Forsterite Crystal Myanmar

Olivine structure c b M2 M1 Two cation sites: M1 M2

Columns of M1 in the olivine structure b a The M1 sites form columns parallel to the c-axis

Compare M1 and M2 sites M1M2Distorted 6-coordination = 2.16 Å = 2.19 Å Smaller siteLarger site

All atoms

Olivine structure

Top layer of oxygens

Almost hexagonal close packing

Problem of site occupancy (I) Into what site do cations go? Does site occupancy make a difference?

Problem of site occupancy (II) M2 site M1 site blocked by Sc 3+ M1 site Ni can be directed into either the M1 or the M2 site by appropriate substitutions in the other site. LiScSiO 4 Ni 2+ occupancy

Optical spectra of olivine Optical absorption spectrum of a 1.0 mm thick olivine from San Carlos, AZ. The three spectra are taken with linearly polarized light vibrating parallel to the three orthorhombic axes. The intense band at 1040 nm is from Fe 2+ in the M(2) site.

Distortion changes the energetics The M(2) site is more distorted than the M(1) site Therefore, the t 2g and e g orbitals will split more in the M(2) site E t 2g egeg distorted octahedral

Conclusion from optical studies MIT says: Fe is dominantly in the M(2) site

X-Ray studies At VPI and University of Chicago a precision X-ray diffraction study to determine the structure. (called a 3-D refinement of the structure)

Conclusion from X-ray study No evidence of M(1) – M(2) disorder M(1) = M(2)

Mössbauer spectra 57 Co  57 Fe(excited state)  57 Fe (ground state) With the emission of a 14.4 keV gamma ray. Gamma sourcesampledetector

Mössbauer results Mössbauer spectrum of Fo 26

Conclusion from Mössbauer Slight preference for Fe in the M1 site

Summary of studies MITM2 >> M1 Chicago/VPI M1 = M2 Carnegie M1 > M2