fissil

Fissil is a term that appears in several scientific and linguistic contexts, but it is not widely used as a standard term in modern English. In many cases, it is considered an alternative or outdated spelling of fissile, and its precise meaning can vary by discipline.

In geology and petrology, fissil (often encountered as a nonstandard variant of fissile) relates to rocks that split easily along planes of weakness. The standard term for this property is fissility, and rocks with this texture are described as fissile rocks, such as shale and slate. Fissility arises from the alignment of platy minerals and fine laminations that create a planes of cleavage allowing smooth, parallel splitting.

In nuclear science, the established term is fissile. Fissile materials are capable of sustaining a nuclear chain reaction with slow neutrons; common examples include uranium-235 and plutonium-239. These materials are central to the design of many nuclear reactors and weapons. The concept is distinct from fertile materials, like uranium-238, which can be converted into fissile isotopes under suitable conditions.

Usage notes: fissil is rarely preferred in contemporary technical writing. When encountered, it is often prudent to interpret fissil as either a variant spelling or a historical/linguistic artifact, and to rely on fissile or fissility for precise scientific contexts. As with many technical terms, regional variations and historical texts may influence spelling.

See also: fissile, fissility, cleavage, shale.