Quiz: Remember the definition of mal de mer?
any member of a rare mineral group consisting of compounds of one or more metals with arsenic (As). The coordination of the metal is almost always octahedral or tetrahedral. In the former case, each metal ion occupies a position within an octahedron composed of six oppositely charged arsenic ions, whereas in the latter each of the metal ions is surrounded by six oppositely charged neighbours arranged tetrahedrally. Structurally the arsenides resemble the sulfides (e.g., galena, sphalerite, and argentite) and are frequently included in that mineral group (see sulfide)