Mineral Composition
The main minerals are alkali feldspar, nepheline, clinopyroxene (±), amphibole (±), and biotite (±). Nepheline is the main feldspathoid. Quartz and orthopyroxene are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10% The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlocking perthite, called anorthoclase. In the alkali feldspar almost pure albite domains are observed. Nepheline generally shows partial alteration into natrolite and cancrinite. The clinopyroxene is sodic whose composition varies from hedenbergite to aegirine-augite. This mineral eventually presents resorption shape. The reaction rim constituted by amphibole and/or biotite is commonly observed. The amphibole is of high alkali, such as alkaline hornblende and riebeckite. The alkaline clinopyroxene and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio. The accessory minerals are magnetite, ilmenite, apatite, and titanite. Eventually, sodalite is found along hydrothermal fractures. Different from granite, zircon is rare and if present it is as xenocrysts. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.
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