Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr43b1081d&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR43B-1081
Physics
3900 Mineral Physics, 3939 Physical Thermodynamics, 3994 Instruments And Techniques
Scientific paper
Orthosilicate olivine in the system Mg2SiO4-Fe2SiO4 (forsterite-fayalite) is a major phase in the solar system. It occurs in several types of meteorites and also on different planets and their moons. On Earth, Fe-Mg olivine is the most abundant phase of Earth's upper mantle and is an important mineral in mafic und ultramafic igneous and metamorphic rocks. Knowledge of its thermodynamic properties is crucial for undertaking many mineralogical, petrological and geophysical investigations. The heat capacities (Cp) of a series of synthetic forsterite (Fo)-fayalite (Fa) olivines have been measured between 5 and 300 K on milligram-sized samples using a new calorimetric method - heat pulse calorimetry. Sharp, λ-type Cp-anomalies are observed in the Fe-rich compositions Fa, Fo10Fa90, Fo20Fa80, Fo30Fa70 and Fo40Fa60. The corresponding Neel temperatures TN decrease linearly from 64.5 K in Fa to 32.8 K in Fo40Fa60 following the relationship TN = 79.02*XFa - 14.07. Fo50Fa50 and Mg-richer olivines show weak broad features in the Cp data around 15-20 K that decrease in magnitude with increasing Fo content in olivine. In order to obtain and separate electronic, magnetic and vibrational heat capacity contributions, Cel, Cmag, and Cvib from the experimental Cp's (Ctot), we used a single- parameter phonon dispersion model to calculate Cvib for the solid-solution members. The Cel + Cmag (i.e., Ctot - Cvib) contributions were fit to expressions describing a Schottky-type electronic anomaly and a paramagnetic-antiferromagnetic transition. For Fo50Fa50 and Mg-richer olivines, our analysis of Ctot shows that also these compositions have a Cmag contribution with a maximum around 25 K. Decomposition of the excess heat capacity (ΔCp^{xs}) into corresponding electronic, magnetic and vibrational contributions yields the largest absolute values for ΔC^{mag,xs}. Excess entropies of mixing, ΔS^{xs}, were also calculated from the Cp data. ΔS^{mag,xs} at 298.15 K is weakly negative for the solid solution (approx. -2 J/Kmol, because positive and negative Cp contributions as a function of temperature largely cancel each other between 0 and 300 K. ΔC^{el,xs} is positive for all temperatures and compositions, ΔS^{el,xs} thus gives a positive contribution with a maximum of 0.8 J/Kmol for Fo50Fa50. ΔS^{vib,xs} is also slightly positive for most members (maximum of 1.2 J/Kmol for Fo40Fa60). The resulting overall excess entropy, ΔS^{xs} = ΔS^{vib,xs} + ΔS^{el,xs} + ΔS^{mag,xs}, at 298.15 K along the Fo-Fa join is zero within 2σ-uncertainty.
Dachs Edgar
Geiger Charles A.
Grodzicki M.
von Seckendorff V.
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