In mineralogy, fracture and cleavage are used to describe the tendency of minerals to break.
Fracture and cleavage are nothing but the positioning of the atoms present in a mineral and how they can break when stress is exerted on it.
To understand how fracture and cleavage are used to identify minerals, it is necessary to understand both properties.
What is a Cleavage?
A cleavage can describe how crystals can break when stress and pressure are exerted on them on a particular plane.
If any part of a crystal breaks as a result of stress and the part that breaks retains a shape that is smooth or like a crystal, you will understand that the mineral has a cleavage.
Minerals that do not produce crystallized fragments after they are broken do not have any cleavages.
Cleavage can be measured by three main factors, which are;
- The quality of the cleavage.
- The habit of the cleavage.
- The number of sides exhibiting the cleavage.
1. Quality of Cleavage
The quality of cleavage can be perfect, good, poor, indistinct, or nothing at all. Minerals that have the perfect cleavage can cleave and do not leave behind any rough surface.
When the crystal is broken, a smooth plane appears. Minerals with good cleavage also leave smooth surfaces; however, they often leave rough surfaces along with mineral residues.
A mineral with poor cleavage will not have a smooth edge, and the surface is rough and easily visible. If minerals have cleavages but cannot be noticed easily, they are known as indistinct cleavages.
Some minerals do not have any cleavage, and hence the broken surfaces will be rough and fractured.
2. Number of Sides that Exhibit Cleavage
Several minerals are known to exhibit cleavage only on a single side. However, a few minerals have cleavages on various sides of the crystal. There can be cleavages in one direction, two directions, three directions, or all directions.
3. Cleavage Habits
Various habits of cleavages exist, which depend on the crystallization mode of these minerals.
Listed below are the forms of cleavages that you should know about;
- Basal Cleavages: A cleavage that can be identified on the horizontal plane, known as a basal cleavage. The best examples of the basal cleavage are Mica.
- Cubic Cleavage: Cleavages that are exhibited on the minerals of an isometric crystal system are known as cubes. In this type of cleavage, the cubes can break off from a cube that already exists. For example, you have Galena.
- Octahedral Cleavage: A cleavage present on the minerals belonging to the isometric crystal system are octahedral cleavages, such as; Fluorite.
- Prismatic Cleavage: Cleavages exhibited on the prismatic minerals are the prismatic cleavages. Take Aegirine as an example.
What is a Fracture?
Fracture is the description of a way how a mineral tends to break. It is not similar to parting and cleavage. Fractures occur in all the minerals that may or may not have cleavages.
However, the direction of cleavage can diminish the presence of a fractured surface. Different minerals tend to break in different manners and leave behind surfaces described uniquely.
A fracture can be described as smooth, splintery, jagged, or irregular.
Types of Fractures
Most of the minerals break similarly, but some have unique fractures that should be studied.
Given below is a list of the types of fractures that are seen in minerals;
- The conchoidal fracture is commonly seen, and it can also be described as the calm shell fracture. This fracture resembles broken glass. Minerals like quartz are known to have this kind of fracture.
- The subconchoidal fracture is also common. It is not curved like the conchoidal fracture, but it is smooth. Andalusite shows this type of fracture.
- Another important type of fracture is an uneven fracture, which is commonly found in minerals like anhydrite.
- A jagged fracture is responsible for sharp edges and points. Metals like copper or a few oxides and sulfides can show this property.
- The splintery fracture normally occurs in minerals that are fibrous or acicular. The chrysotile serpentine is a mineral, which has this splintery fracture.
- The earthy fracture is similar to the texture of the broken clay. This type of fracture is located in minerals that are massive in size and are loosely consolidated. This includes minerals like limonite.
Sometimes fractures can be identified as hackly or irregular as they resemble each other. However, an experienced mineralogist can easily identify the difference between the different kinds of fractures present in a mineral. Fracture plays an important role in identifying a mineral.
How to Use Fracture and Cleavage for Identification of Minerals?
A mineral specimen does not need to be broken down to check if it has cleavage or a fracture. The areas where stress has been exerted on the mineral have to be checked first.
The marks of fracture are hardly present in the minerals that have perfect cleavages. The minerals that have poor cleavages can fracture more in comparison to the ones that have perfect cleavages.
How to Test?
The mineral has to be observed by a mineralogist to determine if there are any fractured edges or cleaved surfaces.
If the mineral has cleavages, the smoothness of the surface of the mineral has to be noted down. There might be no cleaved surfaces visible on the mineral.
This does not mean that there are no cleavages.
To determine a cleavage, a piece has to be chipped off from the mineral. This is something to do carefully and gently so that the value of the mineral does not degrade in any manner.
If fractures are discovered on the surface of the mineral, it can be determined that the mineral does not have cleavage at all.
See? It doesn’t take much know-how to undergo mineral identification if you know well about fracture and cleavage. I hope this article has helped your query. Cheers!
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