Another one that deserves death.
Where the hell did you get the "purity" crap from? Probably from your ass(the place from which you get most of your statements).
There's nothing in the manga that hinted anything about purity.
Stop creating BS.
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Juubi is made of natural energy the one that SM users use to make oneself stronger and up their chakra power.
SM mix nature energy with their own in 1 to 2 proportion thus making it 33.33...% of nature energy.
Juubi doesnt mix nature energy with anything its always pure, more bijus only result in more chakra.