Feldspar is one of the most common mineral groups on Earth, making up a significant portion of the Earth’s crust. This abundance means it is frequently encountered in many common rock types, like granite. Identifying this mineral relies on recognizing its distinct physical properties, such as hardness, color, and luster. A major characteristic used for identification is how the mineral breaks, a property known as cleavage. This article confirms that feldspar possesses this defining feature and details its specific characteristics.
Understanding Mineral Cleavage
Cleavage is the tendency of a crystalline solid to break along smooth, flat surfaces, known as cleavage planes. This property is a direct result of the mineral’s internal atomic structure. The orderly, repeating arrangement of atoms in the crystal lattice creates planes where the chemical bonds holding the atoms together are relatively weaker than in other directions. When a force is applied to the mineral, it preferentially splits along these paths of least resistance.
The quality of the cleavage can vary, described as perfect, good, or poor, based on how smoothly and consistently the surfaces appear. Cleavage is distinct from the mineral’s external crystal shape, which is determined by how it grew. Minerals can exhibit cleavage in one, two, three, or more directions, with the angles between these directions being characteristic. Cleavage is a powerful diagnostic tool in mineral identification.
Feldspar’s Two Distinct Cleavage Planes
Feldspar exhibits cleavage, possessing two distinct sets of cleavage planes. Both the potassium feldspar (K-spar) and plagioclase feldspar subgroups share this defining structural feature.
The two cleavage planes intersect at an angle that is very close to 90 degrees. For monoclinic feldspars, such as Orthoclase, the angle is exactly 90 degrees. In triclinic feldspars, such as Plagioclase, the angle is typically between 86 and 89.5 degrees. This near right-angle intersection is a highly recognizable feature when viewing broken feldspar crystals.
The quality of the cleavage planes is not identical; one direction is typically described as “perfect” and the other as “good.” The perfect cleavage plane is very smooth and highly reflective. The good cleavage plane is flat but may show slightly less consistent breakage. The two planes allow feldspar fragments to break into blocky or nearly rectangular shapes, which aids in identification.
In Plagioclase feldspars, one cleavage surface may also display fine, parallel lines called striations, which are caused by twinning in the crystal structure. These striations are unique to Plagioclase and are not found on potassium feldspar. Observing this dual cleavage intersecting near 90 degrees is the most reliable method for identifying feldspar in a rock sample.
Distinguishing Cleavage from Fracture
While cleavage results in predictable, flat, and smooth surfaces, fracture is the mineral’s tendency to break along irregular or uneven surfaces. Fracture occurs when a mineral is subjected to stress, but its atomic bonds are equally strong in all directions, leaving no preferred planes of weakness. Minerals like quartz, which lack distinct cleavage planes, break by fracturing.
Fracture surfaces are rough, jagged, or sometimes curved, such as the shell-like pattern known as conchoidal fracture. Feldspar, despite having excellent cleavage, may also display fracture surfaces in directions that do not align with its two cleavage planes. It is crucial to recognize the difference between the repeating, smooth, parallel surfaces of cleavage and the random, rough surfaces of fracture.
The presence of well-developed, reflective, planar surfaces is the definitive indicator of cleavage, even if the mineral also contains areas of fracture. For feldspar, finding the two sets of smooth, near right-angle planes confirms the presence of cleavage and distinguishes it from minerals that only fracture.