forsterite crystal myanmar. olivine structure c b m2 m1 two cation sites: m1 m2
Post on 19-Dec-2015
236 views
TRANSCRIPT
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