Silicon Dioxide (silica) is a constituent part of the chemical composition of the clay (Al2
O) from which the quarry tile is formed. (Incidentally, roughly 60% of the earth's crust is silica, aka, "sand.")
I hesitate to point this out, but if the possible toxicity of silica is a concern, exposure from baking with quarry tiles should be the least of your worries.
Silicon Dioxide (SiO2) is a common anti-caking/anti-clumping agent
, so if you're using just about ANY powdered ingredients, dried spices, dry mixes, grated cheese, etc.
, you're already ingesting non-trivial amounts of silica. Not only that, but the dessicating packets commonly found in, e.g.
, pepperoni pillow packs, vitamin and prescrption drug bottles, etc
—the ones that say "Do Not Ingest"—are packed with silica gel.