The element Sulphur (S) is easily recognizable in its pure form, appearing as a bright, luminous yellow substance often found in crystalline masses. This distinct appearance frequently causes people to wonder if it is merely a chemical element or if it also holds the scientific designation of a mineral. Understanding whether this common yellow solid qualifies as a mineral requires examining the precise criteria scientists use to define these fundamental components of the Earth’s crust.
Defining the Term Mineral
For a naturally occurring substance to be classified as a mineral, it must satisfy a strict set of five universally accepted criteria established by organizations like the International Mineralogical Association. The first three requirements are that the substance must be naturally occurring (formed by geological processes), inorganic (excluding materials derived from living organisms like coal), and exist as a solid under normal geological conditions.
The final two criteria relate to the substance’s internal structure and chemistry. Every mineral must possess a definite chemical composition, expressed by a fixed chemical formula. Finally, a mineral must have an ordered internal atomic structure, meaning its atoms are arranged in a repeating, three-dimensional pattern known as a crystal lattice. This crystalline structure dictates many of the mineral’s physical properties, such as its hardness and cleavage.
Sulphur’s Classification as a Native Element
Elemental Sulphur found in geological settings successfully meets all five requirements necessary for mineral classification. When it occurs in its pure, uncombined state in nature, the substance is specifically designated by mineralogists as “Native Sulphur.” Its chemical composition is the simplest possible, consisting only of the element Sulphur, represented by the formula ‘S’.
Native Sulphur is a solid, naturally occurring, and inorganic substance, fulfilling the first three criteria of the definition. Crucially, it also possesses a highly ordered internal structure, crystallizing in the orthorhombic system. This specific, repeating atomic arrangement classifies the pure element as a mineral and places it in the sub-group known as the native elements, alongside substances like gold, silver, and diamond.
How Sulphur Forms and Where It Is Found
The formation of Native Sulphur is tied to several distinct geological environments, reflecting the element’s reactivity and mobility. One primary process is the deposition from volcanic gases, where hydrogen sulfide gas (H2S) reacts with oxygen near fumaroles and volcanic vents. This reaction causes the Sulphur to sublimate, passing directly from a gaseous state to a solid state to form crusts and crystals around the vent openings.
The largest commercial deposits often form through a biogenic process deep underground in sedimentary basins, such as those associated with salt domes. In this environment, sulfate-reducing bacteria utilize hydrocarbons to break down sulfate minerals like gypsum (CaSO4) and anhydrite (CaSO4). This microbial activity reduces the sulfate, releasing elemental Sulphur, which then crystallizes in rock fractures and pore spaces. Significant accumulations of Native Sulphur formed this way have historically been found in places like Sicily, Italy, and the caprock of salt domes along the U.S. Gulf Coast.
Distinguishing Native Sulphur from Sulphur Compounds
While Native Sulphur is a mineral, the majority of the element on Earth is bound up in compounds that are also minerals, which can lead to confusion. Native Sulphur is defined by its pure elemental composition (S), contrasting with the vast number of other Sulphur-bearing minerals that contain additional elements. These related minerals fall into two major chemical groups: the Sulfides and the Sulfates.
Sulfide minerals contain Sulphur bonded directly with a metal, such as in pyrite (FeS2), also known as “fool’s gold,” or galena (PbS), a lead ore. Sulfates, conversely, contain the sulphate ion (SO4), where Sulphur is bonded to oxygen and then to other elements. Common examples of sulfate minerals include gypsum (CaSO4 · 2H2O) and barite (BaSO4). Although these compounds meet all the criteria to be classified as minerals, their complex chemical formulas differ significantly from the simple ‘S’ of Native Sulphur, making them chemically distinct mineral species.