How easily minerals break or deform under stress is determined by the type and strength of the chemical bonds that hold the crystals together. Mineralogists use terms including hardness, cleavage, fracture, and tenacity to describe mineral strength and how minerals break when stress is applied.
One of the most useful diagnostic properties is hardness, a measure of the resistance of a mineral to abrasion or scratching. This property is determined by rubbing a mineral of unknown hardness against one of known hardness or vice versa. A numerical value of hardness can be obtained by using the Mohs scale of hardness, which consists of 10 minerals arranged in order from 1 (softest) to 10 (hardest), as shown in Figure 1A. It should be noted that the Mohs scale is a relative ranking and does not imply that a mineral with a hardness of 2, such as gypsum, is twice as hard as a mineral with a hardness of 1, like talc. In fact, gypsum is only slightly harder than talc, as Figure 1B indicates.
The mineral gypsum, which has a hardness of 2, can be easily scratched with a fingernail. On the other hand, the mineral calcite, which has a hardness of 3, will scratch a fingernail but will not scratch glass. Quartz, one of the hardest common minerals, will easily scratch glass. Diamonds, hardest of all, scratch anything, including other diamonds.
In the crystal structure of many minerals, some atomic bonds are weaker than others. It is along these weak bonds that minerals tend to break when they are stressed. Cleavage (kleiben 5 carve) is the tendency of a mineral to break (cleave) along planes of weak bonding. Not all minerals have cleavage, but those that do can be identified by the relatively smooth, flat surfaces that are produced when the mineral is broken. The simplest type of cleavage is exhibited by the micas (Figure 2).
Because these minerals have very weak bonds in one direction, they cleave to form thin, flat sheets. Some minerals have excellent cleavage in one, two, three, or more directions, whereas others exhibit fair or poor cleavage, and still others have no cleavage at all. When minerals break evenly in more than one direction, cleavage is described by the number of cleavage directions and the angle(s) at which they meet (Figure 3).
Each cleavage surface that has a different orientation is counted as a different direction of cleavage. For example, some minerals cleave to form six-sided cubes. Because cubes are defined by three different sets of parallel planes that intersect at 90-degree angles, cleavage is described as three directions of cleavage that meet at 90 degrees. Do not confuse cleavage with crystal shape. When a mineral exhibits cleavage, it will break into pieces that all have the same geometry. If broken, they fracture into shapes that do not resemble one another or the original crystals.
Minerals having chemical bonds that are equally, or nearly equally, strong in all directions exhibit a property called fracture. When minerals fracture, most produce uneven surfaces and are described as exhibiting irregular fracture (Figure 4A). However, some minerals, including quartz, can break into smooth, curved surfaces resembling broken glass. Such breaks are called conchoidal fractures (Figure 4B). Still other minerals exhibit fractures that produce splinters or fibers, referred to as splintery fracture and fibrous fracture, respectively.
The term tenacity describes a mineral’s resistance to breaking, bending, cutting, or other forms of deformation. As mentioned earlier, nonmetallic minerals such as quartz and halite tend to be brittle and fracture or exhibit cleavage when struck. Minerals that are ionically bonded, such as fluorite and halite, tend to be brittle and shatter into small pieces when struck. By contrast, native metals, such as copper and gold, are malleable, which means they can be hammered without breaking. In addition, minerals that can be cut into thin shavings, including gypsum and talc, are described as sectile. Still others, notably the micas, are elastic and will bend and snap back to their original shape after stress is released.
By E. J. Tarbuck, F. K. Lutgens, Illustrated by D. Tasa