minerals1 - COMMON MINERALS - A PARAGENETIC ORGANIZATION...

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CVEN 3698 - Lecture Notes 4 Page 1 © E. Nuhfer & B. Amadei COMMON MINERALS - A PARAGENETIC ORGANIZATION WITH AN EMPHASIS ON ENGINEERING PROPERTIES 1. Chemistry of Common Earth Materials Although there are over 2000 known minerals, and over 100 chemical elements, the majority of the crust can be characterized with just a few elements and minerals. If we analyze most rocks for ten elements (after heating to drive off any organic matter and carbon dioxide) and express the elements in oxide form, their summation will be close to 100%. The elements are the following: SiO 2 Al 2 O 3 Fe 2 O 3 CaO Na 2 O K 2 O MgO TiO 2 MnO P 2 O 5 _______ Sum = 100% YES! Just ten elements constitute close to 100% of most rocks and soils. Concentrations of other elements occur within the crust, but geologically these are exceptional occurrences. Mining engineers will encounter these and because these often constitute economic deposits, a good part of the geologist's training is with these occurrences. However, civil engineers are builders and therefore deal with common Earth materials. From the standpoint of engineering geology, we want to know how these common elements affect our engineering properties, where they occur and what they mean. It is useful to divide these elements into those that are easily moved in water solutions ( mobile ) and those that normally form insoluble compounds ( immobile elements ): Mobile - K + , Na + , Mg ++, Ca ++ Immobile - Al +++ , Si ++++ , Fe +++ and Fe ++ , Ti ++++ , and Mn ++ Phosphorus lies in about the middle between mobile and immobile. Because we want to see how materials change through time, this is a good initial way to look at earth materials. Most changes will result from attack by air and water.
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CVEN 3698 - Lecture Notes 4 Page 2 © E. Nuhfer & B. Amadei Now, the form these elements appear in has a lot to do with their weathering rates. You probably remember the term, " bonding ". Elements combine with one another in nature to form special compounds called minerals . The minerals have a formula that can be expressed as a chemical compound, a definite crystal structure, and the elements are held together in that structure by bonding from high school chemistry. The common earth materials are put together in two primary ways (two types of bonds - ionic and covalent ). Ionic bonds are easily attacked by polar water molecules and hence a mineral which is dominantly held together with ionic bonds will be reactive, partially soluble, and likely to break down in a reasonably short time. Salt or halite is an extreme example. Covalent bonds, on the other hand, are not much affected by water molecules and hence these materials will be durable and inert. Quartz is an extreme example of a mineral that is dominantly covalently bonded together. A third type of bonding, metallic bonding, is present in many minerals; the civil engineer sometimes sees particularly the ore minerals and these. These are easy to spot because the metallic bond in the mineral gives it a metallic luster.
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This note was uploaded on 02/26/2008 for the course CVEN 3698 taught by Professor Amadei during the Spring '08 term at Colorado.

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minerals1 - COMMON MINERALS - A PARAGENETIC ORGANIZATION...

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