Someone asked me to comment here:
This is all I know:
For my reef club I organized a buy of magnesium chloride. The material is coming from the Dead Sea Works in Israel. It came in 25 kg bags, and was a FLAKE form. I was given an analysis of the material from the manufacturer. This analysis was posted above by Randy. The analysis was for the GRADE of material I received, it was called industrial grade. I have NO idea if other grades are the same or different. When I told the guys was my application was they suggested the industial grade, which apparently is the same grade used by artificial salt mix manufacturers. It was clear from the analysis that the grade I recieved was fine for a reef tank. I have used it in my tanks with all good results.
Now.....it is quite possible that the manufacturer's other grade of MgCl2, the ice melt form, is identical. I have no idea. I was told by the manufacturer that the ice melt form does not have the same quality controls, and this makes sense to me. It is possible (though again I have no idea) that the material is all made in the same manner, and is initially identical chemically. But it is also possible, that after manufacturing the two grades are treated differently. I can imagine the ice melt stuff being dumped into rail cars, or huge cargo ships holds, I have no idea. Since the use is for melting ice, no one cares if there is a little be extra of this or than in it picked up during storage, packaging, etc. The 'little extra of this or that' is/are the things that would not show up in a simple analysis, but would only show up in trace levels likely far below 1%, but these same contaminants (if they exist) might be the ones that would be a problem in a reef tank.
Again...I have no REAL information as to whether the ice melt material is okay. For me, the cost of the high grade was trivial, most of the cost was in shipping. Buyer/user beware.