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CH 9 - Mineralogy & Geological History of the Terrestrial Planets

CH 9 - Mineralogy & Geological History of the Terrestrial Planets

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9 Mineralogy and geological history of the terrestrial planets 9.1 Mineralogy and minerals The chemical elements can combine into molecules and minerals. We normally regard molecules, such as simple water, H 2 O, as having stoichiometrically precise relative composition of elements – in this case 2 atoms of hydrogen for each atom of oxygen. While water might be regarded as a mineral in liquid form with ice, one of its crys- talline forms, many minerals do not follow a precise stoichiometry. Rather minerals may be regarded as structural edi ces into which certain atoms can be easily fi secured. Most minerals can exist in liquid state though we are normally most interested in their solid, typically crystalline state. Rock salt with chemical formula N aC l exists as a typically white crystal. Common table salt, though, is not pure N aC l as iodine is added as N aI for health reasons. Both C l, chlorine, atoms and I , iodine, atoms might well exist within a single crystalline piece of salt. Similarly, the cation K + might be substituted for the N a + cation in a crystal with either the C l or I anion. The cation is an atom or group of atoms which tends to want to yield one or more electrons and so becomes positively charged. The anion is an atom or group of atoms which tends to take on one or more electrons and thus become negatively charged. The negatively charged anions are thus attracted to and complement cations. Much of the study of elementary inorganic chemistry relates to the stable molecules and crystals which can be formed of assemblages of various anions and cations. Here, above, we have roughly described the mineral halite. It is not typically pure N aC l as atoms similar in property to sodium, N a can substitute for the sodium role and atoms similar in property to those of chlorine, C l, can substitute for it. Minerals typically have a range of compositions which is allowed for by such substitutions. Halite is one of the simplest of minerals. The minerals which compose rocks in the Earth’s crust and mantle are more complex. They are typically silicates, oxides and sul des complemented with various fi metallic
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cations such as magnesium, M g, iron, F e, sodium, N a, etc.. The silicates and oxides form lithophile minerals which means that they ”like to be” rock. Certain metals form siderophile alloys. Certain metallic elements easily combine with sulphur; these are called chalcophile elements, iron, zinc and lead being among the common ones. Among the most important of minerals in the solar system is olivine which has a chemical composition ranging between M g 2 S iO 4 and F e 2 S iO 4 . Sometimes we note the generic range as (M g, F e) 2 S iO 4 allowing for any possible mixed composition of magnesium and iron. Magnesium and iron can take on di ff erent silicate compositions too. Orthopyroxene has a generic composition (M g, F e) 2 S i 2 O 6 . Relatively, the chem- 132 istry suggests that a S i atom takes on a role of an O atom in olivine. The minerals are brought to important di ff erence in this ”substitution”. The very structure of the
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