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Antigorite
[edit]Antigorite is a lamellated, platy mineral in the phylosilicate serpentine subgroup with the ideal chemical formula of (Mg,Fe2+)3Si2O5(OH)4[1]. 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 wt.%)[2][3]. It is named after its type locality, the Geisspfad serpentinite, Valle Antigorio in the border region of Italy/Switzerland[4].
| Antigorite | |
|---|---|
 | |
| General | |
| Category | Phylosilicate Serpentine-Kaolinite group |
| Formula (repeating unit) | (Mg, Fe)3Si2O5OH4 |
| Strunz classification | 9.ED.15 |
| Crystal system | Monoclinic |
| Identification | |
| Color | Green, Yellowish-green, Blueish-gray |
| Crystal habit | Massive or platy |
| Cleavage | {001} Good |
| Fracture | Brittle |
| Mohs scale hardness | 3.5-4 |
| Luster | Vitreous - Greasy |
| Streak | Greenish white |
| Specific gravity | 2.5-2.6 |
| Optical properties | Biaxial (-) |
| Birefringence | δ = 0.005-0.006 |
Physical Properties
[edit]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 hardness of 3.5–4 and its lustre is greasy. The monoclinic crystals show micaceous cleavage and fuse with difficulty.
Crystal Structure
[edit]The magnesian serpentines (antigorite, lizardite, chrysotile) are trioctahedral hydrous phyllosilictes. Their structure is based on 1:1 octahedral-tetrahedral layer structures. Antigorite is monoclinic in the space group Pm. The antigorite structure differs from the ideal serpentine structure due to its basic composition having a smaller ratio of octahedral to tetrahedral cations[5].
Geologic Occurrences
[edit]Antigorite is found in low-temperature, high-pressure (or high-deformation) environments, including both extensional and compressional tectonic regimes[6]. 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 magentite, chlorite, and carbonates[7].
See also
[edit]References
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- ^ "AMCSD Search Results". rruff.geo.arizona.edu.
{{cite web}}: CS1 maint: url-status (link) - ^ "Validate User". pubs.geoscienceworld.org. doi:10.2138/am-2002-1022.
{{cite web}}: CS1 maint: url-status (link) - ^ Ulmer, P.; Trommsdorff, V. (1995). "Serpentine Stability to Mantle Depths and Subduction-Related Magmatism". Science. 268 (5212): 858–861. doi:10.1126/science.268.5212.858. ISSN 0036-8075.
- ^ "Antigorite Mineral Data". webmineral.com.
{{cite web}}: CS1 maint: url-status (link) - ^ Wicks, F. J.; O’Hanley, D. S. (1988), "Chapter 5. SERPENTINE MINERALS: STRUCTURES AND PETROLOGY", Hydrous Phyllosilicates, Berlin, Boston: De Gruyter, pp. 91–168, ISBN 978-1-5015-0899-8
- ^ Ribeiro Da Costa, Isabel; Barriga, Fernando J. A. S. Viti; Mellini, Marcello; Wicks, Frederick J. (2008). "Antigorite in deformed serpentinites from the Mid-Atlantic Ridge". European Journal of Mineralogy: 563–572. doi:10.1127/0935-1221/2008/0020-1808.
- ^ Ribeiro Da Costa, Isabel; Barriga, Fernando J. A. S. Viti; Mellini, Marcello; Wicks, Frederick J. (2008). "Antigorite in deformed serpentinites from the Mid-Atlantic Ridge". European Journal of Mineralogy: 563–572. doi:10.1127/0935-1221/2008/0020-1808.