I've purchased a 42" wide glass bi-fold door unit for my fireplace. There is four bracket tabs to fasten this unit to the firebox masonry. The fireplace surround is natural stone and not brick and because of this a simple gasket isn't going to seal the door assembly to the fireplace surround. The stone is irregular and gaps between the glass door frame and fireplace are as much as 2 inches and as little as 1/8". I was considering using a standard masonry mortar, mixed rather stiff and apply this to fill the gaps between the doors and fireplace. The other option was using a masonry caulking product which I think would be easier to apply as a trowel will be difficult to get the mortar deep between the frame and the fireplace surround.
My concerns are if I use a non flexible material (like mortar) will it crack? Will the heat alone crack the product? The door frame is steel which will contract and expand with the heat of the fireplace. I don't want to have to do this job twice so I'm asking the advice of others.