After reading up and deciding on a good alternative to akadama, I settled on montmorillonite clay since I could get a large bag of it for $7 at an auto store as oil absorber. Anyway, I double checked the back of the bag, ingredients listed as being m. clay and silicates (sand particles, presumably), so I poured some into a jar and added water to see how well it absorbed.
The stuff fizzed like I had just poured a can of soda into a glass. No foam, but very loud fizzing and the jar quickly filled with a white vapor. My first thought was nucleation (that the clay was providing sites for the oxygen dissolved in the water to immediately fall out of suspension), but the vapor had a distinct smell and the clay was noticeably warmer to the touch than it had been earlier.
That makes me think there is a chemical reaction that took place, but I don't know what.
Anybody else messed around with this stuff? Didn't seem to continue reacting when I put more water in it, so it may just be that I need to rinse it thoroughly before using it, but it would be more comforting to hear someone else say "Oh yeah, thats normal. Always does that, don't worry about it."