Chemical Composition
The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich in alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica varying from 50 to 56%, while lime, magnesia and iron are never present in great quantity, though somewhat more variable than the other components. A worldwide average of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed as weight percent oxides.
Nepheline syenite is characterized by high ratio of (Na2O+K2O)/SiO2 and (Na2O+K2O)/Al2O3, which are represented respectively by the existence of nepheline and alkaline mafic minerals. Therefore, it is classified geochemically as alkaline rock. This rock has low Fe and Mg contents, in total about 3wt%, and in this sense it is classified to be felsic rock. However, the SiO2 content is not so high, being 53% to 62wt%, which is equivalent to andesite and diorite. In this sense, it corresponds to intermediate rock. Light rare earth elements are highly concentrated, indicating that the magma is highly differentiated.
- SiO2 — 54.99%
- TiO2 — 0.60%
- Al2O3 — 20.96%
- Fe2O3 — 2.25%
- FeO — 2.05%
- MnO — 0.15%
- MgO — 0.77%
- CaO — 2.31%
- Na2O — 8.23%
- K2O — 5.58%
- H2O — 1.47%
- P2O5 — 0.13%
The normative mineralogy of this average composition contains about 22 percent nepheline and 66 percent feldspar.
Because nepheline syenite lacks quartz and is rich in feldspar and nepheline, it is used in the manufacturing of glass and ceramics.
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