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Double Chain Silicates – Amphiboles!T-O-T again, but this time the I-bar structures are composed of 2 linked chains and the O-layer contains 3 octahedral sites (M3, M2, M1)!Double Chain Silicates – Amphiboles!Amphiboles have many different structural sites, which allow cations of various size to substitute easily. !Two mechanisms: simple and coupled subsitution are common.!Sheet Silicates – Micas and Clays!Trioctahedral sheets: Mg2+ or Fe2+ occupies 3/3 O-sites!Dioctahedral sheets: Al3+ occupies 2/3 O-sites!Al3+!Sheet Silicates – Interlayer and Expandable Layer Clays!Miocene-Oligocene sediments of the Gulf Coast of the US. Note changes with burial depth.!K-spar breaks down to make kaolinte + excess K+!K+ incorporated into mixed layer clay increases the illite/smectite ratio!Non-silicate Minerals – Carbonates, Halides, Sulfates, Oxides!Carbonates!Basic structural unit of all carbonate minerals is the CO32- anionic group. Note planar triangular coordination based on sp2 hybrid bonds between C4+ and O2-.!Three basic anhydrous groups: !1) Calcite group – single cation – hexagonal!2) Aragonite group – single cation - orthorhombic!3) Dolomite group – Ca + (Mg, Fe, Mn)!Rhombohedral Carbonates: Calcite and Dolomite groups!Calcite Group structure!Dolomite Group structure!Note planar disposition of the CO32- groups in both structures!Orthorhombic Carbonates: Aragonite Group!Aragonite is the most common carbonate mineral precipitated in modern marine environments; it is NOT thermodynamically stable at earth surface conditions. Some marine fauna also have calcite tests and shells. In contrast, calcite and dolomite are the most common carbonates in ancient limestone sequences.!Large cations like Ca2+, Ba2+, Sr2+, and Pb2+ occupy the 9-fold sites – no large site exists in calcite structure, hence only Mg2+, Fe2+, and Mn2+ substitute.!Oxides: Spinel Group Minerals – XY2O4!Oxygen anions for a ~CCP array with layers parallel to {111}. In each unit cell there are 8 T-sites and 16 O-sites between layers. Al-rich spinels also show Al3+ order-disorder on T and O sites as a function of temperature (more random at high T, like feldspars).!Oxides: Spinel Group Minerals!Spinel group minerals are important industrial materials and common accessory minerals in a wide range of igneous and metamorphic rocks.!One of the few common minerals that will accommodate both ferrous, Fe2+, and ferric, Fe3+ in its mineral structure.!Fe-Ti oxides are useful indicators of fO2 (similar to the partial pressure of oxygen) and temperature in basaltic-andesitic volcanic and igneous rocks.!(Mg,Fe)(Al,Cr)2O4 spinels are found in the earth’s upper mantle at intermediate pressures and are an important sink for Al3+ as feldspar is broken down with increasing pressure first to spinel and then to garnet.!Petrological Uses for Spinels: Oxygen Buffers!<--- Calculated fO2 from Fe-Ti oxides!Fe2TiO4 + Fe2O3 = FeTiO3 + Fe3O4! uvsp hm il


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UT Arlington GEOL 2313 - Lecture Notes

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