Unfortunately science is against you in finding anything bigger than microbes anywhere below the crust, and sadly I learned that apparently silicon isn't nearly as good as carbon in bonding complex molecules, so silica lifeforms are less likely than I had thought, here or elsewhere.
But, given that you're writing a story, you could go past current science, and speculate on possibilities. There's one book about life found on the surface of a neutron star (I can't recall the name), your setting isn't any more extreme than that.
I would suggest go past the crust, and use the fluid mantle as a setting for the creature's world, and the creatures themselves be a collection of solidified, organized minerals (crystalline?)
Have the reason they don't come up where we are be two fold...it's much too cold for them to survive normally, and the pressure lessens, making them lose cohesion (opposite of us, too hot, and crushing pressures).
Just some ideas...I haven't found any good science that discusses anything larger than bacteria.