Anloua, Adamaoua Plateau (Adamawa Plateau), Adamaoua Region, Cameroon

Located near Ngaoundéré (capital of Adamaoua Region).

Lacustrine late tertiary to quaternary sedimentary (sands and clays) basin in a depression of a basaltic plateau.
Black varved argilites that contains the big vivianite crystals are essentially composed of kaolinite, siderite, quartz, organic matter, fluorapatite, crandallite.
The sand lenses that contains the mantienneite are composed of quartz, feldpaths, volcanic glass, chromspinel, in a mantienneite, kaolinite, chlorite-montmorillonite mixed-layer and siderite cement.

Worldwide unique occurrence for the size of vivianite crystals abundant in black argillites http://www.musee.ensmp.fr/gm/60.html).

ref: http://giantcrystals.strahlen.org/africa/anloua.htm