I mix mica with powdered glass a lot. Different colors mixed with different colors of mica give a whole new range of colors than you can get with powders alone. Not to mention the sparkly factor. I use cosmetic micas & have done extensive testing – you can see some of my results here: http://www.steiderstudios.com/mica.html
My web designer isn’t as fast as I am, so the photos aren’t loaded yet… when mixing mica with powder, I found a little goes a long way. I start with a mix of 4:1 (for example) glass:mica, then dilute by half with more powder; dilute that by half, etc, etc.