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 Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

in the phyllosilicate serpentine subgroup 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 serpentinites. 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

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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 ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

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 . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

chlorite The chlorite ion, or chlorine dioxide anion, is the halite (oxyanion), halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as s ...

, and

carbonate A carbonate is a salt of carbonic acid, (), 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 group ...

s. Olivine under hydrothermal action, low grade metamorphism and weathering will transform into antigorite, which is often associate with talc and carbonate. : + + → : + → + +

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 mylonites. The antigorite grains that make up these rocks are very fine (on the order of 1 to 10 microns) and are fibrous, which defines a 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 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 ( ) is a state in the New England region of the Northeastern United States. It borders Connecticut to its west; Massachusetts to its north and east; and the Atlantic Ocean to its south via Rhode Island Sound and Block Is ...

, United States. A bowenite cabochon featured as part of the "Our Mineral Heritage Brooch", was presented to U.S. First Lady Mrs. Lady Bird Johnson in 1967. Williamsite is very translucent and has a medium to deep apple-green color. Somewhat resembling jade, Williamsite is often cut into cabochons and beads.

Crystal structure

The magnesian serpentines (antigorite, lizardite, chrysotile) 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 and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

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

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

Geologic occurrences

Physical properties

Gemstone properties

Crystal structure

See also

References