Pyrite (fool's gold)

Pyrite is a sulfide mineral that naturally forms in hydrothermal, magmatic, metamorphic, and sedimentary rock environments. It is commonly known as fool's gold, owing to its metallic sheen and distinctive bright yellow color. In its natural form, pyrite is easily mistaken for gold by the untrained eye but is significantly harder and frequently contains oblate crystalline faces not found in gold. Pyrite is the most abundant sulfide mineral on Earth, and it takes its name from the ancient Greek phrase pyrite lithos, which translates to "stone that strikes fire." It played a central role in one of the earliest known methods of creating fire, as pyrite generates sparks when struck against hard or metallic surfaces.

Often forming in six-sided, eight-sided, or irregular polyhedrons, pyrite is composed of 53.45 percent sulfur and 46.55 percent iron. Its most common mineral associations are calcite, fluorite, galena, sphalerite, and quartz; small quantities of gold are also sometimes contained in pyrite. However, these gold concentrations are typically in the range of 0.25 percent, and as such, it is rarely profitable to extract any gold that may be found in a pyrite deposit.

Distinguishing Pyrite from Gold

Despite their superficial similarities, pyrite and gold can be distinguished from one another with relative ease. Pyrite is vitreous (glassy), and thin samples will typically break or shatter when prodded with a sharp instrument. Gold, on the other hand, is soft rather than brittle, and while it may develop dents or bends when prodded with a sharp instrument, it will remain whole. Pyrite also leaves a dark streak that is typically black with tinges of green, while gold's streak is distinctively yellow.

Uses and Economic Value

While pyrite contains both iron and sulfur, it is not usually mined or harvested as a source of these elements. Iron is more readily extracted from hematite, magnetite, and other oxide ores, while sulfur is usually extracted from natural salt deposits or harvested as a by-product of petrochemical processing. Beyond a limited scope of application as an ore of gold, pyrite is most frequently used as a decorative metal or low-value gemstone. However, it has limited value in jewelry, given its propensity for breaking into shards or powder upon processing.

Historically, the indigenous peoples of Mesoamerica used carved and polished pieces of pyrite as mirrors, most extensively in the Maya region. It was also used as an ignition source in early firearms, but it was later replaced by flint. Early in the twentieth century, pyrite also saw some use as a semiconductor material in the first radios, helping convert alternating current (AC) electricity into the direct current (DC) electricity needed to operate radio detectors.

Potential Future Applications

In recent years, scientists and engineers have begun to revisit pyrite's potential for a range of applications in the fields of power generation, agriculture, and biological research. In 2014, pyrite was proposed as a possible less expensive alternative for the silicon used in photovoltaic solar panels. The United States Department of Energy financed a research study into the construction of a solar cell from pyrite, with the objective of achieving a minimum efficiency rating of 10 percent. However, succeeding in doing so will likely require advanced engineering, as the currently listed solar energy conversion efficiency rating of pyrite is less than 3 percent. Additional research conducted by the Department of Energy also found that pyrite has intrinsically inferior semiconducting characteristics, owing to a relatively low sulfur content. As a result, it currently generates low voltage output rates. Yet, it has also been demonstrated that pyrite absorbs one hundred times as much light as silicon, leaving the possibility open that its potential as a powerful material in solar panels may yet be unlocked.

Another potential power generation application of pyrite is in the iron sulfide nanocrystals used in the cathodes of hybrid batteries. As pyrite is composed of a near equal amount of both iron and sulfur, is abundant, and very inexpensive, numerous parties have proposed that it could generate significant savings for battery manufacturers.

In the 1970s, pyrite was proposed as a possible supplement to agricultural fertilizer, particularly for use in grass and crop-yielding pastures. Studies dating to 1984 found that pyrite had several noteworthy benefits in such applications, but it failed to gain traction in the agriculture industry. A 2005 study found that adding pyrite to calcareous soils generated a noteworthy increase in the soil's nutritive characteristics. A 2015 research project also found that pyrite derivatives can be used in place of gypsum in the improvement of soils with low alkaline levels; the study concluded that the pyrite by-products were adept at boosting the soil's micronutrient levels, and that wheat grown in pyrite-treated soil had an increased dry weight. This study also noted that pyrite achieved these improvements without generating any harmful by-products.

Pyrite may also play a role as a key indicator of early forms of life on Earth. A 2010 analysis of rock formations in Scotland dating back one billion years found that an examination of the pyrite grains found in the rock samples pointed to isotopes of sulfur that strongly suggested Earth's atmosphere contained enough oxygen to support multicellular organisms, which are currently thought to have first appeared approximately eight hundred million years ago.

Analysis of pyrite crystal formations have already been shown to have applications in the field of geology. David Rickard's book Pyrite: A Natural History of Fool's Gold makes a case for pyrite being a crucially important source of scientific data regarding the volcanic history of the Earth as well as the cooling process that transformed it from a searing hot proto-planet into the familiar habitable environment of today. Rickard also notes that pyrite may have served as one of humankind's earliest known medicines; during the combustion process, pyrite produces sulfur oxides that are known to clear clogged sinus cavities when inhaled through the nose.

Bibliography

Feick, Kathy. "Pyrite." Earth Sciences Museum. University of Waterloo. Web. 18 Feb. 2016. https://uwaterloo.ca/earth-sciences-museum/resources/detailed-rocks-and-minerals-articles/pyrite

"Pyrite." Geology.com. Geology.com. Web. 18 Feb. 2016. http://geology.com/minerals/pyrite.shtml

"Pyrite." University of Minnesota. Regents of the University of Minnesota. Web. 18 Feb. 2016. https://www.esci.umn.edu/courses/1001/minerals/pyrite.shtml

"Pyrite Power: Can We Reinvent 'Fool's Gold'?" New Scientist. Reed Business Information Ltd. 22 July 2015. Web. 18 Feb. 2016. https://www.newscientist.com/article/mg22730310-500-pyrite-power-can-we-reinvent-fools-gold/

"Uses of Pyrite." Pyrite Problem. Pyrite Problem. 29 July 2015. Web. 18 Feb. 2016. http://www.pyriteproblem.com/general/uses-of-pyrite/