Intrusive rocks of the Green Foyaite Suite make up a substantial fraction of the southern part of the Pilanesberg Complex. The rocks range from fine-grained foliated foyaites (nepheline syenites) with clinopyroxenes (cpx;<0.25 mm) as the main ma fic mineral, to coarse-grained heterogeneous and unfoliated foyaites with variations of astrophyllite, cpx, and amphibole as the main constituents. The cpx in the fine-grained foyaites display a shape-preferred orientation. Both varieties contain a range of rare Zr-minerals as well as other accessory minerals identified by SEM-EDS analyses. The petrology and whole rock compositions place the Green Foyaites in two distinct groups which correlate with the two locations where the samples were collected. Clinopyroxene in the Green Foyaite is aegirine, ranging from a more magnesian (Aeg58Hd25Di16) to close to pure aegirine (Aeg99Hd0Di1). Clinopyroxenes are both early and late magmatic, where the early magmatic clinopyroxene is often zoned and has lower aegirine component than the acicular late magmatic pyroxenes with no observed color differences. The aegirine in the Green Foyaites has a high concentration of titanium. In the unfoliated foyaites, the titanium content in aegirine exceeds >0.1 apfu, which suggest that it can be classified ed as titanian-aegirine. Amphibole is a blue-green pleochroic amphibole intergrown with the aegirine. The amphibole ranges from magnesio-arfvedsonite to arfvedsonite with Fetot=(Fetot +Mg) ranging from 0.48 to 0.83. Chondrite-normalized REE patterns in pyroxenes from the Green Foyaite ranges from LREE-enriched, relatively straight patterns to distinctly MREE depleted, trough-shaped patterns. The aegirine component can be correlated with the REE patterns, where low-aegirine samples have a high La/Lu and low Lu/Ho ratio, while the high-aegirine pyroxenes have La/Lu ratios close to 1 and high Lu/Ho ratios. This results in the high-aegirine samples to be more MREE depleted relative to LREE and HREE than the low-aegirine crystals. Amphibole mimics the REE-pattern of the pyroxenes. Because aegirine formed early in the crystallization sequence of the Green Foyaite magma, MREE depletion in the Green Foyaite cannot be explained by in-situ processes and is more likely to be a result of a complex fractionation removing the middle REEs from the melt forming the agpaitic Green Foyaite prior to the final emplacement of the magma. A more primitive amphibole fractionation has previously been suggested, however, fractional crystallization of clinopyroxene is in this thesis the proposed mechanism removing the MREE.