Forsterite CrystalMyanmar

Olivine structure

c

b

M2

M1

Two cation sites:

M1M2

Columns of M1 in the olivine structureb

a

The M1 sites form columns parallel to the c-axis

Compare M1 and M2 sites

M1 M2

Distorted 6-coordination Distorted 6-coordination

<M-O> = 2.16 Å <M-O> = 2.19 Å

Smaller site Larger 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 siteM1 site blocked by Sc3+

M1 site

Ni can be directed into either the M1 or the M2 site by appropriate substitutions in the other site.

LiScSiO4

Ni2+ 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 Fe2+ in the M(2) site.

Distortion changes the energetics

The M(2) site is more distorted than the M(1) siteTherefore, the t2g and eg orbitals will split more in the M(2) site

Et2g

eg

distortedoctahedral

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

57Co 57Fe(excited state) 57Fe (ground state)With the emission of a 14.4 keV gamma ray.

Gamma source sample detector

Mössbauer resultsMössbauer spectrum of Fo26

Conclusion from Mössbauer

Slight preference for Fe in the M1 site

Summary of studies

MIT M2 >> M1Chicago/VPI M1 = M2Carnegie M1 > M2