Fayalite

Fayalite (Fe2SiO4, commonly abbreviated to Fa) is the <a href="/facts/Iron/QK1JYy8E">iron</a>-rich <a href="/facts/Endmember/ftb6zfhy">end-member</a> of the <a href="/facts/Olivine/tLLL4rae">olivine</a> <a href="/facts/Solid_solution/Mla7z8dc">solid-solution</a> series. In common with all minerals in the <a href="/facts/Olivine/tLLL4rae">olivine group</a>, fayalite crystallizes in the <a href="/facts/Orthorhombic/3zo4F8P0">orthorhombic</a> system (<a href="/facts/Space_group/2XQx0J7M">space group</a> 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 <a href="/facts/Forsterite/Qg1HDy5q">forsterite</a> (Mg2SiO4) and also with the <a href="/facts/Manganese/j9ELd1TL">manganese</a> rich olivine endmember <a href="/facts/Tephroite/sxWFxwxu">tephroite</a> (Mn2SiO4).

Iron rich olivine is a relatively common constituent of acidic and <a href="/facts/Alkaline_earth_metal/Ts4U5zIe">alkaline</a> <a href="/facts/Igneous/xeMJhUah">igneous</a> rocks such as volcanic <a href="/facts/Obsidian/GW1DI93A">obsidians</a>, <a href="/facts/Rhyolite/yKeR5Lut">rhyolites</a>, <a href="/facts/Trachyte/TNz4oTQA">trachytes</a> and <a href="/facts/Phonolite/YEGwrroG">phonolites</a> and <a href="/facts/Plutonic/Wl6xaY1a">plutonic</a> quartz <a href="/facts/Syenite/SetBxRcB">syenites</a> where it is associated with <a href="/facts/Amphibole/2Nf6RDRA">amphiboles</a>. Its main occurrence is in <a href="/facts/Ultramafic/7qAapP1d">ultramafic</a> <a href="/facts/Volcanic/c2M5upml">volcanic</a> and <a href="/facts/Plutonic/Wl6xaY1a">plutonic</a> rocks and less commonly in <a href="/facts/Felsic/0jpm1eSA">felsic</a> plutonic rocks and rarely in <a href="/facts/Granite/G2evuVKo">granite</a> <a href="/facts/Pegmatite/qW5BUXPg">pegmatite</a>. It also occurs in <a href="/facts/Lithophysae/TamDs4CW">lithophysae</a> in <a href="/facts/Obsidian/GW1DI93A">obsidian</a>. It also occurs in medium-grade thermally <a href="/facts/Metamorphic_rock/JRp6dsCm">metamorphosed</a> 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 + <a href="/facts/Quartz/4n86tE2G">quartz</a> = <a href="/facts/Orthopyroxene/hWMTLF9o">orthopyroxene</a>. 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 <a href="/facts/Oxygen/acyn6o8r">oxygen</a> to produce <a href="/facts/Magnetite/SVxoGUs2">magnetite</a> + quartz: the three minerals together make up the "FMQ" oxygen buffer. The reaction is used to control the <a href="/facts/Fugacity/C2gdkThT">fugacity</a> 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 <a href="/facts/Ringwoodite/LG79eVdM">ringwoodite</a>, i.e., contrary to forsterite there is no intermediate form analogous to <a href="/facts/Wadsleyite/lQPJNdfT">wadsleyite</a>; under the conditions prevailing in the <a href="/facts/Upper_mantle_(Earth)/5I9jq18K">upper mantle</a> of the Earth, the transition would occur at ca. 6–7 GPa, i.e., at substantially lower pressure than the phase transitions of forsterite. 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 <a href="/facts/Faial_Island/9g9Sqqas">Faial</a> (Fayal) Island in the <a href="/facts/Azores/yWF1EGs8">Azores</a> where it was first described in 1840.

Fayalite open-in-new

Fayalite