Flying over a newly generated mesa biome in 13w39a (seed 4872183138497062395) I noticed a lava pool under the sand (coordinates x=456, y=64, z=292). The sand that fell into the pool is somehow normal sand instead of red sand. I would have expected this to be red sand.

Note that if you knock more red sand in the lava, it stays red sand as expected, so this probably has something to do with how this terrain is generated.