Why is pyrite cubic

Surface energies calculated according to classical and quantum methods, reported in Table 3 , agree in relation to the order of stability of the surfaces. Some differences appear between the results from ab initio calculations reported by Hung et al. Density-functional theory studies of pyrite FeS2 and surfaces.

Surface Science. Journal of Physical Chemistry C. Table 3 but they may be due to the different thickness choices of surfaces, cutoff energies, k-points grid in the Brillouin zone, stoichiometric versus non-stoichiometric models. Using force field methods, we found that the surface is more stable than the stoichiometric surface, a result that cannot be compared with quantum results described by Alfonso 29 29 Alfonso DR.

Indeed, in the latter case, the modeled surfaces contain sulfur in excess which results in stabilization of the surface, according to calculations applying the chemical potential approach; the surface energy of the stoichiometric surface is not given by the author.

The optimized stoichiometric surfaces are represented in Figure 5. Figure 5 Polyhedral representation of surfaces optimized using force field a , b , c , d Assuming the above surface energies calculated with interatomic potentials, one may tentatively explain the observed morphology of the crystal.

Considering first a growth in vacuum involving all surfaces, the cubic growth dominates. However, the relative stabilization of each surface depends on partial pressures and temperature. We delay the respective discussion concerning observed crystal morphologies until the next section and focus here on the structure of the three commonly observed surfaces: , and surfaces.

The surface is also briefly described, being the second most stable of calculated surfaces. On the surface, there is a slight decrease of the Fe-S distances to 2. On the surface , the Fe-S distances decrease to 2. The Fe is 4-coordinated and S 2-coordinated. There is also a 5-coordinated Fe and a 3-coordinated S. On the surface, the Fe atoms are positioned above the S atoms contrary to the and surfaces.

The most external sulfur is bound to 2 Fe atoms, at a distance of 2. In the cell of the surface, there are two 4-coordinated Fe, three 5-coordinated Fe, one doubly-bridging S and six triply-bridging S. The shortest Fe-S is 2. The distances of surface S-S dimers are slightly higher: 2. The crystal structure of pyrite is known and it is easily reproduced using classical and quantum methods.

X-ray diffraction is the key method allowing deducing the structure of the unit cell with the atomic positions. The classical calculations are based on experimental data and reproduce the pyrite structure. In the initial chosen structure 17 17 Brostigen G, Kjekshus A. The calculated dielectric constant is 2. Our results are similar to those given by Sithole et al. Atomistic simulation of the structure and elastic properties of pyrite FeS2 as a function of pressure.

Physics and Chemistry of Minerals. Pyrite growth, whether on the geological scale or synthesized in the laboratory, depends on external conditions. Modelling nanoscale FeS2 formation in sulfur rich conditions. Phase-pure iron pyrite nanocrystals for low-cost photodetectors. Nanoscale Research Letters. The effect of the sulfur content can be evaluated theoretically using the chemical potential method. Several studies based on quantum calculation techniques apply this method to explain the growth of crystals 29 29 Alfonso DR.

Journal of Catalysis. Not only the sulfur content defines the growth direction of the crystal but also the adsorption of species can block the growth of a surface and modify the shape of the crystal.

Relation of trace-element content and crystal form in pyrite. Journal of the Mining Institute of Kyushu. One-step synthesis of pure pyrite FeS2 with different morphologies in water. New Journal of Chemistry. Aqueous pyrite oxidation by dissolved oxygen and by ferric iron. The morphology of pyrite also has an influence on its dissolution and formation of acids.

Framboidal forms aggregates of pyrite spheres similar to a raspberry are more easily oxidized than euhedral forms with well-defined facets 38 38 Weisener CG, Weber PA. Preferential oxidation of pyrite as a function of morphology and relict texture. New Zealand Journal of Geology and Geophysics.

Kinetic and thermodynamic results obtained in the present study, using classical techniques, indicate that the surface is the more stable. The and surfaces are also relatively stable and the morphologies generated from them are pyritohedrons.

The morphology of pyrite thus depends on the sulfur content, but also on the surfactants and solvents. Chemistry of Materials. Controlled growth of pyrite FeS2 crystallites by a facile surfactant-assisted solvothermal method. Crystal Engineering Communications. Fe II ions are more exposed than sulfur ions, being a particularity of the surface The reactivity of iron with arsenic is known, which may explain the specificity of the octahedral growth of pyrite in the presence of this element.

The reactivity of pyrite with arsenic can be used for the removal of the metal As in solution at controlled pH 41 41 Kim EJ, Batchelor B. Synthesis and characterization of pyrite FeS2 using microwave irradiation.

Materials Research Bulletin. The adsorption of As on the pyrite surfaces is thus an important property for removing the toxic concentrations of the metal in the water. A high sulfur content also stabilizes the surface. The ab initio calculations with inclusion of the chemical potential in sulfur allow predicting the evolution of the surface energies as a function of temperature and sulfur content 33 33 Zhang YN, Law M, Wu RQ. Similarly, ab initio calculations considering the chemical potential show that water favors octahedron formation at low temperature and that the cubic morphology is stabilized at high temperature above K 31 31 Barnard AS, Russo SP.

This observation is consistent with the thermodynamic cubic growth favored in vacuum and, conversely, with the transformation of the cubic form in the presence of water, leading to the exposition of the surfaces.

Figure 6 shows two crystal shapes identical to those synthesized by Yuan et al. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why does pyrite form cubic crystals? Ask Question. Asked 6 years, 6 months ago. Active 2 years, 4 months ago.

Viewed 68k times. Improve this question. Gaurang Tandon 8, 10 10 gold badges 55 55 silver badges bronze badges. Joshua Benabou Joshua Benabou 1, 4 4 gold badges 18 18 silver badges 26 26 bronze badges. There isn't an adequate modeling theory as of yet. The mineral calcite has more than crystal forms and thousands of crystal variations.

Add a comment. Active Oldest Votes. Improve this answer. I am not asking why does pyrite have primitive cubic structure on the molecular level. Stacked crystals can form weird and wonderfully-shaped agglomerations like you can see on this sample here.

The perfect cube structure, along with the metallic sheen of the mineral, is why I have chosen this to be my Object of the Month. One common way of forming pyrite is in a deep marine setting after the deposition of organic-rich sediments. Bacteria breaks down the organics forming bisulphide, the S2 component of pyrite.

This then reacts with an iron compound forming pyrite. This can result in fossils being preserved as pyrite as the mineral replaces the decaying organic material preserving the original fossil structure. These cubes can be up to 15 centimeters or more along one side. Pyrite can form these large, perfect cubes because it exhibits a cubic crystal system. This perfect cubic crystal system can be seen in the image above.

The yellow nodes represent the sulfur, while the iron atoms are purple. Together, these form a latticework with bonding points only available at the corners of the structure. Thus, as more chlorite containing rocks are converted to pyrite, the cubes grow in a structured and predictable pattern.

Under other conditions, pyrite can form other shapes such as disks, flat sheets, and less impressive crystal structures. For instance, below is a pyrite formation from Nevada. This formation, created by very different conditions than those that create cubes, forms thin strands of pyrite that overlap like quills on a porcupine. Navajun, Spain is an area where the conditions for forming pyrite have been perfect for millions of years. Navajun is located within the Cameros Basin , on the eastern side.

In this basin, a perfect mixture of aquifers settles above rocks which are subject to low-grade hydrothermal metamorphism. In simpler terms, there is just the right amount of pressure, minerals, and water to produce massive pyrite crystals, such as the one seen below.

Though pyrite can be found all over the world, the mines in Navajun produce some of the biggest and most intricate pyrite clusters available. Sometimes, the natural clusters are so big and interconnected that it seems only an abstract artist could have imagined them.

They are highly aesthetic, sharp, lustrous, and nearly perfect. More than this, the pyrite deposits within the eastern Cameros Basin are thought to be as dense as kg per cubed meter.

This makes Navajun, Spain one of the most important and prolific pyrite mines in the world. Further, unlike in some parts of the world, the pyrite here is nearly all cuboidal and has already been exposed through the efforts of previous miners. Piritas de Navajun is a pyrite mine in Navajun which is open to the public.