In a fine-grained quartz specimen. It is the only known natural pyroarsenate.
Approved IMA 2004 (Dana # Added)
Johanngeorgenstadt, Saxony, Germany. Link to MinDat.org Location Data.
Named for Peter (Pete) Allan Williams (b. 1950), Professor, Minerals and Materials Group, School of Science, Food and Horticulture, University of Western Sydney, Penrith South DC, N ew South Wales, Australia.