Geol 2311 12/05/06 Lab 19 – Optical Mineralogy – Other Mineral Characteristics Due Date: Thurs, December 14. Total Points 15 Objective: Become familiar with the other physical and chemical properties of minerals such as cleavage, twinning, exsolution, and zoning and learn how to determine the anorthite content of plagioclase by the extinction method. Procedure: View the thin sections underlined below and answer the related questions 1) D-37 – Granite: This is a section of a typical granite containing alkali feldspar, plagioclase, quartz, biotite, and hornblende. List these minerals below in order of decreasing relative abundance and list two attributes that help you identify them. Among attributes to consider are: compositional zoning, perthitic exsolution, albite twinning, tartan twinning, birds-eye extinction, 120°cleavage, colorless, brownish pleochroism, greenish pleochroism, uniaxial vs. biaxial optics, optic sign. (5 pts) Mineral Attribute 1 Attribute 2 ____________ _____________________ _______________________ ____________ _____________________ _______________________ ____________ _____________________ _______________________ ____________ _____________________ _______________________ ____________ _____________________ _______________________ 2) ES-10 and GA-3V Porphyritic Basalts: These two sections are an example of porphyritic texture where one or more phases occur as large crystals (phenocrysts) in a fine grained matrix. Porphyritic texture represents two stages of cooling of the magma. A typical mafic magma crystallizes the minerals olivine, plagioclase, clinopyroxene (augite), orthopyroxene (usually hypersthene), and iron oxide (ilmenite or magnetite) in addition to other minor (accessory) phases. Most of these phases occur in the groundmass, but only one of them occurs as the phenocryst phase in each sample. Based on its optical properties, what is this phenocryst mineral in each section and name two optical or physical properties that aid in its identification.(5 pts) ES-10 Phenocryst phase:___________________ Attribute 1:__________________________ Attribute 2:__________________________GA-3V Phenocryst phase:___________________ Attribute 1:__________________________ Attribute 2:__________________________ 3) GKS-1 and GKS-16 Gabbro: Use the symmetrical maximum extinction angle method for plagioclase (see next page) to estimate the An content of plagioclase in this sample (note some sections are thick). Make at least five measurements of albite twin extinction angles (make sure that + and – measurements are within 5º of each other). If you find a perfect ⊥X section, designate that measurement with an asterick * (5 pts) Measurement: 1 2 3 4 5 + direction _____ ______ ______ ______ ______ - direction _____ ______ ______ ______ ______ Maximum avg. ext. angle _______ Estimated An content
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