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Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Two different triangles (top and bottom of Figure 7.50) are used for rocks that contain quartz (the top triangle) and the (less common) rocks that do not contain any quartz due to low SiO2 content (upside-down bottom triangle). The two share the AP line. If rocks do not contain quartz, they often contain a mineral belonging to the feldspathoid group. So, for low-SiO2 rocks, feldspathoids replace quartz, and naming is based on the modes of alkali feldspar, plagioclase, and a feldspathoid. Because quartz and feldspathoids are never found together, it is generally clear whether to use the top or bottom half of the diagram. Some of the rock names in this system include the term foid, which refers to any feldspathoid mineral, the most common being nepheline and leucite.
Industrial minerals
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Nepheline and alkali feldspar constitute nepheline syenite [880] which is a holocrystalline plutonic rock that is generally equigranular, equidirectional and gross with grain size of 2 mm to 5 mm. Nepheline syenite has a high ratio of (Na2O + K2O)/SiO2 and (Na2O + K2O)/Al2O3 representing the existence of nepheline and alkaline mafic minerals, respectively. Nepheline is a feldspathoid which reacts with quartz to produce alkali feldspar and the nepheline syenites are distinguished from ordinary basic syenites by the presence of nepheline and the occurrence of many other minerals rich in alkalis and in rare earths and other incompatible elements. The alkaline feldspar that dominates them is represented by orthoclase and the exsolved lamellar albite and form perthite. Sodalite and nepheline are the principal feldspathoid minerals which are hosts to triclinic aeigmatite, sodium-rich pyroxene, biotite, titanite, iron oxides, apatite, fluorite, melanite garnet, cancritinite and zircon. Magnetite, ilmenite, apatite and titanite are accessory minerals.
Minerals
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Nepheline is a feldspathoid mineral. The feldspathoid minerals are similar in chemistry to the feldspars but have less silica. The formula given above is an ideal formula because all natural nephelines contain some potassium. Nepheline is the commonest of the feldspathoids and its occurrence is an indication that the rock in which it occurs is alkali-rich.
A review of the geological settings, ages and economic potentials of carbonatites in the Democratic Republic of Congo
Published in Applied Earth Science, 2021
Georges M. Kasay, Anthony T. Bolarinwa, Olawale K. Aromolaran, Charles Nzolang, Vikandy S. Mambo
The large areas of the Kirumba Alkaline Complex are underlain by syenites of four different categories (Denaeyer 1959). Micaceous syenites are found in the field in three different types depending on their micas; these are syenites with muscovite, with biotite and with both micas. Feldspathoidal syenites are classified according to their feldspathoid modal abundances: nepheline syenites with both sodalite and cancrinite; syenites with sodalite and cancrinite but without nepheline; and syenites with either sodalite or cancrinite without nepheline. Barkevikite syenites are another type of syenites. The last type is rocks with aegirine which consist of nepheline and aegirine; they are lujaurite cancrinite (also known as a cancrinite-aegirine syenite), ordosite and melteigite.