Stibarsen (previously considered a variety of allemontite but now a valid species) is a rare metallic species composed only of antimony and arsenic. This old locality in Mexico, seemed to have produced the best - a long time ago! These trickle out from old museum collections. Totally covering one side of the matrix is a crust of botryoidal, lustrous, metallic, balls to 1.2 cm across. Ex. Sal Avella collection. http://www.mindat.org/min-3775.html