Paragraph: The reaction of K_ 3 [Fe(CN)_ 6 ] with freshly prepared...

Paragraph: The reaction of $\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$ with freshly prepared $\mathrm{FeSO}_{4}$ solution produces a dark blue precipitate called Turnbull's blue. Reaction of $\mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]$ with the $\mathrm{FeSO}_{4}$ solution in complete absence of air produces a white precipitate $\mathbf{X}$, which turns blue in air. Mixing the $\mathrm{FeSO}_{4}$ solution with $\mathrm{NaNO}_{3}$, followed by a slow addition of concentrated $\mathrm{H}_{2} \mathrm{SO}_{4}$ through the side of the test tube produces a brown ring.
Question: Precipitate $\mathbf{X}$ is