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Fayalite (, commonly abbreviated to Fa) is the 
Iron rich olivine is a relatively common constituent of acidic and alkaline igneous rocks such as volcanic 
At high pressure, fayalite undergoes a phase transition to ahrensite, the iron-bearing analogue of ringwoodite, i.e., contrary to forsterite there is no intermediate form analogous to wadsleyite; under the conditions prevailing in the upper mantle of the Earth, the transition would occur at ca. 6–7 GPa, i.e., at substantially lower pressure than the phase transitions of forsterite.D. C. Presnall (1995): Phase diagrams of Earth-forming minerals. In: Mineral Physics & Crystallography – A Handbook of Physical Constants, ed. by T. J. Ahrens, AGU Reference Shelf vol. 2, American Geophysical Union, Washington, D.C., pp. 248–268 In high-pressure experiments, the transformation may be delayed, so that it may remain stable to pressures of almost 35 GPa (see fig.), at which point it may become amorphous rather than take on a crystalline structure such as ahrensite.
The name ''fayalite'' is derived from Faial (Fayal) Island in the Azores where it was first described in 1840.
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
-rich end-member of the olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...
solid-solution series. In common with all minerals in the olivine group
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly ...
, fayalite crystallizes in the orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
system ( space group ''Pbnm'') with cell parameters ''a'' 4.82 Å, ''b'' 10.48 Å and ''c'' 6.09 Å.
Fayalite forms solid solution series with the magnesium olivine endmember forsterite
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the ort ...
(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 obsidian
Obsidian () is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Obsidian is produced from felsic lava, rich in the lighter elements s ...
s, rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
s, trachytes and phonolite
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...
s and plutonic quartz syenite
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...
s 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
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
. It also occurs in lithophysae in obsidian
Obsidian () is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Obsidian is produced from felsic lava, rich in the lighter elements s ...
. 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
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
. 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.
Fayalite can also react with oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
to produce magnetite + quartz: the three minerals together make up the "FMQ" oxygen buffer. The reaction is used to control the fugacity of oxygen in laboratory experiments. It can also be used to calculate the fugacity of oxygen recorded by mineral assemblages in metamorphic and igneous processes.
At high pressure, fayalite undergoes a phase transition to ahrensite, the iron-bearing analogue of ringwoodite, i.e., contrary to forsterite there is no intermediate form analogous to wadsleyite; under the conditions prevailing in the upper mantle of the Earth, the transition would occur at ca. 6–7 GPa, i.e., at substantially lower pressure than the phase transitions of forsterite.D. C. Presnall (1995): Phase diagrams of Earth-forming minerals. In: Mineral Physics & Crystallography – A Handbook of Physical Constants, ed. by T. J. Ahrens, AGU Reference Shelf vol. 2, American Geophysical Union, Washington, D.C., pp. 248–268 In high-pressure experiments, the transformation may be delayed, so that it may remain stable to pressures of almost 35 GPa (see fig.), at which point it may become amorphous rather than take on a crystalline structure such as ahrensite.
The name ''fayalite'' is derived from Faial (Fayal) Island in the Azores where it was first described in 1840.
See also
*Forsterite
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the ort ...
, (Mg2SiO4), the magnesium-rich end-member of the olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers qui ...
solid-solution series.
* Mineral redox buffer
References
* Deer, W. A., Howie, R. A., and Zussman, J. (1992). ''An introduction to the rock-forming minerals (2nd ed.)''. Harlow: Longman {{Commons category, Fayalite Iron minerals Iron compounds Iron(II) minerals Nesosilicates Orthorhombic minerals Minerals in space group 62