Tyranid eat non-biological materials

• Devastation of Baal:

"Lights from the Splendid Pinion’s subsidiary craft played over the broken station orbiting Sciothopa. Glaring circles of bright light turned bent girders into shining filigree. Large parts of the station structure were missing, and the rest was close to disintegration. Erwin reckoned it would only be a few weeks before the remains were dragged from the sky by Sciothopa Prime and smashed to atoms on the surface.

They take even the metal,’ said Erwin. The Servile of the Watch looked up from his podium over the augur pits, where baseline humans less fortunate than he laboured in unbreakable communion with the ship, their eyes and ears removed and sensory cortexes plugged directly into the auspectoria’s cogitators.

They take minerals of every kind, my lord,’ said the servile. ‘I have compared spectrographic analysis of this world with records of how it was. It shows massive depletion of all main range elements. The devourer remakes the worlds it consumes. Although I notice a small inconsistency with the oldest records of tyrannic-stripped worlds.’

Small enough for me to ignore?’ asked Erwin. The Servile of the Watch was an earnest fellow, genuinely fascinated with his work. He had been known to bore his masters with unnecessary detail.

The servile pulled a neutral expression, making his slave tattoos shift across his face, a sense of motion exaggerated by the low light of the command deck. The Servile of the Watch was unusually expressive for one of his breed. ‘Whether it is relevant or not I shall leave to your deep percipience, my lord.’

Erwin grunted. ‘Edify me then.’

‘The older worlds show a larger loss of mass. The tyranids spent longer on each, digesting parts of the planetary crust. They do not remain so long as they once did. Once the biological components of the world have been devoured, they target only sources of refined metals, such as the Mechanicus station here, in preference to the source minerals."