I say this having read the mailing list and specifically the part that mentions the mirror is made of fused silica:
What the hell is that mirror made of?
Is this kind of behaviour a well-documented/practical property of silica? I would assume that it'd shatter after any amount of shots and a hammer blow.
It Depends(tm). Usually on the grain alignment/lack thereof. As long as the grains are suitably randomized, it can be very durable. [https://en.m.wikipedia.org/wiki/Quartzite]
What the hell is that mirror made of?
Is this kind of behaviour a well-documented/practical property of silica? I would assume that it'd shatter after any amount of shots and a hammer blow.