The mineralogy of Lake Malawi ferromanganese nodules

In Lake Malawi, ferromanganese nodules and oolites occur in oxic areas at water depths of 80 to 160 m at the facies boundary between sands and muds. In most cases, the nodules have been found at the water/sediment interface. The nodules and micronodules range in colour from dark brown to rusty or yellowish. Some tend to be of metallic lustre on the outside. The size of the nodules ranges from about a millimeter to about a centimeter in diameter while the average is 3 mm.

Three main mineral species have been identified in Lake Malawi nodules: 1) manganite (λ-MnOOH); 2) geothite [∝-FeO (OH)] and 3) vivianite [Fe₃ (PO₄)₂. 8H₂O].

A likely origin of Fe and Mn that form the nodules in Lake Malawi is from tthe sediments themselves. Another possible source can be through the movement of deep waters from the geothermally active areas of the lake to the shallow oxic areas where nodules are presently forming.

The vivianite in the vivianite-rich nodules originates from dissolution of apatite (from fish debris) within the sediment and redeposition as Fe-phosphate under reducing conditions. Studies of the vivianite-rich micronodules using a scanning electron microscope (S.E.M.) and an electron microprobe have shown that in some samples, vivianite is associated with nontronite [(Ca Mg).5Fe₂(Si Al)₄O₁₀]. The nontronite occurs as a coating around the vivianite.