Geochemical Classification of Elements PDF

Few lines about the legendry Goldschmidt Victor Goldschmidt (1888-1947) is often considered the ‘father of geochemistry’. Goldschmidt earned a Ph.D. from the University of Oslo in 1911 and remained there until 1929, when he assumed the directorship of the Geo- chemisches Institut a...

Few lines about the legendry Goldschmidt Victor Goldschmidt (1888-1947) is often considered the ‘father of geochemistry’. Goldschmidt earned a Ph.D. from the University of Oslo in 1911 and remained there until 1929, when he assumed the directorship of the Geo- chemisches Institut at the University of Göttingen. Because of the worsening political situation in Germany, he returned to Oslo in 1935. He was for a time imprisoned in a concentration camp after Germany invaded Norway in 1940. In 1942 he fled to Sweden and eventually to England. He returned to Oslo in 1946 but never fully recovered from the effects of imprisonment and died a year later. The Geochemical Society has named its most prestigious medal after him and co-sponsors, along with the European Association of Geochemistry, annual Goldschmidt Conferences.  Geochemical Classification of Elements by Goldschmidt - Based on (i) The way in which elements actually distribute themselves between different kinds of liquids and gas phases. (ii) During the smelting of oxides and sulphides three different kinds of liquids are encountered that are immiscible with each other and that segregate into layers depending on their density. (iii) These liquids are composed of - Molten Fe - Molten sulphide (called matte) - Molten silicates (called slag) - Goldschmidt et al., considered that it is likely that the Earth was initially completely molten and these liquids separated from each other under the influence of gravity to form a Fe-core, a sulphide layer and a silicate layer. - The gases formed the atmosphere which subsequently produced the hydrosphere by condensation of H2O vapour. - The heat required for this melting was provided by (i) Impacts of planetesimals (ii) By compression caused by gravitational contraction (iii) By migration of dense phases towards the centre of Fe-core the earth (iv) By decay of radioactive isotopes of U, Th and K Sulphide layer which were much more abundant at the time of formation of the earth 4.6 Ga ago. Silicate layer - Although the earth doesn’t contain a sulphide layer, Goldschmidt’s classification convey a lot of information about the tendencies of the elements to enter liquids of different composition. - The information on which this classification is based came from the study of meteorites and from smelting of the Kuperschiefer ore ( Copper slate), Germany. - Some of these asteroids are large enough to retain sufficient heat for melting and differentiated into (i) Metallic Fe (ii) Silicate rocks (iii) Sulphide minerals - These bodies subsequently broken up due to gravitational influence of Jupiter and Mars and by collision among themselves - Sometimes some of these fragments are deflected into earth to fall as meteorites. - Goldschmidt et al., analysed metallic, sulphide and silicate phases of many such meteorites and from these results determined how the elements have been partitioned into the three immiscible liquids during the geochemical differentiation of parent bodies of meteorites. - The information derived from this study of meteorites were consistent with the chemical composition of silicate slag, Fe-Cu sulphide matte and metallic Fe all of which form during smelting operations. -The results of these experiments have given rise to the famous “ Goldschimdt’s Geochemical Classification of Elements”. -According to this the elements have been classified as : - (i) Siderophile : Iron Liquid ( Greek letter Sideros : Iron) - (ii) Chalcophile : Sulphide Liquid ( Greek letter Chalcos : Copper) - (iii) Lithophile : Silicate Liquid ( Greek letter Lithos : Stone) - (iv) Atmophile Gas phase (Greek letter Atmos : Vapours and steam) Siderophile elements : Elements which have affinity for metallic iron. In periodic table these are elements of Group VIIIB + Mo, Au, Ge, Sn, C, P. -They are depleted in the silicate portion of the earth and presumably concentrated in the core. Chalcophile elements : Affinity for sulphides. In periodic table these are elements of Group IB, IIB, Ga, In, Tl, Pb, As, Sb, Bi, S, Se, Te, Po, Ag. -They are also depleted in the silicate earth and may be concentrated in the core. Many sulfide ore deposits originated from aqueous fluids rather than sulfide liquid. A chalcophile element need not necessarily be concentrated in such deposits. As it works out, however, they generally are. Most elements that are siderophile are usually also somewhat chalcophile and visa versa. Lithophile elements : Affinity for silicates. Include alkali metals (Gr-1A), alkaline earths(Gr IIA), halogens (Gr-VII A), B, Al, O, Si some transition metals such as Sc, Ti, V, Cr, Mn + U, Th, REE. They are concentrated in the silicate portion (crust and mantle) of the earth. Atmophile elements : Affinity for atmosphere. Include noble gases (Gr-VIII A) They are generally extremely volatile (i.e., they form gases or liquids at the surface of the Earth) and are concentrated in the atmosphere and hydrosphere. Geochemical Classification of Elements In the classification scheme of Goldschmidt, elements are divided according to how they partition between coexisting silicate liquid, sulfide liquid, metallic liquid, and gas phase…defined by examining ore smelting slags and meteorites Melting a chondrite gives 3 immiscible liquids plus vapor: Gas Phase Atmophile H, C, N, O, Noble gases (Gr. VIIIA) Alkalis, Alkaline Earths, Silicate Liquid Lithophile Halogens, B, O, Al, Si, Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Lanthanides, Hf, Ta, Th, U Sulfide Liquid Chalcophile Cu, Zn, Ga, Ag, Cd, In, Hg, Tl, As, S, Sb, Se, Pb, Bi, Te Metallic Liquid Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Siderophile Pt, Mo, Re, Au, C, P, Ge, Sn To first order, the distribution of elements between core and mantle resembles equilibrium partitioning between metal liquid and silicates…confirmed by iron and achondrite meteorites (but at high P, no separate sulfide phase) 5 Geochemical Affinity and Electronic Chemistry OK, but what makes an element siderophile or lithophile? Notably, the Goldschmidt categories are well-grouped in the periodic table of the elements: IA IIA Atmophile Siderophile IIIA IVA VA VIA VIIA VIIIA 1 2 1 H Lithophile Artificial He 3 4 5 6 7 8 9 10 2 Li Be Chalcophile B C N O F Ne 11 12 13 14 15 16 17 18 3 Na Mg IIIB IVB VB VIB VIIB VIIIB IB IIB Al Si P S Cl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 6 Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 87 88 104 105 106 107 108 109 7 Fr Ra Rf Db Sg Bh Hs Mt 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Lanthanides La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Actinides Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 6 Exceptions to Goldschmidt’d rule -O- is lithophile rather than atmophile - C and P dissolve in metallic Fe in absence of O - Tl is chalcophile although it substitutes for K + in silicate melts - Au, Sn and Mo are siderophile and presumably carried away by metallic Fe to form the core of the earth. - Similarly Ni and Co occur as sulphides in ore deposits but prefer Fe-liquid when given a chance. -Ti, Cr, Mn are commonly associated with Fe in igneous rocks even if they are not siderophiles. -Causes behind differential affinity - Chondrite meteorites: Three phases viz. Ni-Fe, Fe-S, siliceous minerals (Ol, Px) - Elements within these phases are distributed according to their relative affinity for metal, sulphide and silicate phase. - Composition of these phases in meteorites have been essentially determined by equilibrium in Fe-Mg-Si-O-S system. -This resulted into 3-immiscible phases: Fe-Mg silicate Fe-sulphide Free Fe - The distribution of rest of the electropositive element is achieved by : M + Fe-silicate → M-silicate+Fe M + Fe-sulphide→ M-sulphide+Fe Factors : (i) Fe- is the dominant constituent it is enriched in all 3 phases (ii) Elements more electropositive than Fe can replace Fe from the silicate and sulphide phases according to this above reaction. - Elements less electropositive than Fe are replaced by Fe from ionic compounds. - Sulphide phases attract only those phases (i) that form essentially homopolar compounds (i.e. covalent bonds) with S, (ii) metalloids and elements those cannot coexist in ionic environment. - eg. U and Th although are of high density are strongly concentrated in earth’s crust as oxides and sulphides. This implies distribution of elements are also controlled by gravity but by their chemical potential. Goldschmidt’s Geochemical Classification of Elements vis-à-vis periodic table: (i) Lithophile : Readily form ions with an outermost 8-electrons eg. Na + and Cl- - (ii) Chalcophile : Readily form ions with an outermost shell of 18-electrons (iii) Siderophile: Outermost shells are for the most part incompletely filled. - Goldschmidt’s Geochemical Classification of Elements vis-à-vis atomic volume : Lithophiles Atomic volume Atmophiles Chalcophiles Siderophiles Atomic Number Goldschmidt’s Geochemical Classification of Elements vis-à-vis geochemical character: -Elements with high +ve potential (1-3volts) : Lithophile, eg. Alkali and Alkaline Earth metals High –ve potential : Siderophiles eg. Noble metals Goldschmidt’s Geochemical Classification of Elements vis-à-vis electronegativity: Lithophile elements :Electronegativity < 1.7 Chalcophile elements :Electronegativity 1.8 to 2.2 ( 1.7 to 2.5) Siderophile elements :Electronegativity > 2.2 -Lithophile elements also have either very low electronegativities or very high ones and tend to form ionic bonds (although the basic silicate bond, the Si—O bond, is only about 50% ionic, metal–oxygen bonds in silicates are dominantly ionic). -Siderophile and chalcophile elements have intermediate electronegativities and tend to form covalent or metallic bonds. Limitations of Goldschmidt’s Geochemical Classification of Elements: - Goldschmidt's classification is relevant mainly to distribution of elements in meteorites and to how elements distribute themselves between the Earth's major geochemical reservoirs: the core, the mantle and crust, and the hydrosphere and atmosphere. - Since there is an over abundance of O in the outer part of the Earth, metallic liquids do not form, and siderophile elements have little opportunity to behave as such. - Similarly, sufficient S is rarely available to form more than trace amount of sulfides. - As a result, siderophile elements such as Ni and chalcophile elements such as Pb occur mainly in silicate phases in the crust and mantle.  The Geochemical Periodic Table -Hence the elements have been grouped based on how they behave in the silicate portion of the Earth, the mantle and crust (see this table below). LILE Lanthanide rare ea Actinide rare earth (The Geochemical Periodic Table, in which elements are grouped according to their geochemical behavior) ThE Volatile Elements : - Includes H, N and the noble gases from He to Xe. -The defining feature of the noble gases is their filled outer electron shell, making them chemically inert as well as volatile. -Hence, they are never chemically bound in rocks and minerals. -Except for He, they have rather large radii and cannot easily be accommodated in either cationic or anionic lattice sites of many minerals. -Thus they are typically present at very low concentrations. -Their concentrations are usually reported in STP cm 3/g at (i.e., cm3/g at standard temperature and pressure: 273 K and 01.MPa; 1 cm3/g = 4.46×10–5 moles/g). -Concentrations in silicate rocks and minerals typically range between 10–4 to 10-–12 STP cm3/g (i.e.10–1 to 10–9 ppm). -Their solubility in silicate melts is a strong function of pressure, as well as both atomic radius and melt composition -Although they cannot form true chemical bonds with other atoms, they can be strongly adsorbed to crystal surfaces through van der Waals forces. -The very strong nature of the N–N bond makes nitrogen relatively unreactive once molecular nitrogen forms; consequently it, like the rare gases, is strongly partitioned into the atmosphere. -However, it is quite capable of forming strong covalent bonds with other elements. -In silicate minerals, N is probably primarily present as the ammonia ion rather than N 2. -It readily substitutes for K+. - As ammonia, it is highly soluble in aqueous fluids and is therefore readily transported by them. - Ammonia, like N2, is quite volatile, so both species partition readily into the gas phase of magmas. - In aqueous solution, nitrogen will be present as nitrate (and trace amounts of ammonia, produced by breakdown of nitrogen-bearing organic compounds), as well as N 2. - Nitrogen is a component of proteins and nucleic acids and as such is vital to all organisms. - However, most plants can utilize only “fixed” nitrogen, that is nitrate or ammonia. - In many natural waters, nitrate concentrations are held at very low concentrations because of biological utilization. - The Semi-Volatile Elements- These include C, halogens, S, As, Sb, Se and Te. - They partition readily into a fluid or gas phase (e.g., Cl, Br) or form compounds that are volatile (e.g., SO2, CO2). - Not all are volatile in a strict sense (volatile in a strict sense means having a high vapor pressure or low boiling point; indeed, carbon is highly refractory in the elemental form). - The partitioning of sulfur between liquid and gas phases is a strong function of ƒ O. - At high oxygen fugacities (i.e.ƒO), sulfur is present primarily as SO2, but at low ƒO it is present primarily as sulfide (i.e S - - ). - The solubility of sulfide in silicate liquids is, however, low. - At sufficiently high sulfur concentrations in magmas, sulfide and silicate liquids will exsolve. - Sulfide liquids are rich in Fe and Ni and other chalcophile metals and are the source of many economically important ore deposits ( Such as Cu, Pb, Zn, Au, Ag, Ni etc.) - Large volumes of sulfide liquid are rare, but microscopic droplets of sulfide liquids commonly occur in mid-ocean ridge magmas. - Similarly, the solubility of CO2 in silicate magmas is limited and is a strong function of pressure. - At low CO2 concentrations, CO2 exsolves from magmas to form a CO2–H2O gas phase. - However, at higher CO2/H2O ratios and total CO2 concentrations, carbonatite magmas can form in which CaCO3 is the dominant component. - On the whole, carbonatites are rare, but over the course of geologic history they have erupted on every continent. - In certain localities, such as the modern East Africa Rift, they can be fairly common. - The remaining elements in this group are always present in trace concentrations and never reach saturation in magmas and hence never exsolve as independent gas or fluid phases. - Rather, they partition into gas phase formed by exsolution of CO2 and H2O. - The Alkali and Alkaline Earth Elements: - The alkali and alkaline earth elements have electronegativities