Crystal form occurs as a mineral grows, while cleavage only forms as a mineral breaks. See Figure 7.10 for the main types of cleavage and an example of each. A mineral may have one or more cleavage planes. Planes that are parallel are considered to be in the same direction of cleavage and should only count as one.
Distinguishing crystal faces from cleavage planes: 1) Crystal faces are normally smooth, whereas cleavage planes, though also smooth, commonly are broken in a step-like fashion; 2) Some crystal faces have fine grooves or ridges on their surfaces, whereas cleavage planes do not.
Cleavage planes occur in parallel sets; crystal faces are solitary, occurring only at the surface of the crystal. Cleavage surfaces arise due to planes of weak bonding within the crystal and can be seen because of crystal breakage (or near breakage). Crystal faces are formed by the growth of the crystal.
Cleavage is the tendency of a crystal to break along weak structural planes. Thus, the way a mineral cleaves provides insight into its crystal structure.
If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage.
Just as wood is easier to split with the grain than against it, gemstone cleavage is the tendency of certain crystals to break along definite plane surfaces. If there are planes in a crystal structure with relatively weak atomic bonds, the crystal is more likely to break along those planes.
Quartz has no cleavage because it has equally strong Si–O bonds in all directions, and feldspar has two cleavages at 90° to each other (Figure 1.5). One of the main difficulties with recognizing and describing cleavage is that it is visible only in individual crystals.
The most common physical properties are crystal form, color, hardness, cleavage, and specific gravity. One of the best ways to identify a mineral is by examining its crystal form (external shape). A crystal is defined as a homogenous solid possessing a three-dimensional internal order defined by the lattice structure.
By-and-large, cleavages at 90 degrees to one another indicate a cubic form, cleavages at 120 and 60 degrees in the same sample indicate a rhombohedral form, and cleavages at acute to obtuse angles over long surfaces indicate a prismatic form – such as in amphiboles.
In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small, even distribution of yolk) or in mesolecithal cells or microlecithal cells (moderate concentration of yolk in a gradient)—bilateral holoblastic, radial holoblastic, rotational holoblastic ...
Minerals that are bonded with equal strength in all directions, such as quartz, have no cleavage, but instead fracture to form irregular surfaces. These minerals break along curved surfaces to form conchoidal fractures, similar to what happens when glass breaks.
Crystals show good cleavage because their constituent particles are arranged in planes.
The cleavage property is shown by crystalline solids because they possess cleavage planes. In a crystalline solid, the cells are neatly stacked. The cleavage planes are areas where the crystal structure is the weakest. It is only along these planes that a crystalline solid can be cut.
Crystal will have a definite, clear ring when tapped or struck, much like a bell. The longer and clearer the ring has the higher quality the crystal. Crystal is a type of glass that contains strengthening minerals like lead-oxide, potassium carbonate, and silica to make the material durable.
Sometimes it is hard to recognize a crystal solely by its name. So identifying them through their hardness, luster, streak, colors, and more is effective. However, the easiest among the rest is color. Colors have different meanings based on culture, traditions, and religions.
When crystals break, they can either split leaving a clean, flat face called a cleavage plane, or fracture leaving a more rough, uneven surface. We can find out more about a crystal by looking at the way it breaks. Cleavage planes form along the weakest area of mineral's structure.
In quartz, all of the bonds are of the same strength (strong silicate bonds), and the geometry of the mineral is such that you can not find a plane that will cut through all of the bonds, therefore the fractures must "wander" creating the conchoidal structures.
The flat faces (also called facets) of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of relatively low Miller index. This occurs because some surface orientations are more stable than others (lower surface energy).
Quartz properties
Does not exhibit cleavage, although crystal faces may be mistaken for cleavage planes. Conchoidal fracture is characteristic of both macrocrystalline and cryptocrystalline quartz varieties. Crystals are vitreous (glass-like), massive form is dull or waxy.
Quartz has crystal surfaces but no cleavage at all. Fluorite forms cubic crystals like those of halite, but it cleaves along planes that differ in orientation from the crystal surfaces.
Cleavage is the ability of single crystals to split easily along specifically oriented planes (Hurlbut & Klein, 1977 ▸). Such planes are usually parallel to the reticular lattice planes with low Miller indices/large interplanar distances.
The term cleavage refers to the way a mineral cleaves, or breaks, in prefered directions. Cleavage directions represent planes of weak bonding in the mineral's atomic structure. Because mineral structures are repetitive, a single cleavage often appears as multiple cracks, all parallel.