|
Olivine [Forsterite, Mg2SiO4; Fayalite, Fe2SiO4]
Structure Orthorhombic
Space Group Pbnm (No. 62)
a=4.7560 Å, b=10.2070
Å,
c=5.9800 Å
a=b=g=90.00
Z=4
Atomic Positional Parameters
Mg1 4a 0.0000 0.0000 0.0000
Mg2 4c 0.9915 0.2774 0.2500
Si1 4c 0.4262 0.0940 0.2500
O1 4c 0.7657 0.0913 0.2500
O2 4c 0.2215 0.4474 0.2500
O3 8d 0.2777 0.1628 0.0331
Reference
JR Smyth and RM Hazen (1973) American
Mineralogist, 58, 588
Mineral Chemistry
Olivine-structured minerals consist
of separate [SiO4] tetrahedra linked
by divalent cations in octahedral coordination.
Complete solid solution exists
from forsterite to fayalite, where replacement of Mg by Fe2+
is accompanied by a linear increase in unit-cell parameters. Two distinct octahedrally
coordinated cation sites are found in the olivine structure, and are described as M1 and M2, with M1 being more distorted than M2. Mechanisms of cation ordering into M1 and M2 sites at high temperatures are a focus of present research on olivine-structured phases.
Magnesium-rich olivine is thought to be abundant in the earth's upper mantle; with increasing depth, olivine transforms to higher density crystal structures. These phase transitions are important for our understanding of the physics and geochemistry of the mantle.
The Chime figure shows oxygens as red spheres, magnesiums as green spheres, and silicons as ochre spheres.