The reason for this formation was first suggested by the great Japanese mineralogist Ichiro Sunagawa in , and I further developed this theory in In order for a crystal to grow, the concentrations of the dissolved constituents must exceed the solubility product of the mineral. Pyrite is a very insoluble mineral, so its solubility product is very low—so low that the concentrations of iron and sulfur in solution are virtually immeasurable.
Nucleation is the key here: It refers to the first stage of the formation of crystals, when atoms and molecules initially coalesce. Pyrite will not nucleate from solution unless there is billion times more iron and sulfur in solution than the equilibrium concentration.
The dodecahedral face requires the greatest amount of energy and thus tends to be preferred at the highest saturations. The octahedral face is the next highest, and the most stable cubic face the least.
So in a situation where the supply of nutrients is limited, crystal growth depletes the concentration of the dissolved components and the crystal faces change with time. The octahedral crystal will grow until the nutrients in solution are used up, and then the cubic faces will take over. So most octahedra are capped by cube faces. My research group designs pyrite crystals with various shapes, with applications in the Earth and environmental sciences and in materials science.
For example, if we understood what controlled the shape of a natural pyrite crystal, we would know what the environment was like when the crystal was formed millions of years ago.
By varying the concentrations of dissolved iron and sulfur, and the hydrodynamics of the solution, a vast array of forms of pyrite crystals can be produced. This explains how a chemically simple mineral such as pyrite may exhibit the greatest variation in natural crystal forms in the mineral kingdom. One of the most common forms of pyrite in nature is as small, globular aggregates of pyrite crystals called framboids, because they look like tiny raspberries. Pyrite framboids are mostly invisible to the naked eye, with diameters usually around 0.
Framboids are found in rocks, especially sediments, of all ages. The oldest reported pyrite framboids may be from 2. They are therefore extremely stable configurations and can last over eons of geologic time. The abundance of pyrite framboids is quite extraordinary. A guesstimate of the total number of framboids in the world suggests that there are around 10 30 , which is 10 billion times the number of sand grains in the world, or about 1 million times the number of stars in the universe.
Today, some 10 12 pyrite framboids are being formed every second. In the early 20th century, improved microscopy showed that these spherules consisted of aggregates of pyrite crystals less than 0. So each framboid may contain more than 1 million tiny crystals of pyrite, each of which has a similar shape and size. Not only that, but they are often beautifully organized and arranged in the framboid. Detailed studies by my group revealed that framboids are not truly spherical but have flattened faces.
They did not grow like normal crystals but aggregated together under the influence of their surface electrical charges. Because these crystals are so small, with 50 million of them usually needed to make up 1 gram of pyrite, these tiny surface electrical forces are sufficient to stick the crystals together.
It may not be generally appreciated how important pyrite has been and still is to the world economy and to providing the basics for our current civilization. Pyrite continues to be mined worldwide and is a major source of sulfur, the basic constituent of sulfuric acid. Sulfuric acid has become one of the most important industrial chemicals, and more of it is made each year than any other manufactured chemical.
World production in was about million tons. Sulfuric acid is used in the chemical industry for production of detergents, synthetic resins, dyestuffs, pharmaceuticals, petroleum catalysts, insecticides, and antifreeze, as well as in various processes such as oil-well acidicizing, aluminum reduction, paper sizing, and water treatment.
It is used in the manufacture of pigments and includes paints, enamels, printing inks, coated fabrics, and paper. The list is endless and includes the production of explosives, cellophane, acetate and viscose textiles, lubricants, nonferrous metals, and batteries.
Sulfuric acid is a relatively recent manufactured chemical. Prior to this, the important analogous chemical substances were the sulfate salts of iron, copper, and aluminum, known to the ancients as the vitriols. These occurred in the lists of minerals compiled by the Sumerians 4, years ago. They were used as mordants in the dyeing industry.
In order for natural dyes to be fixed in the cloth—and not be washed out during the next rainy day—it is necessary to treat the cloth with a mordant. The mordants widely used in dyeing were solutions of the vitriols. The demand for vitriols could not be satisfied from natural supplies, and industries developed to manufacture this substance from pyrite. The production of one mordant, pure alum, from pyrite has been described as the point of origin of the modern chemical industry, because the process required not only the manufacture of a chemical substance but also its purification.
The manufacture of artificial drugs—in contrast to the use of natural remedies—can be traced back to pyrite and strike-a-lights. It is not a big step to drop pyrite from a strike-a-light into the fire. The result is the formation of sulfur oxide gases with their characteristic burnt smell.
These sulfur oxide gases, apart from being poisonous in high doses, can clear clogged-up noses and are very useful in fumigation. Pyrite is a major source of sulfur, the basic constituent of sulfuric acid, which is one of the most important industrial chemicals, and made in greater amounts each year than any other manufactured chemical. One of the earliest descriptions of the medicinal use of sulfur was in The Pharmacopeia of the Heavenly Husbandsman, compiled in the Western Han period BCE—24 CE , which cataloged the medicines invented some 3, years earlier by the legendary emperor Shen Nong.
Medical sulfur had to be produced from pyrite in the absence of deposits of natural sulfur. Sulfur was used mainly in creams, to alleviate conditions such as scabies, ringworm, psoriasis, eczema, and acne. The mechanism of action is unknown—although sulfur does oxidize slowly to sulfurous acid, which in turn through the action of sulfite acts as a mild reducing and antibacterial agent.
The use of alum in medicine has been documented for more than 2, years since the Babylonians listed it in one of the first pharmacopeias. The main medicinal use of alum was, as it still is today, as an astringent to improve wound healing. The modern styptic used to close up razor nicks occurring after wet shaving is alum-based.
It helps reduce swelling of the skin around healing sores. It has also been used as an emetic to treat someone who has ingested a poison. We have seen that pyrite is the raw material from which sulfuric acid can be made, and a major use of sulfuric acid in modern economies is in the production of fertilizers.
About 60 percent is currently consumed for fertilizer manufacture, especially superphosphates, ammonium phosphate, and ammonium sulfates. During the early part of the Industrial Revolution, sulfur in Europe was sourced from natural sulfur deposits associated with volcanic fumaroles in Sicily. In the Sicilian deposits came into the hands of a French company, which raised the price threefold.
This led to other countries reverting to pyrite as a source of sulfur. Roasting of pyrite produces sulfur oxide gases, and these can be dissolved in water to produce sulfuric acid.
Byproducts of the process include copper metal from the pyrite and an iron-based slag that is used in road-building. It has been estimated that the population of Great Britain was constrained to around 6 million in preindustrial times due to the limitations of agricultural productivity.
This compares with more than 60 million today. The excess 54 million people are fed by postindustrial technological advances. This step increase in agricultural productivity was fueled by the development of industrial fertilizers. This, in turn, caused a consequent exponential increase in the demand for sulfuric acid, sulfur, and pyrite.
Gold can even occur as inclusions inside pyrite, sometimes in mineable quantities depending on how effectively the gold can be recovered. Pyrite has long been investigated for its semiconductor properties. Learn about studies underway to develop pyrite as a material to make solar cells.
Pyrite is found in a wide variety of geological settings, from igneous , sedimentary and metamorphic rock to hydrothermal mineral deposits, as well as in coal beds and as a replacement mineral in fossils. Pyrite can be either disseminated throughout igneous rock or concentrated in layers, depending on depositional mechanism and environment.
Pyrite forms in sedimentary rocks in oxygen-poor environments in the presence of iron and sulfur. These are usually organic environments, such as coal and black shale, where decaying organic material consumes oxygen and releases sulfur. Pyrite often replaces plant debris and shells to create pyrite fossils or flattened discs called pyrite dollars. In calcite and quartz veins, pyrite oxidizes to iron oxides or hydroxides such as limonite, an indicator that there is pyrite in the underlying rock.
Gossans can be a good drilling targets for gold and other precious or base metals. Pyrite is unstable and oxidizes easily, which is an issue in controlling acid mine drainage.
Pyrite is a widespread natural source of arsenic, which can leach into ground-water aquifers when geologic strata containing pyrite are exposed to the air and water, during coal mining for example.
Acid mine drainage and groundwater contamination requires close monitoring to ensure that it has been neutralized before being returned to the earth. A question: If you have a shiny and tiny golden color spot in your sample, how would you identify it?
Is it gold? Is it pyrite? Portable x-ray fluorescence XRF analyzers are an important tool in this effort. In seconds, you can identify that grain using a portable XRF. As a final check, rub the sample on a rough, unglazed pottery surface, pyrite leaves a distinctive black streak, while gold leaves a golden streak. Chalcopyrite occurs at numerous localities worldwide. It is the most abundant copper-bearing mineral. Chalcopyrite is a primary mineral in hydrothermal veins, disseminations and massive replacements.
Chalcopyrite and dolomite on dolostone. Unknown Locality. Notice the iridescent colouring on the surface of the chalcopyrite. If you were to rub the mineral vigorously with a hard object then if pyrite it will give off a sulphurous smell like rotten eggs but if gold no odour will be apparent.
As well, if struck with a steel hammer gold will flatten or change shape without breaking but pyrite will give off sparks. Chalcopyrite looks similar to pyrite but it is softer and can be scratched with a knife.
It is a very brassy yellow, often with a bronze or iridescent tarnish whereas pyrite is simply a brassy yellow. As well, pyrite is slightly heavier than pyrite. The name marcasite is derived from the Arabic word for pyrite. This mineral is a common and attractive mineral. It has the same chemical composition as pyrite, but it has a different crystallization system, making is a pseudomorph of pyrite. Without proper analysis aggregates of iron sulphide may be wrongly labelled by dealers.
Crystal habits include the tabular, bladed or prismatic forms. Picher, Oklahoma, USA. Over a period of years, marcasite will oxidize in collection. This process frees sulphur which frees sulphuric acid.
The acid will then attack a paper label or even a cardboard box that the mineral might be kept in. Over a period of decades, most specimens will have disintegrated into a white dust along with deteriorated paper scraps. A sulphur smell will be released during this reaction contaminating other sulphide minerals nearby. Marcasite is common worldwide. It occurs mainly in sedimentary deposits in low temperature ore veins, as well as in skarn metamorphic deposits.
Marcasite is more brittle than gold. It is also lighter and is a brassy yellow mineral with a greenish tint at times or possibly a multi-coloured tarnish which results from oxidation.
Marcasite is difficult to distinguish from pyrite when there is a lack of distinctive crystal habits. As well, marcasite is a brassy yellow with a greenish tint at times.
A multi-coloured tarnish may exist which is the result of oxidation. It is important to note that marcasite is too soft to be used in jewellery. For this reason, marcasite jewellery is actually made from pyrite, contrary to the impression one gets from its name.
The Incas are the earliest known civilization to use marcasite in jewellery. Notable pieces have been found in several burial chambers. As well, marcasite was used by the Incas as mirrors, in sun-worship rituals and as a means of seeing into the future.
During the Georgian period the Swiss began to produce marcasite for the European market to bypass the Sumptuary Laws which forbade the use of diamond by all but the most aristocratic.
When cut in a pyramid shape with a flat back the marcasite had a brilliance resembling diamond. Early cut steel was used in the same way. Arsenopyrite is named for the minerals chemical composition.
It is silver-white to steel-gray in colour with a greyish black streak. The mineral tarnishes dark gray but occasionally also an iridescent pink. Rarely is it seen in igneous basalt rocks.
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