Trowly:
Brown chemicals are almost always impure.
Powder vs Crystaline should not make a difference though. You can't take crystals and make it magically less powerful by grinding it up to a powder ;-) Though it's likely that suppliers who bother to recrystalize it to yield crystaline material are also more rigorous in general, and less likely to give you an impure product.
On the topic of the filthy MDAI, lets have a brief review...
Most people in this thread have been getting it from a single asian supplier. It has only recently become available from other suppliers.
The one supplier first had the HCl, which was a light tan, water soluble powder. It was reasonably pure.
Then around october, they sold out of the HCl and switched to the freebase, i suspect from a different supplier, claiming >98.5% purity. This was a chocolate (or dog shit) brown color.
They now finally ran out of the shit-brown freebase, and are now selling the HCl again, at a lower claimed purity level (97.something%) - lets hope this isn't exaggerated as much as the freebase is!
Initially people fretted that people might take too much of the freebase, because it should be more potent. Of course, this turned out to be unfounded, because that batch of freebase was absolutely filthy. And i of course was one of the poor stooges who bought 5g of it (hey, it's more powerful than the salt, right? Riiiight...). Just the past few days i was trying to clean it up, by dissolving it in heptane and then evaporating the heptane.
I've managed to, out of 2 grams of brown shit MDAI, obtain around 500mg of white powder though i'm not done with it. I think the purity is below 50%, absolutely unacceptable. Typical of chinese exporters, and it really says something that they took that brown shit and sold it off, instead of telling their supplier where to shove it.
EDIT: actually, that 500mg may be all i can get out of this (making it only 25% pure). The last pull has almost nothing in it....
Of course now there are UK vendors with it "real soon now", promising white crystals, at much lower prices.