Mycalex is a composite of mica and glass, one of Dan Gelbart’s favorite ceramics for its cheapness and ease of machining, though it’s much less popular now than it was in the 1940s. (Machinable glass-ceramics that precipitate mica crystals during heat treatment were invented in the 1970s and may be responsible for some of this). But mica is used as a filler in many composite materials.
I dissected a broken microwave last weekend and found that the window protecting the magnetron from spattering food seems to be a mica composite; it’s slightly translucent to light, but when heated with a butane torch to orange heat, it remained intact and barely burned. It did turn black and outgas a little, enough to blister the surface a bit. Wikipedia says phlogopite can withstand 900°, though some other micas only survive to 500°, so this seems likely to be phlogopite-based “mica paper”, as the USGS calls it. It isn’t pure phlogopite, because that wouldn’t turn black. Many vendors offer sheets of “mica” on MercadoLibre for microwave repairs, suggesting to cut them to the correct size with scissors or a razor knife.
I think such sheets (or sheets of pure mica) were the traditional form on which wire resistors were zigzagged, from which we get the schematic symbol.
WP also points out that it’s used in drywall mud, presumably for mechanical strength, and as a filler in paint and plastics, where it has many benefits, including increasing strength and dimensional stability. Dry-ground mica is dull, wet-ground mica is sparkly.
It occurs to me that including ground mica in “Starlite” might help it retain strength when it’s being charred. (And maybe foam up better, too.) Similarly, it seems that it might help keep alabaster from crumbling when dehydrated; see the note on plaster foam. The alabaster would still remain solid to a higher temperature than the mica, but it becomes very friable when dehydrated (at under 200°); retaining substantial strength to 900° could be very valuable.
Ceramics-supply vendors on MercadoLibre sell finely ground mica for about US$2 per kg, but of course they don’t tell you which mica it is. Calcining microwave-oven window panels would be a sure way to get refractory mica, but it’s rather expensive by comparison. The USGS says scrap and flake mica costs US$120–165/tonne at wholesale; this is an interestingly low price because it means that alabaster mixed with mica is still cheaper than muriate of lime; see the note on desiccant climate control. If the admixture of mica were under about 40%, the mixture would cost the same order of magnitude as the alabaster alone.