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Conchoidal fracture describes the way that brittle materials break or
fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
when they do not follow any natural planes of separation.
Mindat.org Mindat.org is a non-commercial online database, claiming to be the largest mineral database and mineralogy, mineralogical reference website on the Internet. It is used by professional mineralogists, geologists, and amateur mineral collecting, mi ...
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 Mindat.org is a non-commercial online database, claiming to be the largest mineral database and mineralogy, mineralogical reference website on the Internet. It is used by professional mineralogists, geologists, and amateur mineral collecting, mi ...
Materials that break in this way include quartz, chert, flint, quartzite, jasper, and other fine-
grain A grain is a small, hard, dry fruit (caryopsis) – with or without an attached 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 legum ...
ed or
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wi ...
materials with a composition of pure silica, 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. Obsidian is produced from felsic lava, rich in the lighter elements s ...
and window glass, 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, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wi ...
, cryptocrystalline, and crystalline 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 flint, with this material property were widely sought after, traded, and fashioned into sharp tools in the
Stone Age The Stone Age was a broad prehistoric period during which stone was widely used to make tools with an edge, a point, or a percussion surface. The period lasted for roughly 3.4 million years, and ended between 4,000 BC and 2,000 BC, with t ...
. 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 ( 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 fissures emanating from the bulb of percussion. They are defined in contrast to the faceted fractures often seen in single crystals such as semiconductor wafers and
gemstone A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...
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. Obsidian is produced from felsic lava, rich in the lighter elements s ...
gives conchoidal fractures. File:2017-Obsidian-conchoidal-fracture.jpg, Conchoidal fracture in
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. Obsidian is produced from felsic lava, rich in the lighter elements s ...
File:Flintasten.JPG, Conchoidal fracture in flint File:Conch fract glass.jpg, Multiple conchoidal fractures in knapped glass


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 Lithic may refer to: *Relating to stone tools **Lithic analysis, the analysis of stone tools and other chipped stone artifacts **Lithic core, the part of a stone which has had flakes removed from it **Lithic flake, the portion of a rock removed to ...
stone tool A stone tool is, in the most general sense, any tool made either partially or entirely out of stone. Although stone tool-dependent societies and cultures still exist today, most stone tools are associated with prehistoric (particularly Stone Ag ...
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 state or condition of not meeting a desirable or intended objective (goal), objective, and may be viewed as the opposite of Success (concept), success. The criteria for failure depends on context, and may be relative to a parti ...
. As conchoidal fractures can be produced only by mechanical impact, rather than
frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a gas) ...
cracking for example, they can be a useful method of differentiating
prehistoric Prehistory, also known as pre-literary history, is the period of human history between the use of the first stone tools by hominins 3.3 million years ago and the beginning of recorded history with the invention of writing systems. The use of ...
stone tools from natural stones.


See also

* Fracture (mineralogy)


References


External links

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