Fayalite
| Fayalite | |
|---|---|
Fayalite crystals on substrate. Sample collected from Ochtendung, Eifel, Germany
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| General | |
| Category | Nesosilicate |
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Formula (repeating unit) |
Fe2SiO4 |
| Strunz classification | 9.AC.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Unit cell | a = 4.8211, b = 10.4779 c = 6.0889 [Å]; Z = 4 |
| Identification | |
| Color | Greenish yellow, yellow-brown, brown; pale yellow to amber in thin section |
| Crystal habit | Commonly granular, compact, or massive |
| Twinning | On [100]; also on [031], as trillings |
| Cleavage | {010} moderate, {100} imperfect |
| Fracture | Conchoidal |
| Mohs scale hardness | 6.5 – 7.0 |
| Luster | Vitreous to resinous on fractures |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 4.392 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.731 – 1.824 nβ = 1.760 – 1.864 nγ = 1.773 – 1.875 |
| Birefringence | δ = 0.042 – 0.051 |
| Pleochroism | Faint |
| 2V angle | Measured: 74° to 47°, Calculated: 54° to 66° |
| References | |
Fayalite (Fe2SiO4; commonly abbreviated to Fa), also called iron chrysolite, is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group Pbnm) with cell parameters a 4.82 Å, b 10.48 Å and c Å 6.09.
Fayalite forms solid solution series with the magnesium olivine endmember forsterite (Mg2SiO4) and also with the manganese rich olivine endmember tephroite (Mn2SiO4).
Iron rich olivine is a relatively common constituent of acidic and alkaline igneous rocks such as volcanic obsidians, rhyolites, trachytes and phonolites and plutonic quartz syenites where it is associated with amphiboles. Its main occurrence is in ultramafic volcanic and plutonic rocks and less commonly in felsic plutonic rocks and rarely in granite pegmatite. It also occurs in lithophysae in obsidian. It also occurs in medium-grade thermally metamorphosed iron-rich sediments and in impure carbonate rocks.
Fayalite is stable with quartz at low pressures, whereas more magnesian olivine is not, because of the reaction olivine + quartz = orthopyroxene. Iron stabilizes the olivine + quartz pair. The pressure and compositional dependence of the reaction can be used to calculate constraints on pressures at which assemblages of olivine + quartz formed.
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Wikipedia