The grooves on the disk are diffracting the light that is being reflected from its silver surface. Diffraction occurs when a wave encounters an obstacle. As the wave hits an object, new waves are produced at all points along the wave front. These waves propagate spherically, and thus light can appear to bend as it passes an object. If there is a narrow slit, light will appear to bend around both edges of the slit. And if the width of the slit approaches the wavelength of the light, the light waves emitted from the slit edges will either be in phase or out of phase: If the diffracted waves are in phase that is, their peaks and troughs are coincident , then the resultant intensity is increased; if the diffracted waves are out of phase, then the peaks are canceled out by the troughs and no light is seen.
In the case of light, the troughs and ridges are represented by a series of bands. These depend on the wavelength of the incident beam and the density of the slits in the object.
The diffraction effect is seen on the fine grooves of a CD disk but not on a grill, for example. In a typical diffraction grating, the number of slits ranges from a few tens to a few thousand per millimeter. Note that because there is a relationship between the wavelength of light and the slit width, each wavelength of the incident beam is sent in a slightly different direction. This can produce a spectrum of colors from white light illumination, visually similar to the operation of a glass prism; this is the shimmering, multicolored effect on the CD surface.
The upshot of all this is that by measuring the angle of the emitted light from a diffraction grating and its wavelength, we can calculate the size and number of the slits in the grating that produced the spectrum. In Max Laue reported that x-rays were diffracted by crystals. As with the CD and other diffraction gratings, the distances between the x-ray bands and their intensities depend on the distances between the atoms in the crystal.
X-rays exited in a pattern determined by the atomic structure. The technique was seized upon by W. Bragg and W. The Braggs realized that the angles and wavelength of the x-rays diffracted by a crystal would be functions of the positions of the planes of atoms in the crystal.
Because there are several such planes in any crystal, this would enable the atomic structure of the crystal to be computed. Pyrite was one of the first crystalline materials investigated by the Braggs. They used it to demonstrate that x-rays behaved in the same manner as light and not as a series of particles.
In , W. Bragg succeeded in solving the pyrite structure and confirmed a theoretical mathematical model of pyrite. Pyrite helped support the foundations of x-ray crystallography, because it showed how the method could be used to determine the structure of a more complex substance. Pyrite is a semiconductor; that is, it is neither a conductor like metal nor an insulator like most rocks. Semiconductors such as pyrite can switch between being a good conductor or insulator under the effects of electric fields or light, or by doping the material with traces of impurities.
In pyrite, only a small amount of energy is required to release electrons from being chained to the atomic nuclei so that they can move freely in the material and conduct electricity. In other words, a small amount of energy will switch pyrite from behaving like an insulator to behaving like a conductor. A suspension of tiny pyrite crystals might be sprayed onto solar panels like paint. Satisfying the increased demand for electricity will be one of the fundamental problems faced by humankind over the next 50 years.
The obvious solution is to capture the energy from the Sun using solar panels. However, current silicon-based solar panels are expensive.
The energy cost, amortized over the year lifetime of the panel, is around twice as much as that of wind- and natural gas—generated electricity. This is where pyrite comes in as the most cost-efficient alternative solar panel material to conventional silicon. Pyrite absorbs times as much light as the present major solar cell material, silicon. A thin layer of pyrite, just 0.
Because only a very thin layer of pyrite is required to collect the sunlight, suspensions of tiny pyrite crystals, such as those that constitute the ubiquitous pyrite framboids, might be mixed in a solvent and sprayed onto panels like paint. Considerable research is going on worldwide at present to synthesize pyrite crystals and films with various compositions in order to produce an optimal solar energy collector. The other way to help resolve the world energy gap is to find a better way to store electricity.
Electric automobiles are wonderful, except for the fact that they are at present limited to a mile working distance and a hour charging cycle. Portable computers are fantastic—for eight hours until the battery runs out. Pyrite is a source material for sulfuric acid, and one use of it is in car batteries: It is the acid in the lead-acid battery. These lead-acid batteries are still used in automobiles, even though the technology is ancient, because they are rechargeable.
However, these lead-acid batteries are cumbersome and not suitable for many applications where a small solid-state battery is required. The problem with these small batteries is that they are not especially powerful or, in many cases, rechargeable. There have been many recent advances in battery technology. One of the most familiar is the development of lithium batteries. In the Energizer series of lithium batteries, lithium metal is the anode the negative electrode , and pyrite is the cathode the positive electrode.
This pyrite has been ground down to 0. The battery works by a redox reaction whereby the lithium metal is oxidized to produce lithium sulfide and the pyrite is reduced to iron. The redox reaction produces electrons, which we use as electricity. The lithium batteries are popular because they are relatively light, so the amount of energy per gram is optimized. At present these basically are not rechargeable, and the development of rechargeable lithium batteries is a major international target of technological research.
Pyrite is an attractive material for the electronics industry: It is widely distributed, cheap, and readily available. It has some environmental benefits in terms of the amount of energy required in transport and manufacture. All of these attributes are the same as those that originally placed pyrite at the core of early industrial development. It is interesting to speculate that the 21st century will see the burgeoning of a pyrite-driven electronics industry, just as earlier periods witnessed the development of pyrite-driven chemical, pharmaceutical, and explosives industries.
A microscopic image below shows gold occurring as tiny blebs entirely enclosed within a pyrite grain. In fact, pyrite is often associated with gold. The solutions in the Earth that transport iron and sulfur to form pyrite are also likely to transport other metals, including gold.
Pyrite is slightly oxidized relative to other metal sulfide minerals. The slightly more oxidized environment in which pyrite precipitates also destroys the sulfide complexes that keep the gold in solution, and the gold precipitates as a metal. For these reasons, most gold deposits in the world contain pyrite as a more or less abundant mineral. In the case of so-called invisible gold, tiny precipitated gold particles have been trapped in the growing crystal of pyrite.
The amount of gold within the grain shown above is probably around 1 percent by weight, because gold is about four times as heavy as pyrite. It is worth mining if the gold can be extracted from within the pyrite. Gold dissolves in cyanide solutions, and more than 90 percent of gold production is based on a cyanide process.
In some low-grade ores, the crushed rock is piled into heaps and sprayed with a dilute cyanide solution. To extract the gold, the mineral is usually oxidised in large reactors, which uses considerable amounts of energy. This idea is still untested, but definitely merits investigation.
Read more: How gold rushes helped make the modern world. Edition: Available editions United Kingdom. Become an author Sign up as a reader Sign in.
Denis Fougerouse , Curtin University. Chemistry Gold Minerals Geochemistry gold mining crystals. All rights reserved. Common gold questions. What is Fool's Gold made of? How to tell Fool's Gold from real gold. How does gold get its name? How is gold mined? How many grams in an ounce of gold? How much gold is found in the human body? How much gold is there in an Olympic gold medal? How much gold is gold worth? What family does the element gold belong to? What gold detectors actually work finding gold?
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