What it is
Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica is similar to granite in its crystalline composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
Mica can be composed of a variety of minerals giving various colours and transparency. Purple, rosy, silver and grey colours come from the mineral called lepidolite.
Dark green, brown and black come from biotite.
Yellowish-brown, green and white come from phlogopite.
Colourless and transparent micas are called muscovite.
All these have a pearly vitreous lustre.
The melting point of mica depends on its exact composition, but ranges from 700⁰C to 1000⁰C.
Glass has a specific gravity of about 2.5, and mica ranges from 2.8-3.1, so it is slightly heavier than glass.
Tips on uses of mica powder and flakes
The naturally occurring colours are largely impervious to kiln forming temperatures. Other added colours have various resistances to the heat of fusing. This is determined by the temperatures used to apply the colour to the mica. Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.
Mica does not combine with glass, but is encased by glass as it sinks into the glass surface. You can use various fluxes to soften the surface of the glass. Borax is one of those. The cleaving of the mica results in only the layer in contact with the glass sticking. The upper layers brush off. This applies to both powder and flakes. One solution is to fire with mica on top in the initial firing and then cap for the final one.
When encasing mica exercise caution. Micas flakes must be applied thinly, as air is easily trapped between layers which leads to large bubbles from between layers of glass. This is the result of the shearing of layers of the flakes allowing air between layers. Although powdered mica is less likely to create large bubbles, air bubbles are often created for the same reason. This is the reason it is most often recommended to fire the mica on top.
Of course, one use of the mica to make complicated designs is to cover the whole area and fuse. Then sandblast a design removing the mica from areas of the glass. You can then fire polish, or cap and re-fire to seal the mica.
MSDS for mica only mentions the inhalation of the dust as a risk. Mica is resistant to acid attack and is largely inert. Inhalation of the dust is a (low level) risk. Any significant health and safety problems relate to the coloured coatings.