Microwave waveguide

I'm not an electronics, geology, or medical guru, but I'd guess yes. Still, most of the people who get hurt by all these toxic things are people who work with it for a living, who mine it or grind it and inhale a tiny bit day after day, year after year. LIke professional waveguide scurbbers.

This assumes it's mica. I haven't seen it but I'm not convinced.

When I inhaled, I think, a piece of vinyl insulation from a piece of wire, my brother, an MD, said don't worry about it. Unnless you're a star athlete or spend time being chsed by lions, you have a lot more lung than you really need.

Particles of mica dispersed by arcing could possibly be hazardous - especially showered onto your dinner.

I certainly wouldn't risk it.

Whether you chose to follow suit is up to you, but in this situation I would remove the mica 'window' and throw it in the bin. The risk you would be taking is that certain foods pop or spit and particles could end up in the waveguide, usually this isn't a big problem unless you explode an egg in the microwave. When I did that a large chunk of egg white blocked the waveguide and overloaded the transformer.

Its also possible you might be able to locate a component supplier for consumer goods and order a new mica - but I'm guessing its pretty old as most use dielectric plastic these days.

The sparkle-looking stuff in toothpast is mica, and harmless.

Just don't snort mica dust like a line of coke or anything like that.