Unfortunately, Chemtura (BioGuard, AquaChem, many other brands) and especially Arch (HTH, other brands) have begun using magnesium sulfate to 'dilute' their calcium hypochlorite products. I think they may be using it elsewhere, too.

Regarding rain water -- I don't know if distilled water (rain water) could dissolve significant amounts of calcium and magnesium in the manner you imply. I'm guessing not, since roofing doesn't fail by losing all its grit. But you could check by testing the water you collect.