Is the structure morphism from an irreducible scheme open?

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Let $X$ be an irreducible (projective) scheme of finite type over $\mathbb{Z}$. Is the structural morphism $\pi \colon X \to \mathrm{Spec}(\mathbb{Z})$ necessarily open?

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No. The morphism $\pi \colon \mathrm{Spec}(\mathbb{Z}/2\mathbb Z) \to \mathrm{Spec}(\mathbb{Z})$ has as its image the closed point $[2\mathbb Z]\in \mathrm{Spec}(\mathbb{Z})$, which is not open.