A conchoidal fracture is a break or

fracture Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

of a

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. ...

material that does 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 Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, chert,

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. Historically, flint was widely used to make stone tools and start ...

, quartzite, jasper, and other fine-

grain A grain is a small, hard, dry fruit (caryopsis) – with or without an attached husk, hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and ...

ed or

amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...

materials with a composition of pure

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

, such as

obsidian Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...

and window

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

, 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 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 In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...

, cryptocrystalline, and

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, macros ...

line 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 chert, or

, with this material property were widely sought after, traded, and fashioned into sharp tools in the

Stone Age The Stone Age was a broad prehistory, prehistoric period during which Rock (geology), stone was widely used to make stone tools with an edge, a point, or a percussion surface. The period lasted for roughly 3.4 million years and ended b ...

. Conchoidal fractures often result in a curved breakage surface that resembles the rippling, gradual curves of a

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

shell; the word "conchoid" is derived from the word for this animal ( ''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 fissures emanating from the bulb of percussion. They are defined in contrast to the faceted fractures often seen in single

s such as semiconductor wafers and

gemstone A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...

s and to the high-energy ductile fracture surfaces desirable in most structural applications. File:Lipari-Obsidienne (5).jpg,

Obsidian Obsidian ( ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter element ...

gives conchoidal fractures. File:2017-Obsidian-conchoidal-fracture.jpg, Conchoidal fracture in

File:Flintasten.JPG, Conchoidal fracture in

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

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 tool Stone tools have been used throughout human history but are most closely associated with prehistoric cultures and in particular those of the Stone Age. Stone tools may be made of either ground stone or knapped stone, the latter fashioned by a ...

s, conchoidal fractures form the basis of flint knapping, 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 Failure is the social concept of not meeting a desirable or intended objective, and is usually viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. On ...

. 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 Prehistory, also called pre-literary history, is the period of human history between the first known use of stone tools by hominins million years ago and the beginning of recorded history with the invention of writing systems. The use o ...

stone tools from natural stones.

References

External links

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

Subsets

Lithics

See also

References

External links