where in canafis to i mine it?

Not canifis, on the road to canifis. There's a road northeast of varrock. Take it past the gate to another road. North on the road should be a limestone mine.

It's just before where you talk to the man for the Rag and Bone quest...so if you can't find it ...check a quest guide for this guys location ...it's closer to Varrock than Canifis