While looking up
thermal expansion coefficients on wikipedia
Invar (a table of Invar's properties
here) caught my eye - it's a Nickel/Iron alloy with a linear coefficient of 1.2 compared to Pyrex's 3.2 (lower is better).
While a mirror the same dimensions as pyrex would be almost 4 times heavier (Specific gravity - Invar: 8.05 vs Pyrex 2.23) I would assume the mass could be reduced to match or exceed that of the equivalent diameter Pyrex mirror by judicious drilling and machining of the back.
Anyone ever worked with it?
FYI: apparently if you have an old CRT sitting around, there is a fair chance the shadow mask inside behind the face is made from Invar