CONCHOIDAL FRACTURE OF OPAL

WHAT IS FRACTURE?

Fracture is a description of the way a mineral tends to break. It is different from cleavage and parting which are generally clean flat breaks along specific directions. Fracture occurs in all minerals even ones with cleavage, although a lot of cleavage directions can diminish the appearance of fracture surfaces. Different minerals will break in different ways and leave a surface that can be described in a recognizable way. Is the broken area smooth? Irregular? Jagged? Splintery? These are some of the ways of describing fracture.

Although many minerals break in similar ways, some have a unique fracture and this can be diagnostic.

• The most common fracture type is conchoidal. This is a smoothly curved fracture that is familiar to people who have examined broken glass. Sometimes described as a clam-shell fracture. Quartz has this fracture type and almost all specimens that have been broken, demonstrate this fracture type very well.

• Another common type is subconchoidal. Similar to conchoidal, just not as curved, but still smooth. Andalusite can show this type.

• Uneven is a type that is basically self explanatory. It is a common type that is found in anhydrite.

• Unlike uneven, jagged has sharp points or edges that catch on a finger that's rubbed across the surface. Usually this indicates a metal such as copper , a metal alloy or some sulfides or oxides.

• Splintery is a fracture type that occurs in fibrous or finely acicular minerals and in minerals that have a relatively stronger structure in one direction than the other two. Chrysotile serpentine is a typical mineral with splintery fracture and kyanite is an example of a non-fibrous mineral that has this fracture.

• Earthy is a fracture that produces a texture similar to broken children's clay. It is found in minerals that are generally massive and loosely consolidated such as limonite.