Science Fiction paperback about an astronaut slowly turning into an alien. Read it in mid 70s.

[David]

Looking for a sci fi paperback I read from my high school library in the mid to late seventies. All I can remember are the bare basics of the plot: a man went into space with a woman to colonize another planet. The man was either somehow contaminated or had been part of an experiment he wasn't aware of (I can't remember), and he slowly began transforming into an alien that could live on the planet they were headed toward. I don't remember how it ended or any more about the protagonist. I do remember one scene where a tray of things fell off a desk or table and the protagonist caught them all in a microsecond - in the order of most fragile to least fragile. This was at the beginning of his transformation.