Physical form of iron powder

Pure metal iron is silvery white, while iron powder is black. This is an optical problem, because the specific surface area of iron powder is small and there is no fixed geometric shape, while the crystal structure of iron block is geometric, so the iron block absorbs part of the visible light and reflects the other part of the visible light mirror, showing white. The light that the iron powder does not absorb is diffusely reflected, so less visible light can enter the human eye, so it is black.