I was under the impression that the cause was chronic inflammation resulting from recruitment and activation of macrophages and other immune cells to site of the fibres [1]. Chronic inflammation causes cancer on its own independent of external factors like free radical formation.
Well, I should have added details. You are correct that asbestos that doesn't contain iron can still cause cancer through a hypothesized inflammatory mechanism.
Honestly, I don't we know the exact mechanism, but have some theories that seem to fit.
And that, perhaps cryptically, was my point. Why should any chemically inert mineral cause cancer? But some apparently do, regardless. Inflammation seems most plausible.
