black arsenic is a rare chemical form of arsenic, which appears as a metastable phase in solid solutions.

Unlike its more common nonmetal counterpart, arsenic is a relatively toxic element that can cause severe health problems when taken in high doses. Its natural abundance is low, but humans can be exposed to it through contaminated soil, water or air.

The element is found in a number of minerals, including the sulfides realgar (As4S4) and orpiment (As2S3), along with sulfoarsenides such as mispickel and loellingite and the sulfoarsenide enargite. It is also present in a wide range of naturally occurring compounds.

Some of these compounds are highly toxic; they can damage the liver, kidneys and lungs. They can also cause cancer.

There are several ways to remediate arsenic contaminated groundwater, using physical, chemical and biological methods. For example, bacteria can transform arsenite, a more toxic form of the element, to the less toxic form, arsenate, under anoxic conditions.

This bioremediation technique is an inexpensive, environmentally friendly way to reduce the risk of inorganic arsenic exposure by consuming contaminated water. However, it has a few drawbacks, primarily that it is difficult to identify the source of the arsenite and to determine its amount.

Another potential use of this material is as a thermoelectric. Studies of a 2D layered semiconductor, black phosphorus, have shown that its properties can be greatly improved by doping with arsenic. This doping causes a structural change that alters the crystal lattice and increases its anharmonicity, slowing down the phonons that carry heat.