Mining also breaks the rocks above and below the coal. This creates more pathways for the movement of oxygenated waters and exposes more surface area to oxidation. Pyrite crystals: Pyrite, cubic crystals in schist from Chester, Vermont. Specimen is approximately 4 inches 10 centimeters across. Crushed stone used to make concrete, concrete block, and asphalt paving materials must be free of pyrite.
Pyrite will oxidize when it is exposed to air and moisture. That oxidation will result in the production of acids and a volume change that will damage the concrete and reduce its strength. This damage can result in failure or maintenance problems. Pyrite should not be present in the base material, subsoil or bedrock under roads, parking lots, or buildings. Oxidation of pyrite can result in damage to pavement, foundations, and floors. In parts of the country where pyrite is commonly found, construction sites should be tested to detect the presence of pyritic materials.
If pyrite is detected, the site can be rejected or the problem materials can be excavated and replaced with quality fill.
Pyrite fossils: Fossil ammonite in which the shell was replaced by pyrite. External view on left and cross-sectional view on right. External view by asterix and cross-sectional view by Henry Chaplin. Both images copyright iStockphoto. The conditions of pyrite formation in the sedimentary environment include a supply of iron, a supply of sulfur, and an oxygen-poor environment. This often occurs in association with decaying organic materials. Organic decay consumes oxygen and releases sulfur. For this reason, pyrite commonly and preferentially occurs in dark-colored organic-rich sediments such as coal and black shale.
The pyrite often replaces organic materials such as plant debris and shells to create interesting fossils composed of pyrite. Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Pyrite can be found in soils and sediments throughout the Earth as myriads of microscopic crystals.
This pyrite is formed by bacteria that remove oxygen from sulfate in the water, producing sulfide that reacts with iron to form pyrite. More than 90 percent of the pyrite on Earth is formed by microbiological processes. Pyrite is already playing a significant role in frontier areas of science and technology, such as nanotechnology and energy conversion.
The remarkable chemical and physical properties of this mineral ensure that it will continue to do so. Likewise, the widespread distribution of huge pyrite concentrations throughout both the land and the oceans of the Earth will ensure that pyrite remains an important source of raw materials needed by a future 10 billion human beings.
The pyrite unit cell, the building block of the pyrite crystal, is basically a cubic structure. Cubes are also the most common pyrite crystal form. The rate of growth of the crystal faces is different on each face, and square prisms are more probable results than perfect cubes. In the extreme case this feature may result in the formation of pyrite wires. This explains the formation of balls of radiating pyrite crystals, commonly found in limestone and chalk, where they are produced from the sulfur and iron in groundwater.
The individual pyrite crystals have simply grown into elongated forms that radiate from a center. The next most common crystal of pyrite is the pentagonal dodecahedron, but it turns out not to be regular; the interfacial angles are not all degrees. Octahedra are the least common natural pyrite crystals. And they are not, in fact, all perfect: The tips are flattened off with cubic faces. 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. Skip to main content. This service is more advanced with JavaScript available.
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