Arsenic (As) is a naturally occurring element widely distributed throughout the Earth’s crust. It is a metalloid, meaning it possesses characteristics of both metals and nonmetals, and is often associated with mining and environmental contamination due to its toxicity. The question of whether arsenic is a mineral requires a precise application of geological and chemical definitions, which distinguish the element itself from the compounds it forms.
The Strict Criteria for Mineral Classification
To be classified as a mineral, a substance must satisfy a strict set of five geological and chemical requirements.
- It must be naturally occurring, meaning it is not created artificially in a laboratory or factory setting.
- It must be a solid under normal Earth surface conditions, which immediately excludes liquids and gases.
- It must be inorganic, separating it from materials derived from living organisms.
- It must have a definite, though sometimes variable, chemical composition that can be expressed by a chemical formula.
- It must possess an ordered internal atomic structure, commonly called a crystalline structure.
This highly organized, three-dimensional pattern of atoms is what gives minerals their characteristic physical properties.
Is Elemental Arsenic Classified as a Mineral?
Native Arsenic
When applying the classification criteria, the element arsenic itself, in its pure native state, does meet the requirements to be considered a mineral. Geologists formally recognize a substance called Native Arsenic (As), which occurs naturally in the Earth and forms a solid with a specific crystalline structure. This material is a relatively rare example of a native element mineral, which are minerals composed of a single element, such as native gold or native copper.
Arsenic Compounds
However, Native Arsenic is not the most common form of arsenic found in the environment. Most arsenic is encountered as a component of various mineral compounds. Arsenic frequently combines with sulfur and other elements to form complex ore minerals like arsenopyrite (\(\text{FeAsS}\)) and orpiment (\(\text{As}_2\text{S}_3\)), which are formally classified as sulfide minerals. The vast majority of arsenic in the Earth’s crust is locked within the chemical structure of these other mineral species.
The Distinction Between Inorganic and Organic Arsenic Forms
Arsenic compounds are broadly divided into two categories: inorganic and organic, based on whether the arsenic atom is bonded to carbon. This distinction is important when discussing arsenic’s environmental presence and biological effects.
Inorganic arsenic compounds do not contain carbon and are the forms most commonly found in groundwater and soil. These forms, such as arsenite (\(\text{As(III)}\)) and arsenate (\(\text{As(V)}\)), are generally the most toxic varieties of arsenic.
Organic arsenic compounds, in contrast, contain arsenic atoms bonded to carbon atoms. These forms are generally considered much less harmful to humans and are frequently found in marine life, often referred to as “fish arsenic.” Common examples include arsenobetaine and arsenocholine, which are largely non-toxic and rapidly eliminated by the human body.