Several times they are listed as non-organic, and that is the main reason that they weren't affected by the Flood, the Halo Arrays, and why they don't care about the "Great Journey." They're mentioned as "Supercomputers," though ONI thought them to be a bio-mechanical species. I can't say it enough, they are NON-ORGANIC. The simplest of searches on Halopedia or the Halo 3 Bestiarium will come up with this.
Spartan R41, prepped for combat insertion.
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Never Forgotten: http://www.youtube.com/watch?v=qGh7JbFQDX4