Olivine

Ternary diagram for the olivine group

Group of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion of mesosiderites, and even in some silicated irons. Olivine and pyroxene are the most common mineral found in stony meteorites.

Meteoriticists usually state the composition of olivine present in a meteorite based on its fayalite content which corresponds to the percentage of iron in the olivine. So, Fa9.7 means the iron content in the olivine is 9.7%.

Complete solid solutions occur in both the forsterite-fayalite series, and the monticellite (CaMgSiO4) to kirschteinite (CaFeSiO4) series. Kirschteinite has been found in CAI and other refractory inclusions, some angrites and even in the matrices of some CV3 oxidixed chondrites where the CV3 parent body experienced hydrothermal alteration. The monticellite-kirschteinite series has larger unit cell than the forsterite-fayalite series, accommodating the larger Ca cation.

 

Fe-Mn substitution forms a complete solid solution between tephroite (Mn2SiO4) and fayalite (no image provided). There is more Mn and Ca substitution in Fe-rich end members than Mg-rich end members. Ni is commonly present in Mg-rich olivine. Tephroite is only a minor component in olivines. The fayalite/tephroite ratio is represented by the Fe/Mn ratio in olivine. All meteoritic olivines have high Fe/Mn ratios.

The olivine structure consists of Isolated (SiO4)4- tetrahedra pointing alternately up and down along rows parallel to c axis. Mg2+ and Fe2+ occur in octahedral coordination (M-cations) in two distinct octahedral sites: a distorted M1 site and a more regular M2 site. Layers occur parallel to (100) with edge-sharing octahedra cross-linked by isolated (SiO4)4- tetrahedra. Mg2+ and Fe2+ appear to occupy the M1 or M2 sites without preference in the Mg-Fe series. In CaMgSiO4 olivine, Ca2+ prefers M2 site with Mg2+ in the M1 site.

Olivine has several polymorphs. The α olivine structure described above is only stable in upper part of Earth’s mantle. At ~400 km depth, olivine transforms into ringwooditespinel structure), which is about 10% denser. At still greater depths (~670 km), ringwoodite decomposes into magnesiowüstite, (Mg,Fe)O and (MgVI,FeVI)SiVIO3, which has a perovskite structure (Si in octahedral coordination).

 


Some or all content above used with permission from J. H. Wittke.


https://www.hou.usra.edu/meetings/lpsc2016/pdf/1752.pdf