Could be barite, selenium or gypsum. Indian's suggestion, desert rose, is pretty close to the mark. These three minerals will crystalize in sands and create the type of mineral formation you have there. Judging by the banding it came from a sedimentary deposit, most likely a dry lake bed, from the desert southwest. My opinion is that its a selenium based formation.
Thanks! I have a few Desert Roses but didn't know they came in two different mineral types. I guess this is a barite type and the others I have are selenite. I put it next to the other ones I have out back on the porch.