F13 quiz 1 condensed notes

F13 quiz 1 condensed notes

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Unformatted text preview: g or losing electrons does not affect the number of protons in the atomic nucleus, and so Na by donating an electron becomes the Na+ ion (a cation), and Cl by receiving an electron becomes the Cl ­ ion (an anion). Positive Na ions and negative Cl ions mutually attract one another—an ionic chemical bond has been created. The mineral halite is composed of cubic unit cells of ionically bonded NaCl. Consider the hardest of all minerals, diamond (C). In this example, all of the carbon atoms are identical, and hence electrons are not donated or received, but rather each atom shares four electrons with the surrounding four atoms. This type of electron sharing interaction is known as a covalent bond. Polymorphs (= “many forms”) are minerals that have the same chemical composition but different atomic structures (three ­dimensional patterns of atoms). Examples are diamond and graphite, both minerals consisting of elemental carbon (C), bonded in different patterns. Approximately 2000 different mineral species are known, but Earth’s crust consists mostly of about 30 common minerals. Just 8 chemical elements, in order of decreasing abundance, oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg) make up more than 98% of crustal rocks (Table 3 ­1, p. 46 of the textbook). Atomic structures of silicate minerals are based around the silicate (or silicon ­oxygen) tetrahedron, in which large oxygen atoms (oranges in our lecture demonstration) occupy the 4 corners of the tetrahedron, and the small silicon atom (grape tomato) is buried in its center. An oxygen ion carries a negative 2 charge, and silicon a positive 4 charge, so the net ionic charge of the SiO4 tetrahedron is (1 x +4) [for silicon] + (4 x –2) [for oxygen] =  ­4. An entire silicate crystal structure can achieve electrical neutrality by one, or another, or a combination of two possible means. (i) The negatively charged SiO44 ­ tetrahedrons may be balanced by positive cations (more tomatoes) of Al, Fe, Ca, Na, K, or Mg positioned around the tetrahedrons. (ii) Si ­O tetrahedrons may be joined corner ­to ­corner to form single chains, double chains, continuous sheets, or complex three ­dimensional networks. The mineral quartz (SiO2) in which Fe, Mg, etc. are absent, exhibits the ultimate degree of linkage. In quartz, every oxygen is shared between two linked tetrahedrons, and so only one ­half as many oxygen atoms are needed. That is, the 4 ­to ­1 ratio (SiO tetrahedron) becomes a 2 ­to ­1 ratio (SiO2) in quartz. From the viewpoint of ionic charge,...
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This document was uploaded on 03/03/2014 for the course GEO 303K at University of Texas at Austin.

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