All that glitters is not gold... it may be... Mica! The name Mica may derive from a root word meaning to glitter. Mica Titanate adds a shimmer of pearlescence. There is quite a bit of diversity within this pigment code, and paint labeling doesn't help to sort it out very much. We think in terms of pearlescents which tend to have a whitish or pastel luster to the color, metallic-looking pigments -- sometimes called iridescents-- which imitate real metals and may be made of powdered metal or of PW20. The metallics don't have the same pastel pearl white that the pearlescents have but may also be made of mica. There also the shiny color change pigments called interference colors which are hard to say which color they are since they have a color change effect depending on the angle of viewing.
A rather serious heads up is given if one is working with dry pigments that this pigment code can contain some crossover with talc, which may contain asbestos. Monona Rossol has done extensive work on talc and asbestos. She writes, "Some talc minerals still contain silica and asbestos," so please consult her research even if you are using something labeled 'asbestos free' talc-- as it may not be, it is worth looking into what she has found.
In terms of labeling, sometimes we see PW20 listed alongside iron oxides, which may gesture to a certain iridescent process, or even just "Pearlescent pigment" listed for a pigment code, or no pigment code at all.
Mica, hydrous aluminium potassium silicate, can be processed in any number of interesting ways, some of which involve coating it with Titanium dioxide white or other pigments. The mica can also be coated with iron oxide, and sometimes we see things like Mica Titanate Iron Oxide.
Mica can also be used on its own as it is shiny, and can depending on the binder, chunks of it can be used whole. Artiscreation mentions that the larger sheets or bits of mica don't really work all that well in an oil film, which may be why some of the larger or coarser iridescent colors are seen in acrylic but not oil. In general there are a lot more acrylic pearlescents.
For more about pearlescent pigments, see What are Pearlescent Pigments and How do they Work? by Natural Pigments.
