Fissilis

Fissilis is a term used in mineralogy and geology to describe a material’s tendency to split readily along planar surfaces, producing thin, flexible sheets. The concept is closely related to fissility, the quality or degree of fissibility, and the word is sometimes encountered in adjectival form as fissile in various languages. The term derives from Latin fissilis, meaning able to be split.

Fissility is most pronounced in minerals with layered structures or weak interlayer bonds, such as sheet silicates (phyllosilicates) like muscovite and talc. It is also observed in certain fine-grained sedimentary rocks, notably shale, which tend to break into laminae. The property results from the alignment of crystal planes that provide planes of weakness, allowing separation with relatively little force. Fissility is a directional attribute and is distinguished from fracture, which describes breakage that is not restricted to cleavage planes.

In practice, identifying fissility involves observing whether a mineral or rock can be split into thin sheets along a preferred orientation, often yielding smooth cleavage surfaces. This characteristic is a useful diagnostic trait in mineral identification and petrological description, aiding the distinction between minerals with pronounced planar cleavage and those that fracture irregularly.

See also: Fissility; Cleavage (mineralogy); Sheet silicates.