Conchoidal fracture describes the way that

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Br ...

materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically slightly concave, showing concentric undulations resembling the lines of growth of a shell".Conchoidal fracture
at Mindat.org Materials that break in this way include

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

flint Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and sta ...

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

jasper Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases,Kostov, R. I. 2010. Review on the mineralogical systematics of jasper and related rocks. – Archaeometry Workshop, 7, 3, 209-213PDF/ref> ...

, and other fine- grained or amorphous materials with a composition of pure

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

, such as obsidian and window

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

, as well as a few metals, such as solid gallium. Crystalline materials such as quartz also exhibit conchoidal fractures when they lack a

cleavage plane Cleavage, in mineralogy and materials science, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in th ...

and do not break along a plane parallel to their crystalline faces. So, a conchoidal, or uneven, fracture is not a specific indication of the amorphous character of a mineral, or a material. Amorphous,

cryptocrystalline Cryptocrystalline is a rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert and flint are crypt ...

, and

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

materials can all present conchoidal fracture when they lack a preferential cleavage plane. Conchoidal fractures can occur in various materials if they are properl
percussed
(struck). Cryptocrystalline silica, such as

, or

, with this material property were widely sought after, traded, and fashioned into sharp tools in the Stone Age. Conchoidal fractures often result in a curved breakage surface that resembles the rippling, gradual curves of a mussel shell; the word "conchoid" is derived from the word for this animal ( grc, κογχοειδής ''konchoeidēs'' < ''konchē'')., . A swelling appears at the point of impact called the ''bulb of percussion''. Shock waves emanating outwards from this point leave their mark on the stone as ripples. Other conchoidal features include small

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure ...

s emanating from the bulb of percussion. They are defined in contrast to the

facet Facets () are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure. Gemstones commonly have facets cut ...

ed fractures often seen in single

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s such as semiconductor

wafers A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...

and gemstones and to the high-energy

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

fracture surfaces desirable in most structural applications. File:Lipari-Obsidienne (5).jpg, Obsidian gives conchoidal fractures. File:2017-Obsidian-conchoidal-fracture.jpg, Conchoidal fracture in obsidian File:Flintasten.JPG, Conchoidal fracture in

File:Conch fract glass.jpg, Multiple conchoidal fractures in

knapped Knapping is the shaping of flint, chert, obsidian, or other conchoidal fracturing stone through the process of lithic reduction to manufacture stone tools, strikers for flintlock firearms, or to produce flat-faced stones for building or facing w ...

Subsets

Several subdefinitions exist, for instance on the Webmineral website:Mineral Tenacity and Fracture
at Webmineral * Brittle—conchoidal: very brittle fracture producing small, conchoidal fragments * Brittle—subconchoidal: brittle fracture with subconchoidal fragments * Conchoidal—irregular: irregular fracture producing small, conchoidal fragments * Conchoidal—uneven: uneven fracture producing small, conchoidal fragments * Subconchoidal: fractures developed in brittle materials characterized by semi-curving surfaces

Lithics

In lithic stone tools, conchoidal fractures form the basis of flint

knapping Knapping is the shaping of flint, chert, obsidian, or other conchoidal fracturing stone through the process of lithic reduction to manufacture stone tools, strikers for flintlock firearms, or to produce flat-faced stones for building or facing w ...

, since the shape of the broken surface is controlled only by the stresses applied, and not by some preferred orientation of the material. This property also makes such fractures useful in engineering, since they provide a permanent record of the stress state at the time of failure. As conchoidal fractures can be produced only by mechanical impact, rather than frost cracking for example, they can be a useful method of differentiating prehistoric stone tools from natural stones.

References

External links

* {{Wiktionary-inline, conchoid Lithics Materials degradation Mineralogy concepts Petrology concepts Stone Age

Subsets

Lithics

See also

References

External links