Antigorite
Monoclinic
Mg3[Si2O5](OH)4
Antigorite is a phyllosilicate of the serpentine group and the high-temperature and pressure polymorph of lizardite and chrysotile. It is a primary constituent of serpentinites and most commonly occurs in subduction complexes with ultramafic rocks that experienced high-pressure metamorphism. Its name derives from Valle di Antigorio, Domodossola, Italy, where the type specimens were collected in 1840 by Matthias Eduard Schweizer.
Structure and chemistry
Antigorite is characterized by a layered structure with alternating tetrahedral (T) sheets consisting of a pseudo-hexagonal network of linked [SiO4] tetrahedra and tri-octahedral (O) brucite-type sheets containing Mg cations surrounded by (OH)– anions. The hydroxyls are replaced by a O2- where the apices of the tetrahedral layer touch the octahedral sheet i.e. one out of three hydroxyls only on one side of the octahedral sheet. The peculiar feature of the structure of antigorite is the curvature of the tetrahedral and octahedral layers, which causes the apical vertices of the tetrahedral layer to flip periodically in the structure. The periodicity of the structure (i.e. the number of tetrahedrons in the cell) is not fixed and varies in antigorite, resulting in a variable length of the cell parameter a. In general, high temperature antigorites tend to have a shorter value of a.
In terms of chemistry, the formula of antigorite, as for all serpentine group minerals, deviates very little from the Mg3[Si2O5](OH)4. The most common substitution is the replacement of Si4+ by Al3+ which is balanced by a substitution of Mg2+ by Al3+ and produces a solid solution towards amesite Mg2Al[AlSiO5](OH)4. Mg2+ can be replaced by Fe2+ but this substitution is very limited as the larger Fe2+ ions distorce the structure of serpentine group minerals. In addition, minor amounts of Fe3+ and Ni2+ can be present.
Properties
Habit: platy, bladed, fibrous
Hardness: 2.5-3.5
Density: 2.6 g/cm3
Cleavage: {001} perfect
Twinning: uncommon
Color: green, dark green, green-blue, white
Luster: vitreous
Streak: white, grayish, slightly greenish
Alteration: chlorite, talc
In thin section…
α(//b): 1.705-1.730
β(^a or //b): 1.708-1.734
γ(γ^c = 2-30°): 1.712-1.740
2Vγ: 37-124°
Color: colorless to pale green
Birefringence (δ): 0.004-0.007 (first-order grey)
Relief: low
Optic sign: –
[Mindat]
[HoM]
Field features
Antigorite occurs in serpentinite rocks, often mixed with or associated with other serpentine polymorphs. Serpentinite minerals all appear green/white to dark colored with fibrous or platy habit, low hardness, and a slight soapy feel on touch. Their distinction requires a thin section and, very often, analysis of the mineral with techniques such as Raman spectroscopy.
Antigorite in thin section
Antigorite and lizardite appear very similar in thin section, as both minerals appear colorless at PPL, show very low first-order grey color at CPL, and have a well developed basal cleavage. In general, antigorite is more crystalline than lizardite and tends to develop interlocking bladed or platy crystals, whereas lizardite more commonly occurs as the product of replacement of olivine and pyroxene in serpentinized peridotites (producing the characteristic mesh textures or bastites together with chrysotile). The relief of antigorite is higher than lizardite and this may be useful to recognize antigorite when it forms crystals that ‘stand out’ in a fine-grained groundmass of serpentine group of minerals. However, it is often necessary to use techniques such as Raman spectroscopy or X-ray diffraction to tell serpentine group minerals apart. Antigorite commonly occurs together with other phyllosilicate minerals, most commonly chlorite and talc: chlorite can be distinguished from antigorite for its pleochroism and typical anomalous blue interference colors, while talc shows very high birefringence (similar to mica) compared to antigorite. Some Mg-rich chlorite compositions may lack pleochroism and appear first-order grey, impossible to distinguish from serpentine group minerals without Raman spectroscopy or analysis at an electron microscope.
Video. Antigorite flakes with the typical first-order grey color and platy habit in a serpentinite. This texture is known as ‘interlocking antigorite’. CPL. Width: 2.5 mm. Antigorite schist, Sanbagawa belt. Sarutagawa, Shikokuchuo, Ehime prefecture, Japan.
CPL + λ
CPL
PPL
⇔ slider. Antigorite rosettes with interlocking texture in a serpentinite. Width: 2.5 mm. Antigorite schist, Sanbagawa belt. Sarutagawa, Shikokuchuo, Ehime prefecture, Japan.
Examples of antigorite-bearing rocks
Antigorite serpentinite from the Sanbagawa belt
The Sanbagawa belt is a metamorphic terrane that crosses Japan from Kyushu to the Tokyo area. It consists of subducted oceanic material (metasediments and metabasites) coupled with ultramafic rocks of the overlying mantle wedge. Hydration of these mantle wedge peridotites at more than 30 km depth and temperatures above 450 °C produced antigorite serpentinites.
Sample: antigorite serpentinite
Assemblage: antigorite, magnetite, talc, dolomite
Locality: Sarutagawa, Shikokuchuo, Ehime prefecture, Shikoku, Japan
Occurrence
Serpentine group minerals form due to the alteration of peridotites in water-rich settings, most commonly (1) mid-ocean ridges, and (2) subduction zones. All three polymorphs have been reported from this geological settings, but antigorite is stabilized by high-pressure and high-temperature conditions. It is, therefore, common in subducted serpentinites, where it replaces lizardite, or mantle wedge serpentinites, where it is produced by hydration of olivine. Antigorite is also a common product of contact metamorphism of serpentinites around igneous intrusions.
Bromiley, G. D., & Pawley, A. R. (2003). The stability of antigorite in the systems MgO-SiO2-H2O (MSH) and MgO-Al2O3-SiO2-H2O (MASH): The effects of Al3+ substitution on high-pressure stability. American Mineralogist, 88(1), 99-108.
Evans, B. W. (2004). The serpentinite multisystem revisited: chrysotile is metastable. International Geology Review, 46(6), 479-506.
Evans, B. W. (2010). Lizardite versus antigorite serpentinite: Magnetite, hydrogen, and life (?). Geology, 38(10), 879-882.
Evans, B. W., Darby Dyar, M., & Kuehner, S. M. (2012). Implications of ferrous and ferric iron in antigorite. American Mineralogist, 97(1), 184-196.
Guillot, S., Schwartz, S., Reynard, B., Agard, P., & Prigent, C. (2015). Tectonic significance of serpentinites. Tectonophysics, 646, 1-19.
Mellini, M., Trommsdorff, V., & Compagnoni, R. (1987). Antigorite polysomatism: behaviour during progressive metamorphism. Contributions to Mineralogy and Petrology, 97(2), 147-155.
Schwartz, S., Guillot, S., Reynard, B., Lafay, R., Debret, B., Nicollet, C., … & Auzende, A. L. (2013). Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos, 178, 197-210.
Wunder, B., Wirth, R., & Gottschalk, M. (2001). Antigorite pressure and temperature dependence of polysomatism and water content. European Journal of Mineralogy, 13(3), 485-496.
Zussman, J. (1954). Investigation of the crystal structure of antigorite. Mineralogical magazine and journal of the Mineralogical Society, 30(227), 498-512.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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