Northwest Africa (NWA) 8599 is a lunar breccia meteorite found in Northwest Africa. We performed detailed petrographic and mineralogical study on this lunar meteorite. Our observations reveal that NWA 8599 is an olivine-rich magnesian anorthositic noritic breccia, with abundant mineral fragments and minor fine-grained magnesian anorthosite clasts. Our NWA 8599 sample contains a coarse-grained highly magnesian olivine fragment (Fo=84-87), which is closely associated with highly magnesian pyroxene (Mg^#=85-89) and geikielite (Mg^#=52-54). The geikielite grains in NWA 8599 are reported for the first time among lunar meteorites and mission-returned lunar samples. The CaO contents in the highly magnesian olivine and thermobarometric calculations of pyroxenes indicate that the highly magnesian mineral assemblage is likely derived from a highly magnesian rock from deep crust of the Moon. Our results show large variations in modal abundance and grain size for the magnesian anorthosites in NWA 8599, indicating diversity of the magnesian anorthositic lithologies in the Moon. We suggest that the magnesian anorthosites may be produced by variable degrees of assimilation of primordial ferroan anorthositic crust by mantle-derived magnesian melt during lunar mantle overturn.