Abstract: Microscopic rods of wüistite and sulphide phases have been observed within silicate crystals from metallurgical slags. The rods, which are distinct from acicular crystals of accessory minerals, tend to be orientated perpendicular to the faces of the host crystal. These rods, and their constant orientation, are a product of the crystallization environment. It is proposed that they result from saturation of the melt with the relevant phase at the growth front of the silicate crystals. This microstructure is compared with known examples of microscopic rods and tubes in rocks.