, and especially so to arsenic, so the tube tests (Martin, 2002) are used for minerals containing those. Not only must any trace of arsenic be avoided, necessitating roasting of samples to oxides if any had been present, but sulfides also should be roasted to oxides, as it is the oxides that work best in bead tests. If one still suspects any trace of arsenic, one may use a graphite rod, usually the lead of a pencil, as a support (Marcin, 1989); although it doesn’t work quite as well as platinum, it is useful in cases. Another method is to make a blank clear bead, flick it out of the looped wire into a jar to catch it, and put the bead on charcoal and do the test there. The technique of blowpiping is discussed by Brush (1890) and by Marcin (1992). One must not blow with the lungs, but only by the natural contraction of the cheeks, which also yields the right force; beginners usually blow too hard. The fluxes used to make beads are borax (available from the drugstore is pure enough for this type of work), salt of phosphorus (also called microcosmic salt), and sodium carbonate (called soda). Salt of phosphorus is NaNH4HPO4·4H2O and when fused on the wire converts to the reactive sodium metaphosphate, NaPO3. One may use sodium biphosphate (sometimes sold in aquarium shops to adjust pH of the water) as that also fuses to sodium metaphosphate. There is also a recipe to make it, and another substitute that works well. To fuse the borax, the end of the wire is held in the flame until red-hot, then quickly dipped into a bit of borax (Note: never return unused reagent to stock bottle as that can contaminate the supply). It sticks when hot and when reheated it intumesces (makes "worms") as it loses its water, but finally settles down to a bead. Repeat this to build up the bead. It is then usually gray, and must be heated with the aid of the blowpipe flame to yellow-hot, it swirling about, until all bubbles are gone and it cools to a clear bead. Then it is clean, and it is reheated and dipped into a bit of powdered sample, and reheated. Fusion must be complete; one sees the particles swirl around in the hot liquid bead until finally being dissolved. Practice, and reference to Pough’s Field Guide, helps. However, in his discussion of cleaning the wire, his term "snapping" does not refer to any snapping of the wire, but one heats the bead hot, until fluid, then with a snap of the wrist one jerks it so that the bead flies off (into a glass jar is best to prevent messes). The bead is diluted repeatedly until clear again. Salt of phosphorus tends to fall off the wire so the bead isn’t made so big. The colors can be a bit different. It is useful to test for silicates, as most of these (with the exception of some zeolites which may slowly dissolve in the bead) are insoluble or incompletely soluble in salt of phosphorus, and the bases go into solution leaving a faint, insoluble skeleton of silica floating like a ghost in the bead. This must be seen to be appreciated. Colors of beads may vary due to amount used, whether the O.F. (oxidizing flame) or R.F. (reducing flame) is used, whether the bead is hot or cold, and such. Not mentioned in many books is that some mixtures can mimic other colors – for example, green is common to copper, but a mixture of much iron and a little cobalt can yield a similar color. Also several colors may not be unique to an element, so it might only narrow down the possibilities. Most texts on the subject give tables of the colors but many are too brief, and no book can perfectly describe the colors in words, so it is best to make beads using known materials and actually see the colors for oneself. O.C. Smith (1946, 1953) gives a color plate of colored beads, but the colors, due to the then-primitive stage of color photography, are off color, so it is best to make ones' own set of real beads for reference. The soda bead is sometimes used, but does not get clear but continues white. It is useful to test for Mn, but this is best done on charcoal, as hot manganates and hot alkalis are not good for platinum, and the test does oxidize the Mn to a bluish green sodium manganate. So soda beads are usually best done on charcoal. They are also useful to break down silicates for further testing. The borax bead test for uranium has the added feature that when fully cooled it is quite fluorescent, bright green under ultraviolet light (Pough, 1976). Only a trace of uranium will do this. This does not work so well, or sometimes not at all, with the Salt of Phosphorus bead. However, it works with the borax bead. Sometimes graphite is black (instead of gray) and then may resemble black manganese oxides. Then it may be tested by performing the borax bead test, in which the tiniest bit of Mn turns the bead black, if too much is used – this may be diluted to get the purple Mn reaction – but if it is graphite the flecks are insoluble and simply swirl around in the hot bead, and freeze in the cold bead, like little specks of gray pencil lead (which is graphite). BRUSH, G.J., 1890, Manual of Determinative Mineralogy and Blow-Pipe Analysis (any edition of this, whether by Brush, or by Brush and Penfield, will do). Most of the above references were published in the U.S. However, there is a book by Firsoff, which includes a few tests, which is written from the Aussie's point of view (uses such terms as fossiking). This may be useful if still available in Australia. --------------------------------------------------------------------------------
From Martin - June 19, 2002 at 17:29:51
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