We have measured the diffusion coefficient of Ca-44 along and perpendi
cular to c direction in natural Fe-bearing (similar to 2 at.%) diopsid
e single crystals. Specimens were annealed at temperatures ranging fro
m 1000 to 1250 degrees C, with controlled oxygen fugacity. Diffusion p
rofiles were analysed by Rutherford Back-Scattering Spectrometry (RBS)
of alpha-particles. The diffusion of Ca is isotropic along c and b di
rections. In addition, the results clearly show two distinct diffusion
al regimes for the natural diopside, revealed by silica precipitates o
ccurrence in the diopside matrix when T greater than or equal to 1150
degrees C. In this case the oxygen partial pressure pO(2) does not inf
luence the self-diffusion coefficient which is characterized by the ac
tivation energy E = 396 +/- 38 kJ/mol. For T less than or equal to 110
0 degrees C the diffusional process has a lower activation energy (E =
264 +/- 33 kJ/mol) and varies as (pO(2))(-0.14+/-0.01) in the investi
gated range (from 10(-16) atm to 10(-6) atm). These results are consis
tent with previously reported results on electrical conductivity (Hueb
ner and Voigt, 1988) and high temperature plastic deformation of natur
al diopside single crystals (Jaoul and Raterrob, 1994). According to t
he point defects model, elaborated by Jaoul and Raterron (1994), the d
iffusional mechanism of Ca should be essentially interstitial. Further
more, this mechanism should be the same for different diopside samples
with iron content ranging from 0.4 to 2.42 at.%. Indeed, for Ca diffu
sion in synthetic diopside (0.4 at.% Fe) the activation enthalpy is ve
ry similar (281 +/- 26 kJ/mol, Dimanov and Ingrin, 1995). On the other
hand, the Fe content indoubtly influences the preexponential factor.
The present paper reports Ca self-diffusion in diopside as a function
of T, pO(2) crystallographic orientation, and Fe content. In fact, amo
ng all diffusion coefficients previously reported in diopside, but Si,
Dc, is the lowest. Thereby, Ca should be a kinetically limiting speci
es for diffusion-controlled processes such as plastic deformation and
cation exchanges. For instance, Ca self-diffusion controls Ca-Mg excha
nges between pyroxenes. Then, our results could be helpful to better u
nderstand the closure behaviour (Dodson, 1973, 1976) of the Clinopyrox
ene-Orthopyroxene geothermometer.