Fireplace construction

I have demolished the brick fascia on my fireplace (ghastly, hard to believe that was once in fashion) and am in the process of replacing it with something more to my taste. I have two questions for the assembled experts....
Firstly, the existing bricks were spaced about 1.5 inches from the fireplace opening (ie the proper chimney bricks), and the gap was filled with mortar. The mortar had begun to crumble in some places (on the top edge where I guess the heat flowed). Does there need to be such a big gap? Can I use a "normal" 0.5 inches to mortar between fascia and fireplace brick? It strikes me that this would make for a more solid structure, but I wonder whether the spacing was there to create distance between the fireplace blocks and the facing brick...
Secondly, given the proximity of this facing structure to the heat, is there any special sort of mortar mix I should use. Looks like the old one was a basic mortar mix, although as I say, it had started to crumble at the top edge.
TIA
Mat
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