Microrods of the ferrosulfide minerals greigite (Fe3S4) and marcasite (FeS2) are selectively synthesized by an in situ magnetic-field-assisted hydrothermal route. Each complex microrod is composed of fine building blocks with different shapes. The unique magnetic properties of the microrods and electrical performance of a single microrod are studied. The results demonstrate that the magnetic,,properties of the ferrosulfide minerals are strongly related to their corresponding microstructures. The value of the low-temperature transition increases as the greigite component in the product decreases. The combination of small-molecule sulfur precursors and an applied magnetic field makes possible the selective synthesis of ferrosulfide minerals with different phases and distinct microstructures, underlining the fact that the magnetic field can be a useful tool as well as an independent parameter for the phase-selective synthesis and self-assembly of inorganic building blocks in solution chemistry.