It's probably a combination of calcium citrate and magnesium citrate. Let me guess: one tablespoon equals 500 mg calcium and 250 mg magnesium? See what it says on the back of the bottle.
Like you, I also wonder if Magnesium capsules can be substituted for the Epsom Salts . Sure would be a lot more pleasant. Have not seen the answer to this question anywhere. Maybe we will just have to experiment, but the epsom works so well I am timid about trying anything else.