To the north and east of Bir Moghrein, within calc-alkaline granites that dominate the geology of this part of northwestern Mauritania, peralkaline granite forms four separate masses within an area of 50x40 km. The largest intrusion is in excess of 30 km in length and up to 10 km wide. The rock weathers to large dome-like features. The homogeneous rock consists of quartz, microcline perthite bordered by albite, hastingsite with riebeckitic margins, biotite and titanite. In places individual riebeckite crystals, and more rarely aegirine, occur; magnetite and zircon are ubiquitous and fluorite occasionally present.