Black, or amorphous arsenic (chemical formula As) is synthesized first through the sublimation of gray arsenic followed by condensation onto a heated surface.  This structure is thought to be the arsenic analogue of red phosphorus. The structure of black arsenic in its crystalline phase, while not synthesized in its pure form, is by extension analogous to black phosphorus, and takes on an orthorhombic structure built from As rings. Black arsenic has as-yet been synthesized only in the presence of atomic impurities including mercury,
phosphorus, and oxygen, though a pure form of black arsenic was found in the Copiapó region of Chile. Mechanical exfoliation of the mineral found in Chilean caves, arsenolamprite, revealed a molecular structure with high in-phase anisotropy and potential as a semiconducting material.