Arsenoclasite (originally arsenoklasite) is a red or dark orange brown mineral with formula Mn5(AsO4)2(OH)4.
[3] A 1971 study identified the mineral's structure as an array of oxygen atoms in a double hexagonal close packed pattern.
However, a 1977 study discovered that all the manganese ions in arsenoclasite are six-coordinated, a constraint this structure cannot permit without severe distortion.
[7] Arsenoclasite was noted from specimens at Långban, Värmland, Sweden, by Gust Flink in 1924 as a mineral that appeared similar to sarkinite, but with one perfect cleavage.
[10] As of 2012[update], arsenoclasite is known from Långban, Sweden, the Valgraveglia Mine in Liguria, Italy, and the Iron Monarch open cut in South Australia.