LECTURE 3 - NATURAL GEOCHEMICAL ENRICHMENTS OF ELEMENTS

Transcription

1 LECTURE 3 - NATURAL GEOCHEMICAL ENRICHMENTS OF ELEMENTS Note: Slide numbers refer to the PowerPoint presentation which accompanies the lecture. Natural Enrichments, slide 1 here INTRODUCTION In lecture two it was observed that the average concentration of many elements, particularly those with odd atomic numbers or those with atomic numbers beyond 27, are very low. Many elements important in modern industrial societies fall into his category. Natural Enrichments, slide 2 here These include copper, widely used for electrical wiring, plumbing, alloys, and other purposes; many elements, such as vanadium, chromium, nickel, niobium, used for alloying iron to make various types of steel; cadmium and nickel, used for rechargeable batteries; lithium, used in newer rechargeable batteries; zinc, used in galvanized steel; gold, silver, platinum, palladium, other similar metals used for jewelry, specialized scientific applications, and as catalysts; mercury, with many specialized uses as a liquid metal; and many other applications of metals. Uranium and thorium, used for nuclear fuels, also are scarce. Natural Enrichments, slide 3 here Most of these elements could not be mined, processed, and formulated into useful products at reasonable costs if they occurred everywhere at their average abundances in the crust. For example, millions of tons of gold exist in seawater, but the cost of obtaining pure gold from seawater is many times the value of the gold. We seek places in nature where these elements have been concentrated by natural processes to reduce the costs of mining and processing. Natural concentrations also mean that less waste will be generated. Natural Enrichments, slide 4 here We need to answer two questions: 1. For any given element, how much enrichment above natural abundance values is needed to produce a mineable ore? 2. What geochemical processes are responsible for producing these natural elemental enrichments? 1

2 Note that even common metals, like iron and aluminum, are usually mined from naturally enriched deposits because it is less costly. HOW MUCH ENRICHMENT IS NECESSARY TO CREATE AN ORE? Natural Enrichments, slide 5 here Ore may be defined as, "The naturally occurring material from which a mineral or minerals of economic value can be extracted at a reasonable profit" (Bates and Jackson, 1980). Profit requires the ability to find, mine, process, and sell the resulting metal at a price above the sum of the costs. Many factors will influence cost. Some of these are: Natural Enrichments, slide 6 here 1. Exploration costs - The location of ores is generally a laborious task. In recent years the tools of exploration geochemistry have been applied to this process. 2. Cost of mineral rights acquisition. 3. Cost of mining, including the cost of compliance with existing environmental regulations. Today, this often includes restoration of the mined land upon completion of the project. Restoration procedures should insure that the mined land will not become environmentally hazardous in the future. In the past, these environmental costs were usually totally ignored. This has resulted in the estimated 500,000 abandoned mines in the United States. Many of these mines are environmentally dangerous and many produce "visual pollution," that is scarring of the land at the surface. 4. Cost of separating and processing the ore, again including the cost of compliance with existing environmental regulations. 5. Cost of transporting the ore and the finished product. Often the transportation costs are the most important factor in determining whether a given natural enrichment is ore or not. Once these, and any other relevant costs, are known we can estimate the grade of material required to make an ore of a given element. Estimating grades as concentration is possible (weight percent or parts per million), but another measure is often more useful. Natural Enrichments, slide 7 here The Clarke (named in honor of F.W. Clarke) is defined as the average abundance of an element in the crust of the earth. 2

3 Natural Enrichments, slide 8 here The Clarke of concentration is the concentration of an element in a rock compared with its average concentration in the earth's crust, or of an element within a particular mineral Natural Enrichments, slide 9 here (Example: The Clarke of copper is about 55 ppm, or 0.006%. In the mineral chalcocite, Cu 2 S, the Cu concentration is 79.8%. Thus, the Clarke of concentration within this mineral is 13,300.) Ores occur when the Clarke of concentration at a particular location is sufficient to meet the aforementioned cost factors. Each site must be evaluated on a case by case basis. Cost factors, particularly the transportation costs, may vary significantly from one location to another. Nevertheless, making some useful generalizations about the clarkes of concentration, usually required to make ore, is possible. Natural Enrichments, slide 10 here Table 3-1 presents the clarkes and the clarkes of concentration required to make an ore body for several elements. It should be noted that the Clarke of concentration can and does change over time. For example the Clarke of concentration for copper is listed as 160 in the literature (Mason and Moore, 1982). By the mid 1980's, copper was being mined at Morenci, Arizona at a Clarke of concentration of about 40. Later copper prices dived on the world market and the Clarke of concentration climbed. Examination of Table 3-1 reveals that elements present with high clarkes (such as Fe and Al) generally have low clarkes of concentration. Elements with very low clarkes (Ag, Au) have high clarkes of concentration. Many deviations from these generalizations exist, particularly among elements with intermediate to low clarkes. To see why this is so we need to examine the geochemical classification of elements. 3

4 Table 3-1 Concentration Clarkes for Ore Bodies of Selected Metals Metal Clarke Minimum metal % for profitable extraction Clarke of Concentration Al Fe Mn Cr Cu Ni Zn Sn Pb U Ag Au After Mason and Moore, 1982; Baumann, 1976; and authors knowledge of more recent figures GEOCHEMICAL CLASSIFICATION OF ELEMENTS Natural Enrichments, slide 11 here The early earth was probably bombarded by solid planetesimals, primarily chondritic meteorites. Chondritic meteorites may be left over from the protoplanet stage of the solar system. Chondritic meteorites are composed of three different phases, or combinations of these phases. Natural Enrichments, slide 12 here 4

5 The phases are nickel-iron metal, iron sulfide, and silicates, largely olivine or pyroxene. Various chemical elements distribute themselves among these phases according to their relative affinity for silicate, for sulfide, or for metallic phases. Iron is more abundant than magnesium or silicon in meteorites. Thus, three immiscible phases formed, with iron present in all three phases. Enough oxygen plus sulfur to combine with all of the cations does not exist, so a metallic phase is present. All other cations had to compete with iron for anions. Reactions of two types were possible, with M representing a non-iron cation. Natural Enrichments, slide 13 here (3-1) (3-2) Thus, the free energies of the metallic silicates or sulfides in relation to the free energy of the iron silicate or sulfide determine the distribution of the non-iron cations. Metals more electropositive than iron can displace iron from the silicate phases. Elements, less electropositive than iron, were displaced by iron from the combined phases and were concentrated in the metallic phase. The sulfide phase attracted elements able to form covalent, rather than ionic, bonds with the sulfur. Study of the meteorites suggests that element distributions are controlled by the affinity of each element for one of the major phases present. These affinities are the result of electronic configurations of the elements, which control their chemical bonding characteristics. These same processes might govern the distribution of elements within the earth. V.M. Goldschmidt (1937, 1954) was the first to point out that the primary differentiation of elements was based on geochemistry, not density. He introduced four new terms to describe the chemical affinity of elements in the earth. These are: Natural Enrichments, slide 14 here 5

6 Siderophile: Elements concentrated in the metallic phase, along with metallic iron. Chalcophile: Elements concentrated in the sulfide phase. Lithophile: Elements concentrated in the silicate phase. Atmophile: Elements concentrated in the atmosphere. Goldschmidt's ideas were first proposed in , when few quantitative data were available to buttress his arguments. Laboratory experiments on the distribution of elements from a liquid phase to metal, sulfide, and silicate phases had not been done, and were of great difficulty. Thus Goldschmidt based his arguments on meteorites, a sort of "fossilized" experiment. Natural Enrichments, slide 15 here Many meteorites consist of the phases nickel-iron (metal), troilite (sulfide), and silicate, and have probably condensed from a liquid phase. By mechanically separating these phases and measuring the chemistry of each, the affinity of various elements for each phase could be determined. In addition, data from smelting processes, in which metals separate from a melt into either a metallic phase or a slag phase (silicates), or a matte rich in sulfides. Based on these measurements, elements can then be separated into Goldschmidt's categories. Natural Enrichments, slide 16 here Table 3-2 presents the classification of elements in these terms. The chart shows that some elements do not show a clear affinity for one category. Chemical affinity depends on temperature, pressure, and the total chemical environment. Elements that belong to one category in meteorites may belong to another category within the earth, particularly within the crust. 6

7 Table 3-2 Geochemical Classification of Elements Siderophile Chalcophile Lithophile Atmophile Fe * Co * Ni * (Cu) Ag Li Na K Rb Cs (H) N (O) Ru Rh Pd Zn Cd Hg Be Mg Ca Sr Ba (Cl) (Br) (I) Os Ir Pt Ga In Tl B Al Sc Y REE He Ne Ar Au Re + Mo + (Ge) (Sn) Pb Si Ti Zr Hf Th Kr Xe Ge * Sn * W ++ (As) (Sb) Bi P V Nb Ta C ++ Cu * Ga * S Se Te O Cr U (P) As + Sb + (Fe) Mo (Os) H F Cl Br I (Ru) (Rh) (Pd) (Fe) Mn (Zn) (Ga) * Elements are chalcophile and lithophile in the earth's crust. + Elements are chalcophile in the earth's crust ++ Elements are lithophile in the earth's crust () Elements show affinity for more than one group. Secondary group(s) are shown in parentheses. After Mason and Moore (1982); Brownlow (1979) Examples of mixed preferences include chromium, which is strongly lithophile in the presence of oxygen, but, under the reducing conditions found in meteorites, is chalcophile. Carbon is lithophile or atmophile under oxidizing conditions, but siderophile under reducing conditions. Phosphorous is lithophile under oxidizing conditions, siderophile under reducing conditions. In the crust, insufficient enough iron for nickel and cobalt to combine exists within a metallic phase, so these elements are chalcophile in the crust. Natural Enrichments, slide 17 here Another problem with Goldschmidt's classification arises for trace elements. No exact definition of a trace element has been established but a useful working definition of a trace element is one whose concentration is less than 0.1%. Trace elements may form their own minerals. Then they usually obey the Goldschmidt classification. For example, all Tl minerals are sulfides. Typically, 7

8 however, trace elements will follow a major element into another mineral, where they replace part of the major element. Thallium usually substitutes for potassium ion. Since potassium is lithophile, this makes thallium seem lithophile. Goldschmidt's classification should be treated as a qualitative indicator only. It cannot be used to make detailed predictions of element distribution under all of the various conditions that are found within the earth. The fact that the classification scheme is generally valid may be attributed to the similarity among the electronic configurations of groups of elements. The electronic configurations that may be associated with each category are as follows: Natural Enrichments, slide 18 here Siderophile: Elements whose valence electrons are not readily available for combination with other elements. Positive charge on the nucleus, at least under certain conditions, exerts a strong attraction on the outer electrons, preventing combination. These elements usually occur in the native state. Natural Enrichments, slide 19 here Chalcophile: Elements whose valence electrons may be shared, but are not electropositive enough to donate electrons or electronegative enough to accept electrons. Thus, the bonds formed are predominantly covalent. Since sulfur is much less electronegative than oxygen, sulfur is prone to form covalent bonds with these elements. Generally the chalcophile elements have their valence electrons outside a shell of 18 electrons. Natural Enrichments, slide 20 here Lithophile: Elements that are strongly electropositive or electronegative and thus typically donate or accept electrons, forming ionic bonds. Most silicate minerals have oxygen ions that can form ionic bonds to metal cations. Generally the lithophile elements have their valence electrons outside a shell of eight electrons. 8

9 Natural Enrichments, slide 21 here Atmophile: Elements that do not readily combine with other elements, or which form diatomic molecules held together in the solid or liquid states only by very weak Van der Waal forces. All of the inert gases, with completed shells or subshells, fall into this category. Natural Enrichments, slide 22 here It should be noted that oxygen, although classified as a secondarily atmophile element, would not occur in the atmosphere of the earth if the earth were at chemical equilibrium. Oxygen is maintained in the atmosphere only by the continual photosynthesis within the biosphere. Indeed, the presence of oxygen in an atmosphere is often regarded as an indicator of life on the planet. Natural Enrichments, slide 23 here Goldschmidt noted one other aspect of atomic behavior that correlates with his classification scheme. If a plot is made of atomic volume versus atomic number, the plot shows various maxima and minima. Siderophile elements occur near the minima. Chalcophile elements occur where the atomic volume is increasing compared with atomic number. Atmophile elements follow the chalcophile and are near the maxima. The lithophile elements are generally from the maxima on down the declining side of the curve. References Robert L. Bates, and Julia A. Jackson, Editors, Glossary of Geology, Second edition, American Geological Institute, Falls Church, Virginia, Ludwig Baumann, Introduction to Ore Deposits, John Wiley and Sons, New York,

10 Arthur H. Brownlow, Geochemistry, Prentice-Hall, Englewood Cliffs, N.J., V.M. Goldschmidt, The principles of the distribution of the chemical elements in minerals and rocks, J. Chem. Soc., , V.M. Goldschmidt, Geochemistry, Oxford University Press, London, Brian Mason, and Carleton B. Moore, Principles of Geochemistry, Fourth edition, John Wiley and Sons, New York, LN03_F16.pdf August 18,