Freakin' awesome marble, looks freakin' awesome with that floor! Sorry I could not be of help, but that is hubba-hubba lovely. What kind of marble is that? LOVE that brown veining!

typically you can set marble on a subtop; thinset not req'd but would also be OK.

Seam adhesive is a little tricky as it can be quite difficult to match the "whiteness" of the stone close enough to be satisfactory. If the seam edges are square and you can get the pieces together so the seam is a hairline then a clear adhesive is a good option. However most "clear" stone adhesives yellow a bit when curing which can be conspicious next to a truly white material. There are stone adhesives specially formulated to stay "water clear". However, the adhesives are usually sold in quantities far larger than you'd need for your fireplace (and it's expensive). Best bet might be to pay a fabricator to install making sure he knows you don't want yellow seams.

if the seams aren't tight another trick is to mash up some of the same stone and use the resulting powder as a coloring agent in the clear adhesive. Properly done this can be near invisible.

maybe Treb has a better option?

For this application, place quarter-sized dabs of silicone every foot or so, run a bead of clear silicone across the pieces and butt the big piece to them. Drive a nail or screw about 1/8" from the front edge and drive a shim between the nail/screw and the front edge, creating a seam clamp. Let the squeeze-out dry, then cut it off with a razor blade to avoid shrinking. I wouldn't worry too much about color match on a seam so far away from view. You can't do anything about the movement, why fuss over the color?

You may want to prime the back of that marble to prevent any adhesive bleed-through. I've heard of this happening. It eventually went away, but the customer panicked in the meantime.

Remember, less is more. You only need to keep this thing from wriggling.

So by prime you mean paint with white primer? Or some other product. Sounds like the silicone would work great. The legs of the mantle are coming down right in this spot so the seams will be mostly covered. Thank you both sooo much for replying. I really appreciate it!

The slab is actually one I found on craigslist for $50. Someone had used it as a counter top in their basement "soda fountain". They didn't know what kind it was but I was definitely drawn to the beautiful tan veining.

kam76:

I think you'd be fine spraying it with some clear finish. Anything that sticks well and prevents the marble from wicking up adhesive.

I can never find such deals. Especially when I need them! Congrats, great find and lovely piece, perfect on that floor. Yum.

This was fascinating, though I'd never have the nerve to try it. Don't we have skilled and generous experts here? I am always amazed.