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 displ ...
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 mineralogical reference website on the Internet. It is used by professional mineralogists, geologists, and amateur mineral collectors alike.
The ...
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 mineralogical reference website on the Internet. It is used by professional mineralogists, geologists, and amateur mineral collectors alike.
The ...
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 f ...
,
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 start ...
,
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-
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 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 (quenchin ...
, 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,
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 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
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 ...
, or
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 start ...
, 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 ...
.
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
faceted 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 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 gives conchoidal fractures.
File:2017-Obsidian-conchoidal-fracture.jpg, Conchoidal fracture in obsidian
File:Flintasten.JPG, Conchoidal fracture in 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 start ...
File:Conch fract glass.jpg, Multiple conchoidal fractures in knapped 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 (quenchin ...
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
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 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 state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. One ...
. 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.
See also
*
Fracture (mineralogy)
In the field of mineralogy, fracture is the texture and shape of a rock's surface formed when a mineral is fractured. Minerals often have a highly distinctive fracture, making it a principal feature used in their identification.
Fracture differ ...
References
External links
* {{Wiktionary-inline, conchoid
Lithics
Materials degradation
Mineralogy concepts
Petrology concepts
Stone Age