The microstructure of fibrous iron-bearing brucite (nemalite, (Mg,Fe)(OH)₂) has been investigated using high-resolution SEM and TEM to reveal the origin of its morphology. The fiber consists of aggregates of flat, lath-like brucite single crystals elongated parallel to <10-10>. Chrysotile tubes, which are also parallel to the fiber, frequently exist, mainly at the grain boundaries of brucite and surrounded by brucite crystals. It is suggested that this unexpected morphology of brucite was formed by crystal growth along the preexisting chrysotile tube surfaces. However, the interface between brucite and chrysotile is not coherent and the arrays of hydroxyl on the surfaces of two minerals are rotated by 30°C to each other. Such orientation relationship is often reported in aggregates of sheet silicates. Brucite crystals frequently contain 180° rotational twins on (0001).