Mineralogy 101: Fusibility
By Don Peck
Fusibility
Nearly all crystalline solids, including minerals, have a specific melting point. The meltingpoint is determined by the nature of the chemical bonding within the crystal. Metallic bonds tend to produce lower melting points, while covalent bonds usually produce the higher melting points. The differences often can be exploited as a diagnostic property in identifying minerals.
A numerical scale, similar to the one that Frederic Mohs set up for hardness, was established by Von Kobell. It has been modified slightly, as he usednatrolite to define the value, 2, whereas today we use chalcopyrite. The scale of fusibility is as follows.
1 Stibnite Fuses easily in a candle flame, alcohol flame or match flame
2 Chalcopyrite Fuses easily in a blowpipe flame; difficult in an alcohol flame
3 Almandine Fuses in a blowpipe flame; splinters rounded in a gas flame
4 Actinolite Thin edges fuse easily in a blowpipe flame
5 Orthoclase Thin edges fuse with difficulty in a blowpipe flame
6 Enstatite Fuses only on the thinnest of edges in a blowpipe flame
7 Quartz Will not fuse in a blowpipe flame
The only way to become familiar with the fusibility scale is to try it with minerals having known fusibility. Some experience will produce confidence that your technique and equipment will give reproducable results.
You should use only the thinnest of splinters, held at one end in metal forceps. The idea is to keep, as much as possible, the metal forceps from conducting the heat away. The splinter is heated in the top of the luminous portion of the flame (not the light blue cone). This is the oxidizing flame. If a blow pipe is used, the tip should be inserted just *above* the light blue cone of the flame. A propane torch may be used as an alternative to a blowpipe. As with the blowpipe, try it with minerals with known fusibility to get a feel for the results.
Borax, cryolite, and cerrusite all fuse in a candle flame. Most sulfides and some oxides fuse easily a with a blowpipe. The thinnest of edges on pyroxenes fuse with great difficulty.
There are some corallary properties that you may observe while heating a mineral. Some, such as borax and stilbite, swell and form glassy or blebby globules as they melt. Others, like halite decrepitate, or shatter almost explosively, on heating. Calcite and many other minerals that do not fuse glow when heated to high temperatures, and some even phosphoresce. Vermiculite exfoliates, or separates into leaves. Some of the beads that result from fusing metallic sulfides become magnetic. Often, the melting fragment will color a flame (this is particularly true if hydrochloric acid is dropped on it as it is heated).
Fusibility is just one more tool in the toolbox when attempting to identify that troublesome mineral.

From Dave Jacobson - June 01, 2001 at 20:52:07
Email: dnj1[ ]juno.com