Image credit: Scott Brande
The Bottom Line on Fracture - Here on Top!
One particular pattern on the surface of a broken mineral fragment is called conchoidal fracture. "Conchoidal" (like "conch" a large and edible marine snail) is the term that relates the broken surface of a mineral to the curved surfaces of snails. This particular feature is present in only a few common minerals, such as pyrite, quartz and garnet.
Bottom Line: Conchoidal fracture is an important property when used in combination with others that helps you to identify a mineral by name. Check the Mineral Bank for this and additional information.
Errors/Suggestions: Contact Scott Brande (see footer).
What Is Conchoidal Fracture?
Background - Cleavage and fracture are both patterns of breakage.
Minerals with cleavage break along planar (flat) surfaces of weaker chemical bonds. When minerals break to expose large, flat surfaces, cleavage is said to be good to excellent.
Conchoidal fracture differs from cleavage because the broken surface is not a smooth, flat plane.
Conchoidal fracture is easily recognized and named as "conchoidal" because the curved pattern resembles the shape of the outside surface of snail shells.
Examples of Conchoidal Fracture
Conch shell. Note curved surfaces that are concave into the shell.
Image credit: Pearson Scott Foresman (https://commons.wikimedia.org/wiki/File:Conch_2_(PSF).png), „Conch 2 (PSF)“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-author
Manmade glass, knapped. Note the concave scars scooped into the glass, curved in shape. These are termed conchoidal.
Image credit: Tchenguise at English Wikipedia (https://commons.wikimedia.org/wiki/File:Conch_fract_glass.jpg), „Conch fract glass“, marked as public domain, more details on Wikimedia Commons: https://commons.wikimedia.org/wiki/Template:PD-user
Conchoidal fracture in rose quartz. Note the reflection of light off an irregular surface - the light breaks up into different sized bright spots, of irregular shape.
Image by R.Weller/Cochise College. http://skywalker.cochise.edu/wellerr/mineral/quartz/rose-quartz3.htm
Conchoidal fracture, close-up in rose quartz. Note the particular concave curved shapes to the fracture patterns.
Image by R.Weller/Cochise College. http://skywalker.cochise.edu/wellerr/mineral/quartz/quartz14.htm
Conchoidal fracture identified in crystalline sulfur by the presence of curved, concave surfaces.
Image by R.Weller/Cochise College. http://skywalker.cochise.edu/wellerr/mineral/sulfur/6sulfur-conchoidal1.jpg
"Subconchoidal" fracture in spodumene. Note the presence of concave breakage patterns, but not as well developed as in quartz or sulfur. The surface is distinctly not flat, and the pattern of light reflection includes different shapes and sizes.
Image by R.Weller/Cochise College. http://skywalker.cochise.edu/wellerr/mineral/hiddenite/hiddeniteL.htm
Conchoidal fracture in a variety of feldspar called albite. Note the reflection of light off the non-planar (not flat) surface - the light breaks up into different sized bright spots, of irregular shape.
Image by R.Weller/Cochise College. http://skywalker.cochise.edu/wellerr/mineral/feldspar/albite-cleavage3.htm
Instructions: How To Observe Fracture
Materials for test
sample that has been broken or fragmented from a larger sample
Procedure for the test
Observe a broken surface by angling the sample back and forth under a light
Describe the pattern, if any, of the reflection, and compare it to descriptions in your textbook reference table or other resource
Possible test results and interpretation
Positive result - the broken surface exhibits a conchoidal pattern. We conclude that the sample is one of only a few of the common minerals that fracture conchoidally - typically quartz, garnet, or pyrite.
Negative result - the broken surface does not exhibit a conchoidal pattern. We may see flat, reflective surfaces that indicate the sample exhibits one or more cleavage directions.