Update to this, from a friend of mine with a handy electron microscope
The chimney breast was uncontaminated vermiculite, well done to those who picked it
The kitchen tile fragment I sent him as an afterthought was chrysotile
Marvellous :( I've been hacking these up for a few weeks...
Yep, that would describe it well enough
Was it in use in the 50's or is it likely to be more recent?
I suspect this fireplace opening was reduced in size at some point with vertical house bricks around the edge...
Some bits look and feel like a very soft pale wood, but flake away to the touch. I guess a microscope would be needed but they don't really look like fibres
Other bits look more crystalline, perhaps where it has been exposed to more heat
I'm just hacking out a firebrick in a 1950s council house and there's some odd material behind it...
Initially looked like very crumbly mortar but on closer inspection looks to have insulating properties... fused together spherical lumps, part mortar maybe, part a kind a scaly crystalline...