Olivine is a common, high-temperature silicate mineral known for its distinctive olive-green color, which gives the gemstone peridot its appeal. Understanding how this mineral breaks is an important part of identifying it in a geological setting. The way olivine breaks—whether by cleavage or fracture—provides insight into its internal atomic structure compared to many other silicate minerals.
Understanding Cleavage vs. Fracture
The terms cleavage and fracture describe the characteristic surfaces produced when a mineral specimen breaks. Cleavage is the tendency of a mineral to break along smooth, flat planes of weakness that exist within its crystalline structure. These planes occur where the atomic bonds holding the crystal together are significantly weaker in one direction than in others. A classic example is mica, which breaks easily into thin, parallel sheets due to one perfect cleavage direction.
Fracture, in contrast, describes breakage along irregular, rough, or uneven surfaces. This pattern occurs when the mineral’s internal atomic bonds are of relatively uniform strength in all directions, meaning there are no pre-existing planes of weakness for the break to follow. Minerals like quartz commonly exhibit fracture, producing a characteristic curved surface rather than a flat plane. The type of fracture—whether conchoidal (shell-like), hackly (jagged), or uneven—is also a diagnostic property for mineral identification.
Olivine’s Dominant Breakage Pattern
Olivine is overwhelmingly characterized by fracture and generally lacks the distinct, flat planes associated with true cleavage. When broken, olivine typically exhibits conchoidal fracture, leaving behind curved, shell-like surfaces similar to broken glass. The resulting broken surfaces are jagged and uneven rather than smooth and predictable.
While some sources acknowledge a poor or indistinct cleavage in two directions, this feature is rarely visible in hand specimens and does not control the mineral’s fragmentation. Therefore, the dominant breakage characteristic used by geologists to identify olivine is its high tendency to fracture conchoidally. This absence of well-developed cleavage helps distinguish it from other green silicate minerals like pyroxene.
How Olivine’s Crystal Structure Dictates Breakage
The reason olivine fractures instead of cleaving is directly related to its specific internal atomic arrangement. Olivine is classified as a nesosilicate, meaning its crystal structure is built around isolated silicate tetrahedra (\(\text{SiO}_4\)). These tetrahedra, where a silicon atom is bonded to four oxygen atoms, are strongly bonded and act as individual, disconnected units.
These isolated silicate groups are held together by strong ionic bonds to divalent cations, specifically magnesium (\(\text{Mg}^{2+}\)) and iron (\(\text{Fe}^{2+}\)) ions, giving olivine the generalized chemical formula \((\text{Mg, Fe})_2\text{SiO}_4\). Because the bonds between the isolated tetrahedra and the metal ions are relatively uniform in strength in all directions, the crystal lattice lacks continuous planes of significant weakness. This structural uniformity causes the mineral to break randomly through the uniform bond network when stress is applied, creating the jagged, curved surfaces of conchoidal fracture.
Key Physical Properties of Olivine
Beyond its characteristic fracture, olivine possesses several other defining physical properties that aid in its identification. The mineral’s color is typically a pale olive green to a yellowish-green, which is the source of its name. The gem-quality variety, known as peridot, is prized for this vibrant green hue.
Olivine is a relatively hard mineral, registering between 6.5 and 7.0 on the Mohs scale, meaning it can easily scratch glass. It exhibits a vitreous or glassy luster and is transparent to translucent. Due to the presence of magnesium and iron, olivine has a high specific gravity, ranging from 3.2 to 4.5, giving it noticeable density.
Olivine is one of the most abundant minerals in the Earth’s mantle. It is a common component of mafic igneous rocks, such as basalt and gabbro, which form from high-temperature magma.