Arsenoclasite (originally arsenoklasite) is a red or dark orange brown mineral with formula Mn₅(AsO₄)₂(OH)₄. The name comes from the Greek words αρσενικόν (for arsenic) and κλάσις (for cleavage), as arsenoclasite contains arsenic and has perfect cleavage. The mineral was discovered in 1931 in Långban, Sweden.