Lab+06+Minerals+and+Rocks+F17.pdf - Name Lab section EAS...

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1 Name____________________________________________ Lab section _______________ EAS 1600 Fall 2017 Lab 06 : “Minerals and Rocks” Introduction. Minerals: Composition, Structure, and Properties Minerals make up the majority of rocks on the Earth’s crust. But, what is a mineral? The name mineral has many different connotations, from nutritional supplements to ores in mines. Geologists have a specific definition for minerals. Specimens must follow five major points to be considered minerals: 1 Mineral is a naturally occurring solid with an orderly internal arrangement of atoms (called crystalline structure ) and a definite, but not fixed, chemical composition. 2 Most minerals are compounds consisting of two or more elements. 3 Minerals have specific physical properties. 4 Minerals are inorganic. CHEMICAL COMPOSITION Minerals are composed of one or more elements. Minerals from one element are called Native Elements. Minerals from multiple elements are characterized by their elemental composition, or by which ions constitute the mineral. For example, calcite contains two ions, Ca 2+ and CO 3 2- , so its chemical composition is CaCO 3 . The major categories are Sulfides, Sulfates, Oxides, Phosphates, Silicates (with other sub-categories), and Carbonates. Other categories exist, but will not be studied in this introductory lab. The ions and ratio of ions can determine a mineral’s crystalline structure and color. Sometimes ions can substitute for each other to change the composition slightly, changing the color dramatically. These types of minerals are called “solid solutions” and form mineral groups. Ions of roughly the same size will exchange themselves without changing the chemical structure. Olivine is an example of a mineral group. Olivine’s chemical composition is (Mg,Fe) 2 SiO 4 . The magnesium and iron can combine in any ratio from 0:1 to 1:0. So, one end-member of olivine (0:1) is Fayalite (Fe 2 SiO 4 ), while the other end-member (1:0) is Forsterite (Mg 2 SiO 4 ). See See Figure 1 for the “ratio line” of olivine. Figure 1: Compositional ratio for the olivine mineral group. Fa stands for Fayalite, and Fo stands for Forsterite. The numbers for each endmember represent the percent composition of each species.
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2 CRYSTALLINE STRUCTURE The chemical compositions of a mineral incorporate into specific crystalline structures. PHYSICAL PROPERTIES The chemical composition along with the crystalline structure determines a mineral’s physical properties. The physical properties of minerals are usually easy to ascertain and one can usually identify a mineral through careful observation/notation of these properties. The most common physical properties use to differentiate between minerals are summarized below: Crystal Form A crystal is composed of multiple “faces” that meet at specific angles, interfacial angles, to create a complete geometric form. Then number of faces and the interfacial angles may be used as a diagnostic tool for identification. For example, galena has a diagnostic cubic form. Sometimes
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