Antigorite is a lamellated,

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

in the phylosilicate

serpentine subgroup Serpentine subgroup (part of the kaolinite-serpentine group in the category of phyllosilicates) are greenish, brownish, or spotted minerals commonly found in serpentinite. They are used as a source of magnesium and asbestos, and as decorative ...

with the ideal chemical formula of (Mg,Fe²⁺)₃Si₂O₅(OH)₄. It is the high-pressure polymorph of serpentine and is commonly found in metamorphosed

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''se ...

s. Antigorite, and its serpentine polymorphs, play an important role in subduction zone dynamics due to their relative weakness and high weight percent of water (up to 13 weight % H₂O). It is named after its type locality, the Geisspfad serpentinite, Valle Antigorio in the border region of

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

/ Switzerland and is commonly used as a gemstone in jewelry and carvings.

Geologic occurrences

Antigorite is found in low-temperature, high-pressure (or high-deformation) environments, including both extensional and compressional

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

regimes. Serpentines are commonly found in the ultramafic greenschist facies of subduction zones, and are visible on the Earth's surface through secondary exhumation. Serpentinites that contain antigorite are usually highly deformed and show distinct textures, indicative of the dynamic region where they were formed. Antigorite serpentinites commonly have associated minerals of

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

, chlorite, and

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

s. Olivine under hydrothermal action, low grade metamorphism and weathering will transform into antigorite, which is often associate with talc and carbonate. : 3Mg₃SiO₄ (olivine) + H₂O + SiO₂ → 2Mg₃Si₂O₅OH₄ (serpentine) : 2Mg₃Si₂O₅OH₄ (serpentine) + CO₂ → 2Mg₃Si₄O₁₀OH₂ (talc) + 3MgCO₃ (magnesite) + H₂O

Physical properties

Lamellated antigorite occurs in tough, pleated masses. It is usually dark green in color, but may also be yellowish, gray, brown or black. It has a Mohs scale hardness of 3.5–4 and its lustre is vitreous to greasy. Antigorite has a specific gravity of 2.5-2.6. The monoclinic crystals show micaceous cleavage, a distinguished property of phyllosilicates, and fuse with difficulty. Serpentinite rocks that consist of mostly antigorite are commonly

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

s. The antigorite grains that make up these rocks are very fine (on the order of 1 to 10

microns The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

) and are fibrous, which defines a

texture Texture may refer to: Science and technology * Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object * Texture (roads), road surface characteristics with waves shorter than road roughness * Texture ...

in the rock caused by lattice preferred orientation.

Gemstone properties

Antigorite is used as gemstones or for carvings when it appears pure and translucent, although many crystals have black specks of magnetite suspended within. The gem types of antigorite are

Bowenite Bowenite is a hard, compact variety of the serpentinite species antigorite, (Mg3(OH)O4Si2O5). Classed as semi-precious gemstone it has been used for tools, weapons and jewellery by the Māori in New Zealand, and for jewellery by Fabergé. Deposits ...

and Williamsite. Bowenite, known for George T. Bowen from Rhode Island (the variety's type locality), who first analyzed the mineral, is translucent and light to dark green, often mottled with cloudy white patches and darker veining. It is the serpentine most frequently encountered in carving and jewelry, and is the state mineral of

Rhode Island Rhode Island (, like ''road'') is a state in the New England region of the Northeastern United States. It is the smallest U.S. state by area and the seventh-least populous, with slightly fewer than 1.1 million residents as of 2020, but it ...

, United States. A bowenite

cabochon A cabochon (; ) is a gemstone that has been shaped and polished, as opposed to faceted. The resulting form is usually a convex (rounded) obverse with a flat reverse. Cabochon was the default method of preparing gemstones before gemstone cutt ...

featured as part of the "Our Mineral Heritage Brooch", was presented to U.S. First Lady Mrs.

Lady Bird Johnson Claudia Alta "Lady Bird" Johnson (''née'' Taylor; December 22, 1912 – July 11, 2007) was First Lady of the United States from 1963 to 1969 as the wife of President Lyndon B. Johnson. She previously served as Second Lady from 1961 to 1963 whe ...

in 1967. Williamsite is very translucent and has a medium to deep apple-green color. Somewhat resembling jade, Williamsite is often cut into

s and beads.

Crystal structure

The magnesian serpentines (antigorite, lizardite,

chrysotile Chrysotile or white asbestos is the most commonly encountered form of asbestos, accounting for approximately 95% of the asbestos in the United StatesOccupational Safety and Health Administration, U.S. Department of Labor (2007)29 C.F.R.&nb ...

) are trioctahedral hydrous phyllosilicates. Their structure is based on 1:1 octahedral-tetrahedral layer structures. Antigorite is monoclinic in the space group Pm. Although the magnesian serpentines have similar compositions, they have significantly different crystallographic structures, which are dependent on how the SiO₄ tetrahedra sheets fit in with the octahedral sheets. Antigorite's basic composition has a smaller ratio of octahedral to tetrahedral cations (relative to lizardite and chrysotile), allowing the structure to compensate for the misfit of sheets through periodic flipping of the curved tetrahedra layers, and subsequently their polarity. Polysomes of antigorite are defined by the number of individual tetrahedra (denoted as the value ''m'') which span a

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

of the direction of curvature. The sheets of tetrahedra allow the platy, fibrous crystals to separate parallel to the 001 (basal) plane, giving antigorite its perfect cleavage.

References

{{reflist Serpentine group Iron(II) minerals Magnesium minerals

Geologic occurrences

Physical properties

Gemstone properties

Crystal structure

See also

References