In 862, the Greek mother of Abbasid Caliph al-Muntasir built a mausoleum on a hill site on the western shore of the Tigris River to honor his death. Such a building tradition was previously unknown in Islamic history. Constructed in a stone-like material consisting of clay and quartz, what remains of the mausoleum is octagonal in shape with three zones to the structure: an outer octagon (almost half destroyed), an inner octagon, and a central, square chamber measuring about 6.3 square meters, that through the use of squinches is modeled into an octagonal hall. A vaulted walkway with sixteen transverse arches runs between the two outer octagons. Each façade of the outermost wall features an arched doorway whereas there were only four entrances, designed on a cardinal axis, to the inner domed sanctuary. Small semicircular alcoves adorn the other four faces of this area. The Qubbat as-Sulaibiya also contains the tombs of the succeeding caliphs, al-Mu'tazz and al-Muhtadi. It is the only building of its type in the Middle East to use an octagonal hallway.

Sources:

Creswell, K. A. C.1989. A Short Account of Early Muslim Architecture. Rev. ed. Allan, James W. Aldershot: Scolar Press, 372-374.

Ettinghausen, Richard and Grabar, Oleg. 1987. The Art and Architecture of Islam 650-1250. New Haven and London: Yale University Press, 101.