Stibarsen or allemontite is a natural form of arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 antimonide

Antimonide

Antimonides are compounds of antimony with more electropositive elements. The antimonide ion is Sb3−.Many of them are flammable or decomposed by oxygen when heated since the antimonide ion is a reducing agent....

 (AsSb) or antimony arsenide (SbAs). The name stibarsen is derived from Latin stibium (antimony) and arsenic, whereas allemonite refers to the locality Allemont

Monêtier-Allemont

Monêtier-Allemont is a commune in the Hautes-Alpes department in southeastern France.-Population:-References:*...

 in France where the mineral was discovered. It is found in veins at Allemont, Isère, France; Valtellina

Valtellina

Valtellina or the Valtelline valley ; is a valley in the Lombardy region of northern Italy, bordering Switzerland. Today it is known for its skiing, its hot spring spas, its cheeses and its wines...

, Italy; and the Comstock Lode

Comstock Lode

The Comstock Lode was the first major U.S. discovery of silver ore, located under what is now Virginia City, Nevada, on the eastern slope of Mount Davidson, a peak in the Virginia Range. After the discovery was made public in 1859, prospectors rushed to the area and scrambled to stake their claims...

, Nevada; and in a lithium pegmatite

Pegmatite

A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....

s at Varuträsk, Sweden. Stibarsen is often mixed with pure arsenic or antimony, and the original description in 1941 proposed to use stibarsen for AsSb and allemontite for the mixtures. Since 1982, the International Mineralogical Association

International Mineralogical Association

The International Mineralogical Association is an international group of 38 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 4000 plus known mineral species...

 considers stibarsen as the correct mineral name.

Structure

Stibarsen has the same crystal structure as arsenic and antimony, with the intermediate values of the lattice parameters. This structure (space group

Space group

In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...

 Rm No. 166) is variably described as hexagonal, trigonal and rhombohedral because of the overlap between these terms (see trigonal crystal system). Simulation of the X-ray diffraction intensities reveals that the Sb and As atoms form ordered (or partly ordered) sublattices in SbAs. The atoms are arranged in warped graphite-like sheets, which extend normal to the c axis. Week bonding between the sheets accounts for the relatively low hardness of As, Sb and AsSb.

Mineral	a (nm)	c (nm)	ρ (g/cm3)	ref.
As	0.376	1.055	5.78
AsSb	0.4025	1.084	6.37
Sb	0.43056	1.125	6.72