I'm looking for a series of books I read back in maybe 2005 - 2010. I think there were three books, either in the children's category or YA.
They featured magic users who I believe were hidden in normal modern society and used magical dust which they carried with them in pouches to power their spells. The dust came in different colors, with the good magic users having white dust and the evil magic users having black dust.
The plot of the second book featured the evil magic users discovering a suit of armor that had been made by an evil magic user long ago entirely out of black dust, and they had ground up the gloves giving them an ample supply of black dust.
I believe the plot of the third book involved a giant cavern hidden in the earth (perhaps below Antarctica?) filled with all sorts of magical things.