Why are sulphides important

The definition is commonly widened to include minerals in which the anion is As or Sb, sometimes together with S, and to include Se and Te minerals. They generally contain pyramidal TS 3 groups in their structures.

These five are pyrite, pyrrhotite, galena, sphalerite and chalcopyrite, and it is the iron sulfides pyrite and pyrrhotite which are dominant. The very fine particulate iron sulfides found in reducing environments beneath the surfaces of some sediments and soils are also important.

Both mackinawite and greigite are metastable compared to pyrite and pyrrhotite. Above all, the sulfides are the most important group of ore minerals because they are responsible for the concentration of a wide range of metals as mineable deposits. They are also potential sources of pollution, be it of the air, surface waters, or soils. Air pollution may arise both from the smelting of sulfide ores and from the burning of coal, which contains sulfur mainly as sulfide impurities. This form of pollution may arise from mine wastes acid mine drainage or sulfide-containing natural rocks acid rock drainage.

The literature on sulfide minerals is extensive, with a number of overview textbooks and monographs. Comprehensive reviews can be found in Ribbe , Vaughan and Craig , and, most recently, in Vaughan The present article provides a brief overview of the compositions and crystal structures of the major sulfide minerals, aspects of their chemistries bulk and surface and their occurrence.

In addition to the sulfides mentioned above, pentlandite [ Fe,Ni 9 S 8 ] and its alteration product, violarite FeNi 2 S 4 , are important as the major ore minerals of nickel; bornite Cu 5 FeS 4 and chalcocite Cu 2 S as major copper minerals; and molybdenite MoS 2 is the primary source of molybdenum.

Tetrahedrite Cu 12 Sb 4 S 13 is notable because of the large range of metals, silver in particular, which can substitute at percent levels for copper or antimony in its structure. In contrast, arsenopyrite FeAsS is the major natural source of arsenic, an extremely toxic pollutant.

The most important sulfides are categorized into groups based on major structure types or having key structural features in common Table 1 see Makovicky for detailed classification. Commonly, these are the structures exhibited by a much larger group of crystalline solids, such as the rocksalt structure of the galena group Fig.

The disulfide group of minerals Fig. In the pyrite structure, FeS 6 octahedra share corners along the c-axis direction, whereas in the marcasite form of FeS 2 , the octahedra share edges to form chains of octahedra along the c-axis. Loellingite FeAs 2 and arsenopyrite FeAsS are variants of the marcasite structure with, respectively, shorter or alternately long and short metal—metal distances across the shared octahedral edge.

Sulfides such as covellite CuS Fig. A diverse group, defined as the metal-excess group by Vaughan and Craig , have metal:sulfur ratios greater than and structures of the type illustrated by pentlandite, the major ore mineral of nickel Fig. Figure 1. Crystal structures of the major sulfides. A Galena PbS. B Sphalerite ZnS. C Wurtzite ZnS. D Niccolite NiAs. F Covellite CuS. G Cube cluster of tetrahedrally coordinated metals in the pentlandite structure.

Adapted from Craig and Vaughan The relationship between derivatives and parents can involve:. The chemical compositions of sulfide minerals have been well characterized by numerous analyses of natural samples and laboratory investigations of phase equilibria Table 1 gives names and formulae of all common, and many less common, sulfides.

Such impurities may include toxic elements such as arsenic, cadmium and mercury. The more extensive substitutions associated with solid solution are also found in the sulfides: for example, the complete solid solution between pyrite FeS 2 and vaesite NiS 2 to give the intermediate composition mineral bravoite [ Fe,Ni S 2 ].

Certain sulfides also exhibit non-stoichiometry deviation of the formula from an integral ratio. The varying compositions correspond to varying concentrations of vacancies in iron atom sites. However, in systems such as these, ordering of the vacancies occurs at low temperatures, and the result may be a series of stoichiometric phases of slightly different compositions.

Although Fe 7 S 8 has a monoclinic superstructure that results from vacancy ordering Fig. Some of these pyrrhotites may represent ordered phases with clearly defined compositions Fe 9 S 10 , Fe 11 S 12 and so on , but more complex and partial ordering in these systems may occur.

Experimental studies of the phase relations in sulfide systems have done much to inform our understanding of the crystallization of sulfides from melts and high-temperature fluids. A few quaternary systems are particularly important, notably the Fe—Zn—As—S system.

Details of the work done in these areas can be found in Vaughan and Craig , and Fleet In the example illustrated in Figure 3, exsolved chalcopyrite yellow occurs as laths in host bornite brown ; the orientation of the laths is crystallographically controlled by the bornite host. The blue-grey phase is chalcocite, which was formed by later alteration of the bornite.

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.

An abundance of iron oxides will give the water a red color, which is most common. The water can also have other colors, for example green for copper.

In general, AMD can be hard to spot with the naked eye, requiring monitoring of recipients surface water and groundwater for analyzing pH and metal content. Even if ARD has been in progress since the start of a mining project, it can in some cases take years to detect. The ARD-process can start slowly, but as the pH is lowered and feedback mechanisms start it can rapidly increase and quickly get out of hand.

At this stage, the generation of AMD at a mine site can become difficult and expensive to mitigate. Acid mine drainage at an open pit. Acidic, metal-rich water can be seen at both the bottom lake of the pit and the pit walls. Photo: Pontus Westrin. A prediction of ARD will preferentially start during the exploration phase and continue during the whole life of mine.

Prediction of drainage quality is to be made both qualitatively and quantitatively. A qualitative prediction will evaluate if acidic conditions might develop in the mine waste.

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. The massive sulphide occurrences of the East Pacific Rise, and in part those of the Mid-Atlantic Ridge, contain mostly iron sulphide, which has no economic value.

The deposits in the Bismarck Sea east of Papua New Guinea are one example of economically promising massive sulphides. They have high contents of copper and zinc. The contents of gold and silver are also considerable. The concentration of gold in some of the deposits here is around 15 grams per tonne.

That is about 3 times as much as in typical deposits on land. The silver content here is commonly between and grams per tonne, with peak values of grams per tonne in the Solwara Field in the western Bismarck Sea. This is significantly higher than the concentrations in manganese nodules and cobalt crusts, which only reach values of about one gram of silver per tonne.

The highest proportions found on land are to grams of silver per tonne. Many chemical elements are found in relatively small amounts in massive sulphides, including manganese, bismuth, cadmium, gallium, germanium, antimony, tellurium, thallium and indium. In some deposits, however, especially at island-arc volcanoes, these elements can be more highly concentrated.

But by no means every massive sulphide occurrence is rich in precious metals. Even within a single region such as the Manus Basin of Papua New Guinea, occurrences are found with highly variable gold and silver contents.

Massive sulphides are notable for their high gold and silver content, which in part greatly exceeds that of manganese nodules and cobalt crusts. The contents fluctuate, therefore, not only from region to region, but also within a single massive sulphide occurrence or at an individual black smoker. This is because the temperature drops with increasing distance from the hydrothermal vent.

Minerals that are rich in copper often form in the core of the smoker. In the outer zone of the porous smokers the hot fluids are mixed with the cold seawater, and minerals with other metals are deposited, for example pyrite, sphalerite, or marcasite, which are rich in iron and zinc. This zonation is also observable at larger scales: at the margins of massive sulphide occurrences the smokers have lower outflow temperatures, so these precipitate different minerals.

Because expeditions in the past have often only taken massive sulphide samples directly from the chimneys themselves, it is still not well known how the metals are distributed within an area.

The composition of the massive sulphides varies not only with distance from the hot vent, however, but also with depth, and there is little data available regarding this. Only small numbers of expeditions or research ships have special drilling equipment available for taking samples.

In order to assess how profitable a deposit is and how high the metal content is, much additional drilling will be necessary in the future. First activity in the South Pacific Like the cobalt crust occurrences, important massive sulphide deposits are found not only in international waters of the high seas, but also in the Exclusive Economic Zones EEZ of a number of island states.

Here the appropriate local governments and not the International Seabed Authority will determine the conditions for future extraction activities.

The government there is working with a Canadian company which, in turn, includes participation by large commodities companies from Canada, Russia and South Africa. The plans were temporarily on hold due to arbitration proceedings related to the payment of project costs.

An arbitrator was finally able to bring the parties to an agreement in October It now appears that a contract will be awarded to a shipyard in the spring of for the construction of a special ship for massive sulphide mining. The seabed crawlers for working on the bottom have already been built. In the future, vehicles weighing from 3 to tonnes will be used: one large and one small rock cutter plus a collecting machine to retrieve the pieces of massive sulphide. According to the manufacturer, the technical challenges can be easily overcome.

The company has been producing heavy crawler vehicles called trenchers that are used to lay underwater cables. These have been operated in even deeper waters. The rock mixture will be pumped from the collecting machine into a large container that rises and sinks between the ship and sea floor.

The container is filled with blocks of massive sulphides on the bottom and then raised to the ship, emptied, and lowered to the sea floor again.

The partners expect mining operations to begin around The heavy chassis of the rock cutter, which will work on the sea floor, is ready. Off Papua New Guinea, the mining of massive sulphides should begin by Other states have just recently applied for, or will soon apply for exploration licences.

Germany is planning for the Indian Ocean, for example. The ISA will first have to rule on these applications. Overall, however, the same scenario is expected for massive sulphide deposits as will likely occur for the mining of cobalt crusts and manganese nodules: while mining in international waters will not happen in the immediate future, individual states, in cooperation with mining or resource concerns, could get a head start by beginning to mine in their own EEZs.

For Papua New Guinea, for example, mining is interesting because the massive sulphide deposits off their coast have high gold and silver contents. Extreme habitat, many specialized species Hydrothermal vents are not only providers of resources, but also extraordinary habitats. In spite of the hostile conditions, such as temperatures over degrees Celsius and the slightly acidic hydrothermal fluids enriched in toxic metal compounds, a unique natural community has evolved here over millions of years, perfectly adapted to the inhospitable environment.

Normally the sun is the source of energy for life in the ocean. It causes algae to flourish, which use the sunlight and photosynthesis to construct high-energy molecules like sugar.

This is called primary production , and is the base of the food web in the ocean. But it is dark at the hydrothermal vents. Primary production here is performed by chemoautotrophic bacteria, which exploit the energy-rich chemical compounds found at the hydrothermal vents and alter them into molecules that can also be used by other organisms.

The bacteria can endure water temperatures greater than degrees Celsius and thus occur near the smokers. The bacteria or the products of their metabolism provide nourishment for higher organisms such as mussels, and these in turn for other organisms.

Communities with up to different species can thus be found at the vents, including, for example, snails of the genus Alviniconcha, which can tolerate temperatures up to 45 degrees Celsius. Many of these animal groups live exclusively at hydrothermal vents. Because of the continuous influx of nutrients, the organisms are present in great numbers.

Expeditions have sometimes recorded hundreds to thousands of animals within one square metre of sea floor. Whether there are endemic species living at the hydrothermal vents in the deep sea that only occur in a limited area, or in extreme cases only at a single massive sulphide deposit, is a vital question for mining, because it could bring about their extinction.

Biologists are thus trying to determine the extent of distribution of certain species — whether they live in a larger oceanic region like the Indian Ocean at numerous hydrothermal vents or are limited to a smaller region such as the Bismarck Sea. In fact, scientists have found differences between different ocean regions.