P.A. Cox - Inorganic chemistry (793955), страница 59
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The formation ofplanetary systems may be common in the Universe. Studies suggest that the Earth and other planets formed about thesame time as the Sun (4.5 billion years ago). While the Sun formed at the center, chemical reactions in the cooler outerregions of the gas concentration produced solid particles, which gathered under gravitational forces, first into smallbodies known as planetesimals, and subsequently into the planets. In the outer regions of the Solar Systemtemperatures were low enough to form ‘ices’ of water, solid methane, carbon dioxide and ammonia, which areconstituents of the giant planets Jupiter and Saturn.
The inner planets such as Venus, Earth and Mars formed at highertemperatures, and their composition is dominated by elements that form metallic solids, such as iron and nickel, andones with stable involatile oxides, such as SiO2. Many other electropositive elements were incorporated as silicates, andsome also formed sulfides and halides. The molecular compounds of H, C and N were still gaseous at the temperatureat which the Earth was formed, so that these elements largely escaped, except for relatively small amounts of H2O,CH4, CO2 and NH3, which were trapped in solid silicates. Noble gas elements (group 18) are rare on Earth.Abundant elements on Earth are therefore ones which were both made efficiently in nuclear reactions in stars, andalso formed involatile metals or compounds when the Solar System was formed.
Subsequent heating by radioactivedecay allowed the denser metals (Fe, Co and Ni combined with some S) to melt and sink towards the center, formingthe core. Silicates and other complex oxides remained as the dominant constituents of the outer layers.Section J—Environmental, biological and industrial aspectsJ2GEOCHEMISTRYKey NotesElement classificationCrust formationWeathering andsedimentationAtmosphere and oceansRelated topicsLithophilic elements are those present on Earth in oxide (mostlycomplex, e.g. silicate) and halide minerals, chalcophiles in sulfideminerals, and siderophiles in metallic form.The crust is formed by melting and recrystallization of minerals in themantle. Compatible lithophilic elements (Mg, Fe, Cr) are commonerin the mantle, incompatible ones (e.g.
Na, K, Al) in the crust.Chemical reactions in molten rocks and in water at high temperaturelead to the concentration of many elements in particular minerals.The breakdown of rocks by water and CO2 gives insoluble resistates(e.g. Al, Ti, Sn oxides) and soluble ions. Some ions oxidize to formsolids (e.g. Fe2O3); others pass into the ocean and eventually formevaporite minerals (e.g. NaCl).The atmosphere was formed by outgassing of minerals. O2 comesfrom photosynthesis. Ions common in sea water are ones that do notform insoluble salts.Chemical periodicity (B2)Origin and abundance of theelements (J1)Element classificationGeochemistry is the study of chemistry in the Earth’s natural environment.
Most elements available to us come from thesolid rocks of the Earth’s crust. Underlying the crust is the mantle of rather similar composition to the crust, insidewhich is a metallic core. Overlying the crust are the atmosphere and the aqueous environment or hydrosphere ofthe oceans, lakes and rivers.The chemical processes taking place in the crust are especially important as they have formed ores, concentratedmineral deposits that are exploited industrially as sources of specific elements and their compounds.
Figure 1summarizes the principal chemical forms in which each element occurs. At least half the elements occur in the crust asoxides (mostly complex ones such as silicates) or less commonly as halides, and are called lithophiles. All the highlyelectropositive metals are in this class. Chalcophiles are elements present in sulfide minerals; these include someelements chemically similar to sulfur (Se, As) together with less electropositive metals of the later transition and posttransition metal groups (see Topics G1 and H1). A few metallic elements of low reactivity are found in native(uncombined) form on Earth. They are known as siderophiles and are commoner in the Earth’s metallic core.
A fewnonmetallic elements (N, noble gases) occur in uncombined form. As Fig. 1 shows, some elements have intermediate258J2—GEOCHEMISTRYFig. 1. The periodic table showing the principal types of chemical compound occurring for elements at the Earth’s surface. Oxides include manycomplex forms, especially silicates.behavior and fall in more than one class. For example, iron is found in both lithophilic (Fe silicates, Fe2O3, etc.) andchalcophilic (FeS2) states.Crust formationNew crust is formed by tectonic processes caused by upwelling convection currents in the mantle, driven ultimatelyby heat from radioactive decay of elements in the Earth (see Topic A1). The melting of rocks and subsequentrecrystallization leads to fractionation of some lithophilic elements. Magnesium tends to remain in the mantle, and withit some other compatible elements, which form ions of fairly similar charge and size to Mg2+ (e.g.
Fe2+ and Cr3+).Incompatible elements (e.g. Na, K, Ti) do not remain with the magnesium silicate but pass easily into the melt andhence are more concentrated in crustal rocks.Whereas the rocks of the mantle contain mostly orthosilicates with nonpolymerizedions, and chainsilicates such as MgSiO3 (see Topics D5 and F4), the minerals of the crust mostly contain more highly polymerizedsilicate units. Some of the commonest crustal rocks are feldspars, three-dimensional framework silicates consistingof corner-sharing [SiO4] groups, like SiO2 but with some Si is replaced by Al.
Some idealized formulae are KAlSi3O8and CaAl2Si2O8, but in reality these minerals are much more complex, with many other elements present in smallconcentrations.Many less common elements (e.g. Ga and Ge) are incorporated to some extent into the crystal structures of majorminerals, and thus may be rather thinly spread over the crust.
Others are concentrated by forming individual minerals.Native gold and cinnabar (HgS) were known in antiquity although Au and Hg are very rare elements. On the otherhand, the less rare Ga and Ge were not discovered until the late 19th century.The chemical processes leading to different minerals are diverse. Highly incompatible lithophilic elements (e.g. Li,Be, Zr and lanthanides) are concentrated in the final stages of solidification of molten rocks, known as pegmatites.Many sulfide minerals (e.g. of Cu, Zn, Mo and Pb) are formed by hydrothermal processes, in which water circulatesdeep in the crust and at high temperatures and pressures, and forms soluble complexes of these elements with anionssuch as Cl− and HS−, which may subsequently precipitate solids when they cool.SECTION J—ENVIRONMENTAL, BIOLOGICAL AND INDUSTRIAL ASPECTS259Weathering and sedimentationSedimentary processes begin with weathering of rocks, a chemical breakdown produced by the action of waterand atmospheric CO2.
A typical reaction is the weathering of potassium feldspar (KAlSi3O8) to form the clay mineralkaolinite:CO2 acts to provide acid in this reaction, and weathering is accelerated by living organisms that provide CO2 throughrespiration and decay. A further step in this process leads to very insoluble Al(OH)3:Rocks are therefore transformed by weathering, with soluble ions such as K+ being washed out and insoluble resistatesremaining. Some important sources of elements are of this form, including bauxite Al(OH)3, rutile TiO2 and cassiteriteSnO2.The action of atmospheric oxygen on soluble ions may produce insoluble oxidates such as Fe(OH)3 and MnO2 fromFe2+ and Mn2+, respectively.
Other elements pass into the ocean and become deposited in various ways: as biogenicdeposits such as CaCO3 and SiO2, which originate as the shells of marine organisms (see Topic J3), or as evaporitessuch as NaCl produced by evaporation of salt lakes.Atmosphere and oceansThe atmosphere was originally formed by outgassing of crustal minerals that decomposed under heating.
N2 and CO2were probably the main original constituents. Water vapor condensed to form the liquid oceans. O2 is a very unusualconstituent of our own atmosphere by comparison with other planets. Nearly all of it comes from photosynthesis, theprocess by which green plants obtain their organic carbon from CO2 with the help of energy from sunlight (see TopicsJ3 and J6).The major dissolved constituents of the oceans are ions that do not form very insoluble compounds. Large amountsof many common elements such as Ca and Si are carried into the sea in soluble form by rivers, but many are precipitatedeither by inorganic or by biological processes (see above).
The remaining ions of high abundance (Na+, Cl−, Mg2+) formsoluble compounds and are removed only by evaporation.Section J—Environmental, biological and industrial aspectsJ3BIOINORGANIC CHEMISTRYKey NotesThe elements in biologyMajor elementsTrace metalsToxic andelementsRelated topicsmedicinalAround 25 elements are known to be essential for life. There are 11major elements with a concentration greater than one part in 104, theothers being known as trace elements.Major elements form constituents of biological molecules (C, N, O,P, S), ions either in solution or complexed to biomolecules (Na, K,Mg, Ca, Cl), and solids such as bones (e.g. calcium phosphate).Essential d-block elements (e.g. Fe, Zn, Cu) are mostly constituentsof metallo-enzymes, which act in the transport and chemistry of O2,and perform many catalytic functions including redox and acid-basereactions.Some strongly complexing nontransition metals (e.g.
Hg, Pb) arevery toxic. Elements used in medicine include Li, Pt, Au andradioactive Tc.Chemical periodicity (B2)Environmental cycling andpollution (J6)The elements in biologyLife is sometimes thought of as ‘carbon chemistry’, but around 25 elements are essential for life. It is normal to dividethese into major elements and trace elements according to their concentration (greater or less than one part in 104by mass).
Table 1 shows elements classified in this way and according to their diverse roles. Nearly all known elementscan be detected in the human body by modern analytical methods (see Topic B7), but most are presumed to be thereadventitiously without playing an essential role. To establish whether an element is essential is therefore difficult,especially as some essential elements (e.g.
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