Help identify this mystery plant in my garden

[csziggy]

It was a volunteer but I've never seen anything like it in my garden. I did get a load of topsoil last winter so the seeds may have come in that. It is low growing, the blossoms only are about 4" above the ground, but it has spread to cover about half a square yard. The foliage is delicate. When we pulled weeds and mulched I left it because it is sort of attractive, even if does have purple flowers.