Rocks come in many different colors, shapes, sizes, and compositions. While some rocks maintain a relatively uniform appearance, others feature an intricate blend of various hues and tones. One type of rock in particular is known for its distinct green and brown coloration – serpentinite. Serpentinite is a metamorphic rock that forms when olivine-rich rocks like peridotite undergo hydrothermal alteration. The green color comes from various magnesium-rich minerals like antigorite and chrysotile, while the brown comes from the presence of iron-rich minerals like magnetite. In this article, we will explore serpentinite in more detail – from how it forms to where it can be found. We will also cover its various uses, both historic and modern.
How Serpentinite Forms
Serpentinite forms in a process known as serpentinization, which occurs when olivine-rich rocks are altered by heat, pressure, and chemically active fluids like water. This typically happens at tectonic plate boundaries deep underground where fluid access is increased. As hot water penetrates the peridotite, it causes the unstable mineral olivine to break down and convert into more stable serpentine group minerals like antigorite, chrysotile, and lizardite. Magnetite and brucite are also common products of this reaction.
The exact chemical reaction looks like this:
2Mg2SiO4 + 3H2O -> Mg3Si2O5(OH)4 + Mg(OH)2
Olivine + Water -> Serpentine + Brucite
As this hydration reaction occurs, the peridotite takes on a mesh-like texture called a mesh texture. Over time, the peridotite is completely replaced by an interlocking matrix of serpentine minerals, creating the metamorphic rock serpentinite. The green serpentine minerals provide the characteristic green color, while magnetite contributes the brown and black tones.
Additional heat and pressure can also compress and deform the serpentinite texture even further. This can orient the platy serpentine minerals into parallel alignment, creating an appearance called antigorite schist. So in some cases, serpentinite may take on a more schistose texture while still retaining the green and brown colors.
Where to Find Serpentinite
Serpentinite can be found in many regions of the world, particularly along major fault zones where subduction has occurred. Some prominent examples include:
– California – The California Coast Ranges contain extensive serpentinite bodies due to the active tectonic setting along the boundary between the Pacific and North American plates. The California state rock, serpentine, is abundant here.
– Canada – The Tablelands in Gros Morne National Park, Newfoundland showcase a classic example of intact oceanic mantle peridotite that has been obducted onto land. Serpentinization altered much of the peridotite into serpentinite.
– Italy – The Alps contain bodies of serpentinite that formed when the Apulian plate was thrust over continental Europe. The Corsica-Sardinia ophiolite complex also contains serpentinite.
– Greece – Serpentinite is found in the Pindos Mountains and on the island of Lesbos, associated with the closure of the Tethys Ocean.
– Turkey – Large bodies of serpentinite occur near Istanbul and the Denizli province related to the convergence between the African and Eurasian plates.
– Cuba – A serpentinite body covers over 60% of the island and originally formed below the seafloor along the southern margin of the North American plate.
– New Zealand – Serpentinite is found along the Alpine Fault zone running through the South Island, marking the boundary between the Australian and Pacific plates.
– Afghanistan – The Nangarhar serpentinite body formed from oceanic lithosphere that accreted to Eurasia prior to the Himalayan orogeny.
In many cases, serpentinite bodies mark suture zones where oceanic lithosphere has been emplaced onto continental margins. Serpentinite’s distinct appearance makes it a useful indicator rock for geologists to identify paleo-subduction zones.
Uses of Serpentinite
In addition to providing clues about geologic history, serpentinite also has a number of practical uses:
Historic Uses
– Carvings and ornaments – Serpentinite’s smooth texture allowed it to be carved into bowls, urns, and other decorative objects by early craftspeople. The Romans and Greeks used serpentinite for ornamental carvings.
– Building material – The durability and polishability of serpentinite made it useful as an architectural stone. Historical buildings made with serpentinite exist in places like Portugal, England, Turkey, and Italy.
Modern Uses
– Construction – Serpentinite remains in use today as a dimension stone for building facades, countertops, and floor tiling.
– Industry – Powdered serpentinite is used as a refractory material and as a filler in paints, plastics, and friction products like brake pads and clutches.
– Agriculture – The magnesium in serpentinite makes it an attractive soil supplement to replenish magnesium deficient farmland.
– Carbon sequestration – Serpentinite’s natural ability to absorb CO2 is being tested in carbon capture and storage research.
– Remediation – Serpentinite can help immobilize toxic metals in contaminated soils through mineral fixation.
Conclusion
In summary, the green and brown rock known as serpentinite forms by hydration of peridotite primarily at subduction zones. It occurs in faulted blocks around the world, providing evidence of past tectonic plate interactions. While valued historically as an ornamental stone, serpentinite also has important modern applications ranging from construction to carbon sequestration. Its distinctive green and brown appearance continues to make serpentinite an important rock in both geology and culture. Understanding serpentinite provides insight into our dynamic planet.
