Mineral Properties: Cleavage
Cleavage and Parting in Gemstones
The term "cleavage" refers to the splitting, or parting of a mineral along defined "planes" that are related their crystalline structure. In certain varieties of gemstones the chemical bond, or molecular cohesion between layers of atoms is weaker in certain crystallographic directions, and breakage will occur along smooth, flat surfaces that are parallel to the zones of weakness.
The crystalline structure of a mineral will determine wether it will separate in only a single direction, or in two, three, four, five, or even six different directions. Some gemstone varieties have no cleavage at all. These minerals can fracture, but the breakage is not due to any inherent weakness in their atomic structure. Additionally, a fracture typically has none of the geometric sharpness of cleavage.
The fundamental difference between a "fracture" and a "cleave," is that a fracture can occur virtually anywhere on the gem-body, and in any spatial direction, with little chance of another identical fracture occurring in the same manner. When a gem-body has a defined "cleavage-plane" it can be fractured, or "cleaved," predictably and repeatedly, with each fracture taking on an identical form, and being parallel to one another. A good analogy would be the splitting of a piece of wood, which will occur more easily when it is split with the grain, as opposed to splitting it against the grain. In the field of materials science, cleavage is sometimes referred to as a "faceted fracture."
A mineral can cleave multiple times along its defined cleavage-plane, and each of these separations will typically be parallel to one another because each fracture is parallel to the same zone of weakness. Of particular significance is the number of directions that a given mineral can be cleaved, and the angles that their cleavage-planes occur.
Each cleavage plane within a mineral has a grade or rating which indicates the relative ease with which the crystal can be separated, or "cleaved." These cleavage ratings are separated into six commonly-used categories:
- Perfect: Cleaving occurs along a perfectly smooth plane
- Excellent: Cleaving occurs along a very smooth plane
- Distinct: Cleaving plane is well defined
- Good: The cleaved surface has some imperfections
- Poor (Indistinct, Imperfect): Cleaving leaves a rough surface that is close to an irregular fracture
- None: The mineral will fracture along random paths, leaving a jagged surface
There are several categories, or types of cleavage, which are descriptive of the way in which the material separates under stress. They are:
- Basal Cleavage: Cleavage occurs on a horizontal or "peeled" plane in one direction only
- Cubic Cleavage: Cleavage occurs in three directions, making smaller cubes from larger cubes
- Octahedral Cleavage: Cleavage occurs in a triangular, or flat pattern from an octahedral mineral
- Pinicoidal Cleavage: Cleavage occurs in three or more directions in prismatic or tabular minerals
- Prismatic Cleavage: Cleavage occurs in two directions, leaving a "stair-step" pattern
- Rhombohedral Cleavage: Cleavage occurs in oblique, or obtuse angles
A mineral that possesses only a single cleavage direction that is parallel to its base is described as having "basal" cleavage, and the fracture will be in flat sheets (peeled) across its entire surface (i.e., mica or muscovite). When a mineral possesses two or more cleavage directions the fracture will typically have a "stair-step" appearance when viewed under magnification, but will not always break perfectly along two or more cleavage planes, creating secondary, irregular fractures.
Cleavage of Halite at the molecular level |
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Perfect cleavage in calcite (photo: public domain) |
Cleavage planes should not necessarily be confused with the directions of crystal faces, although crystal faces can be a clue as to the angle of a cleavage plane. If a rough mineral's crystal faces show telltale signs of alteration or dullness (tarnish), and are not parallel, they may not provide enough information (if any) as to the cleavage angle. However, if a rough crystal has multiple parallel faces, and/or they show little sign of alteration or dullness, they may be a good indicator of cleavage direction.
When the molecular bond between a mineral's atoms is equal in all six directions, separation or breakage will create irregular or splintery surfaces, or it will create curved (concave) surfaces known as a conchoidal fracture.
Parting
The term "parting" is sometimes confused with the term "cleavage," and parting does refer to a physical characteristic that is somewhat similar to cleavage, but it can exist in minerals that do not exhibit any form of cleavage. A telltale sign that a separation occurred as the result of parting, and not from a plane of cleavage, is the existence of conchoidal, jagged or splintery fracturing.
Although parting is not linked to the molecular structure of the mineral, it is directly linked to inherent structural defects that occurred as the result of crystal formation. Common causes for such structural defects include crystal twinning and/or hydrostatic pressure that was exerted during formation. Each of these can cause internal planes of weakness that can lead to separation.
Minerals and their Cleavage Types
Lapidary and Setting Considerations
In centuries past, taking advantage of these cleavage planes had distinct advantages when it came to cutting nature's "hardest" gem, the diamond. Before the days of modern diamond saws a cutter first "cleaved" the rough stone into smaller stones that would be closer to the approximate final shape that was desired. This was accomplished by striking the stone at its weakest point of cleavage, with a chisel and mallet [4]. Due to its octahedral (cubic) crystalline structure a diamond can be cleaved in four directions.
For the lapidary, it is critical to ascertain the angular relationship between multiple cleavage-planes, determining wether they are perpendicular to each other at perfect right angles (as is the case with gems that have a cubic crystalline structure), or converging at acute, or obtuse angles (as is the case with gems that have a rhombohedral crystalline structure).
In gemstones with perfect cleavage such as diamond, tanzanite, or topaz the faceting must be angled so that it runs transverse to the cleavage-plane, making the cut stone less vulnerable to breakage. Any drilling of the stone should be done perpendicular to the cleavage plane.
When setting a stone that has cleavage, care should be taken to insure that the stone is not exposed to too much heat from soldering as this could create fissures along the cleavage-planes, thereby weakening the gem.
Bibliography on Cleavage in Gemstones
1. Philip A. Candela Fractures and Fracturing  . www.geol.umd.edu
2. Edward Jay Epstein The Diamond Cut . www.edwardjayepstein.com
3. Bill Long, Fracture and Cleavage . www.drbilllong.com
4. Zangerl, Loew, Eberhardt, Brittle discontinuities in anisotropic crystals . Birkhäuser Basel
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