Flooring Tile - Replacing marble tiles with granite tiles

I currently have (well, HAD, as of this afternoon ) 12 x 12 marble tiles covering the hearth of my fireplace. The hearth is fairly low to the ground (2" tall). Well, of course the green marble tiles don't match what we want to do with the place, so black galaxy granite tiles (also 12 x 12) are going in the place of the marble. So, this afternoon, I beat out the old marble tiles and found that the hearth is made out of bricks. The marble tiles were set into a mortar bed that appears to have been laid either directly on the bricks or there may be something very thin between the brick and mortar bed, but it's difficult to tell. Anyway, about 95% of the mortar bed is still on the bricks of the hearth.

My question is, now what do I do? I had planned on putting 1/2" cement backerboard down (in a layer of thinset), then thinset the granite tiles to the backerboard. Do I need to remove the old mortar? Or can I smooth out the old mortar bed with fresh thinset (or floor leveler or something), then once dry apply my thinset for the cementboard? If I have to remove the old mortar, any tricks? I really do not want to spend the next 2 weeks beating the old mortar out with my wonderbar. There's got to be an easier way.

You can grind down the mortar bed pretty easily, but the dust will be unbearable. Smooth out the top of the mortar bed with thinset and then set your stone. Use white modifed thinset to set the stone, no cement board needed at all.

If you dont mind spending about $40 on a tool rental you can get a demolition hammer from Home Depot Tool Rental. Put a 2" spade on the business end of that baby and you will be finished in 15 minutes.

Thanks for the info. Tilebri, would the fresh thinset adhere to the old mortar that is already on the bricks? And why white thinset, rather than gray thinset? I have some leftover gray thinset from when I tiled my bathroom floor a few weeks ago...I was hoping to use that (not just because I'm cheap, which I am, but because I want that stuff out of my woodshop). I thought I read something in another thread about staining if using the gray, but that might be the coffee (and my bladder) talking. Is that a real problem? Again, thanks for everyone's advice.