Chalcophilic metals

Occurrence. Selenium is occasionally found as native. Minerals are rare and occur together with the sulphides of chalcophilic metals. Sometimes the minerals are partially oxidized (MSe03 2H20 with M = Ni, Cu, Pb). 

We used a Thermo X7 ICPMS coupled to a New Wave Research 213 nm UV laser to determine the PGE and Au in the sulfides. Other chalcophile metals (Ag, As, Bi, Cd, Co, Cu, Fe, Ni, Pb, Re, Sb, Se, Sn, Te and Zn) were also monitored. Analytical conditions were beam size of 80 pm laser pulse rate of 10 Hz laser output power of 0.3 mJ/pulse to ablate the sulfide for 60s after a 20s gas blank was collected. Sulfide standards were used to... 

Lithophile (silicate) Siderophile and chalcophile (metal and sulfide)... 

None of the three elements is particularly abundant in the earth s crust though several minerals contain them as major constituents. As can be seen from Table 13.1, arsenic occurs about halfway down the elements in order of abundance, grouped with several others near 2 ppm. Antimony has only one-tenth of this abundance and Bi, down by a further factor of 20 or more, is about as unabundant as several of the commoner platinum metals and gold. In common with all the post-transition-element metals. As, Sb and Bi are chalcophiles, i.e. they occur in association with the chalcogens S, Se and Te rather than as oxides and silicates. 

Note It is sometimes convenient to describe the element distribution between sulfides and oxides as chalcophiles (occurring in the Earth s crust as sulfides) and lithophiles (predominating as oxides and halides in the Earth s crust) (see Fig. 1.5). This geochemical classification includes also the siderophiles (remaining as metals or alloys, especially in the Earth s core) and the atmophiles (which occurs largely in volatile form in the atmosphere and dissolved in the oceans). 

Occurrence. Thallium can be associated to heavy metals that occur in sulphidic ores (chalcophilic element behaviour) or to alkali metals in minerals such as car-nallite, sylvite, mica (the Tl+1 ion behaves as an alkali metal ion), or in true, but very rare, thallium minerals such as lorandite (T1AsS2), chalcothallite (Cu3T1S2). 

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

Estimates of the Mars core composition by the authors listed above suggest it is made of metal plus iron sulfide, the latter varying from 29 to 44 wt.%. Abundances of siderophile (tungsten, phosphorus, cobalt, molybdenum, nickel) and chalcophile (indium, copper) elements in the mantle (Fig. 13.23) are consistent with equilibrium between sulfide, metal, and mantle silicate at high temperature and pressure (Righter and Drake, 1996). 

Most chalcophile elements (i.e, S and other elements with an affinity for S in nature such as Cu, As, Se, Cd, In, and W), boron, and the halogens are enriched in coal with respect to soil, and this accounts in part for their enrichment in emitted particles. Differences between eastern and western coals are apparent for many elements, especially the alkali and alkaline earth metals, As, and In. This accounts for some of the large plant-to-plant variability that we observe below. 

Goldschmidt s ideas on the primary distribution of the elements in the Earth have not been seriously challenged (see, however, Bums and Fyfe, 1966a). From studies of minerals in meteorites and phases from blast furnaces, Golschmidt classified the elements as siderophilic if they are inert (relative to iron) and enter the metallic phase, chalcophilic if they are concentrated in sulphides, lithophilic if they are concentrated in silicates and atmophilic if they are gaseous and are present in the atmosphere. Those elements enriched in organisms were also classed as biophilic. 

Another manifestation of covalent bonding relates to the sulphide mineralogy of the transition elements. Although earlier chapters have stressed properties of transition metal ions in oxides and silicates, an important feature of these elements is the frequency of their geochemical association with B-sub-group non-metal and metalloid elements such as sulphur, selenium, tellurium, phophorus, arsenic and antimony. The chalcophilic properties of iron, cobalt, nickel and copper in the crust are well known and are important eco-... 

The third case includes those ligands which possess both filled and empty n orbitals. Examples are the Br, Cl" and CN" anions. However, in order to understand the chalcophilic properties of transition elements, only the second case involving n-bonding with metal t2g orbitals needs to be considered. 

Many less electropositive metals known as chalcophiles are found commonly as sulphide minerals some important examples are pyrites (FeS2), sphalerite (zinc blende, ZnS), molybdenite (MoS2), cinnabar (HgS) and galena (PbS). Volatile sulphur compounds such as H2S and organic compounds are also found... 

In geochemistry, these two principles resulted in the differentiation of the metallic elements as lithophiles (hard acids see Lithophiles) and chalcophiles (borderline and soft acids see Chalcophiles) during the formation of the earth, and continue to affect the chemistry that occurs in the black smokers at the midoceanic ridges these are also the organizing principles behind the familiar qualitative analysis scheme of the cations. ... 

The question of the similarities and differences between the metal-oxygen bond and the metal-sulfur bond is fundamental in geochemistry, and relates to the lithophile versus chalcophile nature of particular elements. A full discussion is presented in Chapter 8. 

Figure 3 Cl-normalized siderophile-chalcophile element ratios in acapulcoite-lodranite clan (ale), winonaite-IAB-iron silicate inclusion clan (wic) and some unique meteorites showing the effects of melting in the metal-sulhde system. Residual solid metal will have high Ir/Ni and low Se/Co, while metallic melt will have the opposite characteristics. Data from sources listed in the text, except for Divnoe (Petaev et al., 1994 Weigel et al., 1997) and Enon (Kallemeyn and Wasson, 1985).

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