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How do arsenate minerals form in open cavities and mineral veins?

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A labeled diagram of the periodic table entry for arsenic, showing its atomic number 33, symbol As, atomic weight 74.921595, electron configuration, and classification as an other nonmetal. Additional icons indicate arsenic is solid at room temperature, has a rhombohedral crystal structure, and weakly acidic oxides. — arsenic Properties of arsenic.

Arsenate minerals form in open cavities and mineral veins through specific chemical processes occurring at moderate to low temperatures. Their formation generally follows two primary pathways:1

Chemical Reaction: In some localities, such as the Långban mines in Sweden, arsenate minerals result from the reaction of arsenic acid ( $H_{3} A s O_{4}$ ) with minerals like pyrochroite [manganese hydroxide; $M n (O H)_{2}$ ].1
Oxidation: In many other environments, arsenate minerals are deposited as oxidation products of arsenide ores. These minerals precipitate from solutions within late-stage veins and open cavities.1

These minerals typically develop in open spaces—such as fissures or fracture zones—where solutions can circulate and deposit their mineral load. Because these environments provide the necessary free space, the resulting arsenate minerals are often able to develop well-formed crystal structures. These deposits are frequently associated with the oxidation of primary ores, serving as indicators for the presence of metals like cobalt, nickel, and copper.21