Abstract Jacobsite is a relatively rare mineral of composition MnFe2O4, found in Urandi (Bahia State) in Brazil. It is also a common species in the deep-sea manganese nodules, attracting the interest of many mineral-extracting companies. Because of its spinel constitution similar to magnetite, Jacobsite is commonly called a manganese-ferrite. However, the manganese/iron content may vary substantially according to its origin, demanding specific studies in each case. The Jacobsite mineral inspired our laboratory synthesis of the analogous manganese ferrite nanoparticles. The direct synthesis by the coprecipitation method has not been successful; however, it can be carried in the presence of citrate ions, yielding strongly magnetic nanoparticles, with a maximum magnetization of 45.6 emu.g1. Although they were structurally identical to Jacobsite, the mineral from Bahia exhibited a rather weak magnetism, because it involves a ferrimagnetic coupling. For this reason, the synthetic method seems to provide a better way of obtaining strongly magnetic manganese ferrites. These magnetic nanoparticles have been investigated in detail, including their interaction with diatoms, providing interesting magnetic bio-silicate carriers in drug delivery. MnFe2O4 MnFeO MnFe O State Brazil deepsea deep sea nodules mineralextracting extracting companies magnetite manganeseferrite. manganeseferrite ferrite. manganese-ferrite However manganeseiron iron origin case successful however ions 456 45 6 45. emug1 emug emu g1 g emu.g1 magnetism coupling reason ferrites detail diatoms biosilicate bio silicate delivery MnFe2O 4 emu.g