Arsenic minerals, Table

Arsenopyrite (FeAsS), an arsenosulfide, is the most common arsenic mineral on Earth (Welch et al., 2000), 597. The mineral occurs in a variety of hydrothermal deposits and some metamorphic and intrusive igneous rocks Table 2.5 (Klein, 2002), 369. As mentioned earlier in this chapter, the crystalline structure of... 

The results of simultaneous weighted least-squares regression of the data and some of the unfitted but derived quantities are shown in Tables la, lb, and 2. Table la displays elemental arsenic, its simple oxides, and the reactions for arsenic oxidizing to arsenic trioxides. Table lb introduces the hydrolysis species for As(III) and As(V) in solution, the hydrolysis reactions, and the solubility reactions for the simple oxides. Single species are shown at the top of each table with the reactions underneath. The following discussion describes some of the mineral occurrences for these substances, describes their relative stabilities from field observations, and considers the implications of the evaluated thermodynamic data in terms of these occurrences. 

Raman spectroscopy was used to characterise a range of arsenate minerals of the vivianite type.589 IR and Raman spectra were assigned on the basis of factor group analysis for Cd2As207, Table 13.590 Ab initio calculations have given values for the vibrational wavenumbers for As4Ofi.591... 

The affinity of arsenic for sulfur is seen in the list of common minerals. Table 1. Readers may recall that arsenic is usually precipitated as the sulfide in qualitative analysis schemes. Many proteins contain -SH groups and these readily react with arsenic compounds in both oxidation states. As(V) is reduced to As(III), which then binds to the protein and, as a consequence, interferes with its normal function. 

Arsenic is widely distributed about the earth and has a terrestrial abundance of approximately 5 g/t (4). Over 150 arsenic-bearing minerals are known (1). Table 2 fists the most common minerals. The most important commercial source of arsenic, however, is as a by-product from the treatment of copper, lead, cobalt, and gold ores. The quantity of arsenic usually associated with lead and copper ores may range from a trace to 2 —3%, whereas the gold ores found in Sweden contain 7—11% arsenic. Small quantities of elemental arsenic have been found in a number of localities. 

Table 2. Naturally Occurring Arsenic-Bearing Minerals...

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. 

Arsenic is a major constituent of at least 245 mineral species, of which arsenopyrite is the most common (NAS 1977). In general, background concentrations of arsenic are 0.2 to 15 mg/kg in the lithosphere, 0.005 to 0.1 pg/m3 in air, <10 pg/L in water, and <15 mg/kg in soil (NRCC 1978 ATSDR 1992). The commercial use and production of arsenic compounds have raised local concentrations in the environment far above the natural background concentrations (Table 28.1). 

Occurrence. Nearly all the silver ores are compounds of silver with sulphur and the neighbours in the Periodic Table arsenic, antimony and bismuth (argentite Ag2S, the most common silver compound, pyrargyrite Ag3SbS3, proustite Ag3AsS3). Other silver minerals include the halides. Silver is found sometimes as the free metal. Secondary silver (from catalysts, scraps, photographic films, etc.) is an important source. 

Certain magnesium calcium arsenates, sometimes associated with other metals, are found in Nature and are enumerated in the table of minerals, pp. 14-16. 

Native Arsenic—Compounds of Arsenic—Tables of Minerals Containing Arsenic —The Ubiquity of Arsenic. 

Table 2.5 Relatively common arsenic-bearing minerals (Klein, 2002 O Day, 2006 Mandal and Suzuki, 2002 Smedley and Kinniburgh, 2002 Mozgova et al., 2005 Utsunomiya et dl., 2003 Dunn, Pecor and Newberry, 1980).

Table 2.7 Typical arsenic concentrations in selected minerals and other solid substances where arsenic is not a major component. In synthetic and rare natural samples, arsenic concentrations may be much higher (e.g. jarosites in Savage, Bird and O Day (2005) and calcite in Di Benedetto et ol. (2006)). 

As discussed in Chapters 5 and 7, the use of lime to precipitate calcium arsenates is a common method for removing inorganic As(V) from water or flue gases. Calcium arsenates were also once extensively used in pesticides (Chapter 5). The compositions of some calcium arsenates, such as johnbaumite (Ca5(As04)3(0H) Table 2.5), resemble the very common phosphate mineral, apatite (Ca5(P04)3(F,Cl,0H)), where arsenate replaces phosphate. Some lead arsenates, such as mimetite (Pb5(As04)3Cl Table 2.5), also have crystalline structures that are related to apatite. Mimetite may occur in oxidized lead-rich hydrothermal deposits. 

Temperature, humidity, precipitation, and evaporation are important factors that contribute to the oxidation of sulfide minerals. In warm and wet climates, excessive precipitation may produce persistently high water tables and extensive biological activity that may create reducing conditions in the shallow subsurface and hinder sulfide oxidation (Seal et al., 2002, 208). At the surface, high humidity and temperatures would promote the oxidation of sulfide minerals (Williams, 2001, 274). Frequent precipitation would also suppress evaporation and the formation of arsenic salt deposits (Seal et al., 2002, 208). Furthermore, precipitation and groundwater, which are controlled by climate, are the major sources of water for the production of arsenic-contaminated runoff from sulfide-bearing rock outcrops. 

Groundwaters in the loess aquifers of La Pampa, Argentina, may contain > 5 mgL-1 of arsenic ((Smed-ley et al., 2005 Smedley et al., 2002) Table 3.13). The aquifers are very oxidizing and As(V) is the dominant arsenic species. The alkalinity of the groundwaters (pH 7.0-8.8) and the unusual presence of vanadium probably hinders the sorption of As(V) onto iron and manganese (oxy)(hydr)oxides. The alkalinity and vanadium may also desorb arsenic from the minerals (Smedley et al., 2005). 

Some limestones and dolostones are relatively rich in arsenic because they contain significant hydrothermal or diagenetic sulfide minerals. In particular, the Suwannee Limestone of Florida, USA, contains up to 54 mg kg-1 of arsenic (Table 3.23). Almost all of the arsenic is associated with diagenetic pyrite. The pyrites typically contain 100-11 200 mg kg-1 of arsenic (an average of 2300 mg kg-1 for 25 samples) (Price and Pichler, 2006). 

Phosphorites are sedimentary rocks that contain at least 15-20 wt % P2O5 (Boggs, 1995), 266. The phosphate in phosphorites primarily occurs as apatite (Ca5(P04)3(F,Cl,0H)). Typically, phosphorites chemically precipitate in deep, cold marine waters. Due to chemical similarities, arsenate may partially substitute for phosphate and the arsenic concentrations of phosphorites may exceed 100 mg kg-1 ((Matschullat, 2000), 299 Table 3.23). However, arsenic concentrations in some phosphorites (e.g. southeast Jordan) are relatively low (7-9 mg kg-1) and the arsenic is mostly associated with clay and carbonate minerals rather than phosphates (Al-Hwaiti, Matheis and Saffarini, 2005). 

In metamorphosed sedimentary rocks, arsenic tends to occur in oxide and sulfide minerals (Bebout et al., 1999), 69-70. Many metamorphic rocks simply inherit their arsenic from their precursor rocks. That is, unless arsenic-rich metamorphic fluids are introduced, quartzites metamorphosed from low-arsenic quartz-rich sandstones and marbles metamorphosed from low-arsenic limestones should have relatively little arsenic. In contrast, shales often contain more arsenic than sandstones and limestones (Table 3.23). Therefore, slates and phyllites that form from the metamorphism of shales should inherit at least some of the arsenic (Table 3.24). 

Of the metal sorbents, amorphous to poorly crystalline iron (oxy)(hydr)oxides are most efficient at sorption because of their large surface areas (Chapters 2,3, and 7). However, as these compounds crystallize into hematite, magnetite, or other minerals, their surface areas decrease. Although the affinity of the iron (oxy)(hydr)oxides to sorb arsenic may not always change very much as a result of crystallization (Dixit and Hering, 2003), the reduction of surface area may lead to the release of surface-complexed arsenic (O Shea, 2006). Smedley and Kinniburgh (2002) provide a detailed list of sorption studies dealing with metal (oxy)(hydr)oxides (Table 6.1). 

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