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GEOL_notes1 - GEOL 201 FALL 2008 NOTES 1.0 Minerals...

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GEOL 201 FALL 2008 NOTES 1.0 Minerals Chemistry review/definitions Cation - an atom having a positive charged (missing an electron) Anion - an atom having a negative charge (has an extra electron) Remember that the defining attribute of the atom of a given element is the number of protons in its nucleus Mineral : a naturally occurring inorganic solid that has a specific elemental composition and a specific 3 dimensional structure. Rock : are aggregates of minerals. Molecular bonds (the first two types are really "end members” of a single type of bond): Covalent bonds : “electron sharing” mineral example carbon. Strongest bond. Ionic bonds : – “electron exchange” mineral example is halite. Weaker than covalent bonds stronger than weak bonds. Weak bonds : for this course, bonds formally known as electron spin, opposite charge attraction, ect., will be collectively referred to as “weak bonds” Silicate minerals : Silicate minerals are the most significant group of rock-forming minerals Silicate minerals contain the silica tetrahedron (SiO 4 ) 4- arranged in a variety of structures. The silica tetrahedron readily forms covalent bonds with many cations. The shape of silica tetrahedrons (SiO 4 ) 4- is best described as a 4 sided pyramid. See chapter 2 in your text book for illustrations and information regarding silica tetrahedral and the different ways they are arranged in minerals. Silicate minerals having a higher proportion of silica have more complex structures. Physical properties of minerals C LEAVAGE (property #1): cleavage planes: are planes of weakness in a mineral structure. If force is applied to a mineral, it will break along a cleavage plane. Examples of cleavage: Mica (muscovite and Biotite) : has perfect cleavage in one direction because it is a sheet silicate. Halite : has perfect cleavage in 3 directions at 90 degrees because of its highly symmetrical structure and consists of relatively weak bonds. Quartz has a very symmetrical structure consisting of relatively strong bonds, so it has no cleavage.
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GEOL 201 FALL 2008 NOTES 1.0 HARDNESS (property #2) hardness refers to the capacity of a mineral to resist abrasion. packing" of atoms in a mineral : atoms of minerals that form at high pressure will be more close together than atoms of minerals formed at low pressure: example: the minerals graphite and diamond Both graphite and diamond consist of carbon graphite is very soft (low resistance to abrasion) because it consists of covalently bonded layers of hexagons linked by weak bonds diamond is very hard (high resistance to abrasion) because it consists has symmetrically distributed covalent bonds (“puckered hexagonal” structure). S PECIAL PHYSICAL PROPERTY (property #3): the miraculous, asbestiform (fiber-like) mineral known as chrysotile Chrysotile or “white” asbestos provides a dramatic example of the relationship between the structure of a mineral and its physical properties.
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