Seems to have to do with that it's "hard rock" instead of something else, and the purity? Not sure, not a chemist/geologist, but here is what they say in the article about it:
> The Freemans and researchers at Plumbago North knew the crystals they found were extraordinary as soon as they uncovered them. To determine the concentration of lithium, they sent bulk samples for analysis at labs in Australia and Germany.
> The results, said Simmons, were astonishing. The deposit showed an average lithium oxide content of 4.68% by weight. That’s three times the average concentration of the world’s top 10 hard rock lithium mines. By comparison, the world’s second-richest hard rock lithium deposit, the Bernic Lake mine in Manitoba, has a lithium oxide content of 2.76% by weight and contains roughly eight million tons of ore.
> The Freemans and researchers at Plumbago North knew the crystals they found were extraordinary as soon as they uncovered them. To determine the concentration of lithium, they sent bulk samples for analysis at labs in Australia and Germany.
> The results, said Simmons, were astonishing. The deposit showed an average lithium oxide content of 4.68% by weight. That’s three times the average concentration of the world’s top 10 hard rock lithium mines. By comparison, the world’s second-richest hard rock lithium deposit, the Bernic Lake mine in Manitoba, has a lithium oxide content of 2.76% by weight and contains roughly eight million tons of ore.