Extremely rare. Listed on mindat.org, but not by Gebhard (1999) or Lombaard et al.(1986).
Paragenetic and General Notes
Presumably encountered usually in polished section. However, a specimen of arsenopyrite (EDS and XRD confirmed) associated with mimetite is in the collection of Paul De Bondt (http://www.mindat.org/gallery.php?loc=2428&min=305; accessed November 2015).