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Malphite should have no problem farming gold with his AoE Skill. You should easilly be able to land 2 or 3 last hits on each wave with it. When you get good at it (and judging the damage the enemy creep wave is taking, so you know which creep to smack before you AoE) you can get all or almost all of the enemy creep wave. Stack some armor in runes and masteries, maybe cloth armor and potions to start and it should do PLENTY of damage, making it very easy to last hit multiple creeps with.
The shard is not a great harassing tool. The damage it does is too low. Its far better off spending your early mana AoE'ing (and possibly hitting the enemy as well) then throwing shards at them. Save the shard for when they screw up and the speed change can net you a kill or assist.
You really shouldnt have any trouble farming with malphite. As far as tanks go, hes one of the best at it.