Fool's gold often carries the real stuff

Pyrites are not just "fool's gold", but carry actual gold particles too!

 Fool's gold often carries the real stuff

---

• The story: Pyrite - or Fool’s gold — often disappointed prospectors looking for golden nuggets. But modern miners now love it, because inside its crystal lattice of iron and sulphur there is often actual gold to be found.
• How much: It is never much, at most a few tenths of a percent. Extracting it is hard work too, which involves grinding and heating the mineral, and then treating it with sodium cyanide, extremely toxic. But it is worthwhile. Some 5-10% of the world’s gold production derives from pyrite. And this is likely to increase. Today, estimates suggest 24% of the world’s gold reserves are “refractory”, meaning the gold must be pulled from the clutches of some mineral, in most cases pyrite.
• How formed: This extraction process is generally blind. Why? Because it has never been properly established quite how the gold is lodged in the pyrite. One theory is that it forms tiny inclusions — nanonuggets. Another is that it is integrated, atom by atom, within the crystal, thus forming a kind of iron-sulphur-gold alloy.
• A new explanation: Researchers say they used a scanning electron microscope and an atom probe (a device which combines a field-ion microscope and a mass spectrometer) to investigate samples from a Chinese gold mine. They discovered that the gold in pyrite (along with many other “foreign” atoms, including arsenic, copper, nickel and lead) is concentrated in places known as dislocations, where the crystal, having been exposed to high pressure or high temperature at some point in its past, has got kinked.

1. This concentration happens when foreign atoms, which because of their odd sizes do not fit properly into the crystal, migrate through the lattice to relieve the tensions that result when it is bent.
2. That migration herds them towards particular places, and when there are enough of them in such a place the lattice buckles and the tension is released.
3. This process must be quite efficient. Aliens made up just over 1% of the metal atoms in the sample (the rest being iron). In the deformation zones they were three times as abundant.
4. That is useful information. Experiments with other types of crystal have shown that impurities in such deformed areas are easier to dislodge than those elsewhere in a lattice. If this is also true for pyrite, it might point the way to a more efficient way of extracting its gold — perhaps by using a harmless solvent like thiosulphate, rather than cyanide, or even employing bacteria to do the job. And only a fool would not be interested in seeing how that panned out.

• More on Pyrites: Pyrite is found associated with other sulfides or oxides in quartz veins, sedimentary and metamorphic rock, in coal beds and as a replacement mineral in fossils. A substantial proportion of the gold is "invisible gold" incorporated into the pyrite. During the early years of the 20th century, pyrite was used as a mineral detector in radio receivers, and is used by crystal radio hobbyists. Until the vacuum tube matured, the crystal detector was the most sensitive and dependable detector available. Pyrite detectors occupied a midway point between galena detectors and the more mechanically complicated perikon mineral pairs. Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-shaped masses called framboids. Iron pyrite is unstable at Earth's surface: iron pyrite exposed to atmospheric oxygen and water decomposes into iron oxides and sulfate. Notable deposits are found as lenticular masses in Virginia, U.S., and in smaller quantities in many other locations. Large deposits are mined at Rio Tinto in Spain and elsewhere in the Iberian Peninsula.

 

Join PT courses | PT Youtube | PT App | Contribute |  Testimonials