Which minerals are sulfides

Figure 3. There are two large fields of solid solution also coloured blue , one centred around bornite bn and the other at the so-called intermediate solid solution iss , which centres around chalcopyrite and related minerals. New phases now stable are covellite cv , chalcocite cc and idaite id.

Chalcopyrite ccp exsolves from the iss as it shrinks on cooling. The blue-grey areas are chalcocite alteration of the bornite. The sulfides are in a silicate mineral host and the sample is typical of a porphyry copper deposit see Table 2. In another example, work on the Fe—As—S system has shown that the As content of arsenopyrite, when formed in equilibrium with pyrite and pyrrhotite, varies as a function of temperature and can be used as a geothermometer.

On the other hand, work on the Fe—Zn—S system has demonstrated that increasing pressure reduces the iron content of sphalerite, and this can be used, under favourable circumstances, as a geobarometer. Sulfide surface chemistry is particularly important because of its relevance to the oxidation and breakdown of sulfide minerals and to the processing of mined ores using froth flotation or leaching.

Investigations using spectroscopic and imaging studies of pristine surfaces in ultra-high vacuum UHV conditions have provided information on structure and reactivity at an atomic resolution. Micron-scale studies have investigated reacted surfaces and reaction products.

Comprehensive reviews are provided by Rosso and Vaughan a, b. The most studied sulfide with respect to surface chemistry is pyrite. At the surface, a complex microtopography has been observed in UHV, defined by flat, stepped terraces, commonly with a high step density Rosso et al. Spectroscopic studies of this surface in vacuum indicate that, upon cleavage, disulfide bonds break to form monosulfide species Nesbitt et al.

The redox chemistry of pyrite in aqueous solution involves further complexities. Rimstidt and Vaughan note that oxidation of a disulfide, such as pyrite, to release sulfate requires transfer of seven electrons and, hence, up to seven elementary reaction steps. Furthermore, pyrite is a semiconductor, so the reactions are electrochemical in nature. This electrochemical reaction may involve three distinct steps: 1 cathodic reaction, 2 electron transport, and 3 anodic reaction.

The cathodic reaction is probably the rate-determining step. Figure 4. Images of sulfide surface structures. B Step terraces on pyrite commonly present at a high step density. After Rosso et al. After Becker et al. D Bacterial leach pits on the surface of arsenopyrite FeAsS reacted in the presence of the acid mine drainage bacterium Leptospirillum ferrooxidans, which were found below a layer of E extracellular polymeric substance, hypothesised to act as a dual direct and indirect oxidation mechanism for enhanced arsenic release.

Figures 4D and 4E after Corkhill et al. Sulfides such as monoclinic pyrrhotite Fe 7 S 8 also have complex surfaces. Superstructures within the pyrrhotite family arise from vacancy ordering in layers parallel to the basal plane Fig.

In the most Fe-deficient end-member, Fe vacancies occur in every other Fe atom layer and in alternate rows within that layer; in every S atom layer, one in four S atoms relaxes into an Fe vacancy. Such complex surfaces then give rise to different oxidation mechanisms. Due to the deficiency in Fe atoms at the pyrrhotite surface, oxidation proceeds via the formation of a sulfur-rich layer Chirita and Rimstidt , whereas for the Fe-rich surface layers of pyrite, ferric oxyhydroxide forms during the initial oxidation.

There is significant environmental relevance in understanding the reactions at the surface of arsenopyrite during aqueous oxidation see Corkhill and Vaughan Like pyrite oxidation, these are complex, multistage electrochemical reactions.

Spectroscopic studies Schaufuss et al. Oxidation products, including ferric- oxy hydroxides, form an oxidized surface layer that is controlled by diffusion of species from the bulk mineral Schaufuss et al. Oxidative leaching of arsenopyrite in the presence of common acid mine drainage bacteria e. Leptospirillum ferrooxidans greatly enhances the release of As from the surface when compared to abiotic dissolution. The mechanisms were a combination of direct leaching, as evidenced from cell-shaped etch pits Fig.

The uptake of metal ions by sulfide surfaces is an important process in the transport and mobility of metals in the subsurface, in ore formation, and on the mobility of contaminants and other pollutants. Again, pyrite is the most studied phase, along with the readily studied galena cleavage surface and the environmentally important mackinawite Rosso and Vaughan b.

Investigation of the sorption of heavy metals e. As, Mo, Hg and radionuclides U, Tc by pyrite surfaces has identified a number of complex reactions that lead to sorption. The problematic radionuclide 99 Tc was found sorbed to framboidal pyrite that was present in a clay formation that itself was used as host rock for the geological disposal of nuclear waste.

The mechanism in this case was via oxidation—reduction, whereby Tc IV —sulfur-type phases were formed Bruggeman et al. Table 2 shows the main types of ore deposits that contain significant amounts of sulfide minerals, their major ore minerals, the metals extracted from them, and some specific examples see Cox and Singer ; Craig and Vaughan , Pyrite is abundant in nearly all of these deposits.

These latter minerals are regarded as having formed via crystallization from an immiscible sulfide melt that separated from the main silicate melt following injection into the country rock. In the Bushveld Igneous Complex South Africa , the dominant sulfides are pyrrhotite, pentlandite and chalcopyrite.

Pyrite is the dominant sulfide in porphyry copper deposits, though it is chalcopyrite that is the most important ore mineral, along with bornite and various binary copper sulfides. In the related porphyry molybdenum deposits, it is molybdenite that dominates.

The sulfides in such deposits occur as veinlets or disseminated grains in host intrusions. If so, this will lead to a release of metals and acidity to the mine drainage. A quantitative prediction will evaluate the extent of acidity and metals released during a certain time frame. This can be done by several different methods, but generally includes a waste characterization whereas samples are tested in a lab in different ways to see if they will produce acid. Mine waste characterization is conducted to predict future quality of the leachate.

Performing a geochemical characterization of mine waste generally means following a certain test programme to predict ARD generation and mine drainage chemistry. There are mainly two types of tests applied for this purpose. Static tests often concerned as screening tests evaluate if a sample will produce an acidic environment.

If so, other tests such as kinetic tests will follow. Kinetic tests, evaluates, when the acidification will take place and which elements that are to be liberated from the sample. There are several types of tests used to get this information, and some are seen as standard methods. Overall, characterization would require an understanding of physical, chemical characteristics as well as the potential for acid generation, acid neutralization and leaching of harmful loads of metals. Waste characterization must be preceded by representative sampling of the material that is to be analyzed and evaluated.

Representative sampling is crucial for characterization results, although there are few guidelines available about this matter, especially concerning waste rock. Representative sampling is further explained in chapter Sulfide minerals Sulfide minerals are compounds of sulphur with one or several metals.

How to cite. This is a preview of subscription content, log in to check access. Berry, L. San Francisco: W. Google Scholar. Craig, J. Wuensch, B. New York: Springer-Verlag, pp.