A new three-dimensional hydroxide-arsenate compound called compound 2 has been synthesized by heating (in air) of the sarkinite phase, Mn-2(OH)AsO4 (compound 1), with temperature and time control. The crystal structure of this high-temperature compound has been solved by Patterson-function direct methods. A relevant feature of this new material is that it is actually the first member of the adamite-type family with mixed-valence manganese(II,III) and electronic conductivity. Crystal data: a = 6.7367(5) angstrom, b = 7.5220(6) angstrom, c = 9.8117(6) angstrom, alpha = 92.410(4)degrees, beta = 109.840(4)degrees, gamma = 115.946(4)degrees, P (1) over bar. The unit cell content derived from Rietveld refinement is Mn-8(O4Hx)(AsO4)(4). Its framework, projected along [111], is characterized by rings of eight Mn atoms with the OH-/O-2(-) inside the rings. These rings form an almost perfect hexagonal arrangement with the AsO4 groups placed in between. Bond-valence analysis indicates both partial deprotonation (x congruent to 3) and the presence of Mn in two different oxidation states (II and III), which is consistent with the electronic conductivity above 300 degrees C from electrochemical measurements. The electron paramagnetic resonance spectra of compound 1 and of its high-temperature form compound 2 show the presence of antiferromagnetic interactions with stronger magnetic coupling for the high-temperature phase. Magnetization measurements of room-temperature compound 1 show a complex magnetic behavior, with a three-dimensional antiferromagnetic ordering and magnetic anomalies at low temperatures, whereas for compound 2, an ordered state is not reached. Magnetostructural correlations indicate that superexchange interactions via oxygen are present in both compounds. The values of the magnetic exchange pathways [Mn-O-Mn] are characteristic of antiferromagnetic couplings. Notwithstanding, the existence of competition between different magnetic interactions through superexchange pathways can cause the complex magnetic behavior of compound 1. The loss of three-dimensional magnetic ordering by heating of compound 1 could well be based on the presence of Mn3+ ions (d(4)) in compound 2.