The fragility of minerals.
Under the action of mechanical factors, such as hitting or pressing down with the blade of a knife in a certain direction, crushing, sudden cooling down or heating up, some minerals decompose properly into pieces bounded by flat surfaces. This property is called cleavage. It is clearly a vector property; for minerals having it can only be correctly divided in certain directions. Fragility is strictly dependent on the internal structure.
Based on the number of majors, in which minerals show cleavage, it is divided into a single-plane structure (sometimes called unidirectional), two- and three-plane, less often four- and the six-plane. An example of single-plane cleavage is the cleavage of gypsum and mica. The crumbs of these minerals can be easily divided into pieces with the blade of a pocket knife; however, this can only be done in one plane, called the cleavage plane. An example of minerals that can be divided correctly in two planes, i.e.. having two cleavage planes, there are feldspars, e.g.. orthoclase, the main component of granite. If a crumb of this mineral is struck with a hammer, it will break down into two or more lumps delimited by flat walls. This mineral has more than one, but two planes of cleavage, characterized by smoothness and gloss. An important distinguishing feature is sometimes the angle of inclination of the splitting planes. In orthoclase it was found, that they are at right angles. Hence the name of this mineral from the Greek orthos - simple, klasis - odłamanie.
Different types of cleavage seen in microscopic slides: a - excellent, b - pronounced, c - not clear.
Other minerals also show fragility in two planes, e.g.. common pyroxenes and amphiboles. They can be distinguished by the different cleavage angles. Although both pyroxenes, and amphiboles have two cleavage planes, then the angle between the cleavage planes of the two groups of minerals is different. The angle between the cleavage planes of pyroxenes is about 90 °, in amphiboles it is about 124 °.
There are also minerals with three cleavage planes. An example is rock salt with the so-called. ankle cleavage. When struck, the salt crystal breaks into smaller pieces, each of which is a cuboid with faces set at right angles. The rhombohedral cleavage of calcite is different. It is also a three-dimensional cleavage; however, the cleavage planes are inclined at less than right angles. A heavily impacted piece of calcite breaks down into smaller pieces, each of which is shaped like a diamond. The mineral dolomite exhibits a similar cleavage. An example of four-dimensional cleavage is fluorspar and diamond cleavage. These minerals have cleavage parallel to the octahedron faces.
Depending on the ease of dividing a given mineral into parts limited by flat surfaces, perfect cleavage is distinguished, clear and indistinct. The minerals with the best cleavage are mica, which can be easily divided into thin ones with a pocket knife, transparent or almost transparent plates with a thickness of hundredths of a millimeter. Due to this property, in the Middle Ages, thin tablets of transparent mica were used as glass.
Analysis of the spatial networks of minerals shows, that the cleavage planes correspond to these planes of their lattice structures, in which the atoms are arranged particularly densely. Based on the knowledge of the structure of the analyzed mineral, one can therefore predict, how many cleavage planes it has. In a regular pattern, cleavage occurs parallel to the faces of the cube, e.g.. in rock salt, octahedron in fluorite, or rhombic dodecahedron and sphalerite. In the hexagonal arrangement, cleavage is usually parallel to the base wall, e.g.. not very pronounced in beryllium. In a trigonal pattern, cleavage most often occurs parallel to the faces of the rhombus, e.g.. in calcite and dolomite. In a tetragonal pattern, the cleavage is parallel to the base face, and also to the walls of the column. In the orthorhombic pattern, cleavage parallel to one of the walls of the double-wall is the most common, e.g.. in topaz - basic double-wall. In a monoclinic arrangement, many crystals have cleavage parallel to the plane of symmetry (cast, orthoclase), it is not uncommon and perpendicular to this plane of orthoclase). The planes of cleavage in a triangular pattern are usually taken as base planes, e.g.. in plagioclase, whose forms are similar to orthoclase.