Stibarsen or allemontite is a natural form of arsenic antimonide (AsSb) or antimony arsenide (SbAs).

The name stibarsen is derived from Latin stibium (antimony) and arsenic, whereas allemontite refers to the locality Allemont in France where the mineral was discovered.

[3][5] It is found in veins at Allemont, Isère, France; Valtellina, Italy; and the Comstock Lode, United States; and in a lithium pegmatites at Varuträsk, Sweden.

[8] Stibarsen has the same crystal structure as arsenic and antimony, with the intermediate values of the lattice parameters.

Weak bonding between the sheets accounts for the relatively low hardness of As, Sb and AsSb.