Heat capacities and entropies of Mg/sub 2/SiO/sub 4/, Mn/sub 2/SiO/sub 4/, and Co/sub 2/SiO/sub 4/ between 5 and 380 K

Journal Article · Sat May 01 00:00:00 EDT 1982 · Am. Mineral.; (United States)

OSTI ID:6873605

Robie, R A ^[1]; Hemingway, B S; Takei, H

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1. U.s. Geological Survey, Reston, VA

The heat capacities of synthetic single crystals of Mg/sub 2/SiO/sub 4/ (foresterite), Mn/sub 2/SiO/sub 4/ (tephroite), and Co/sub 2/SiO/sub 4/ (cobalt olivine), were measured between 5 and 380 K using an adiabatically shielded calorimeter. Mg/sub 2/SiO/sub 4/ is diamagnetic, and its heat capacity follows a normal sigmoidal curve at low temperatures. Co/sub 2/SiO/sub 4/ shows a single sharp lambda-type transition at 49.85 +/- 0.02 K associated with the antiferromagnetic ordering of the magnetic moments of the Co/sup 2 +/ ions into a collinear spin arrangement below 49.8 K. In contrast to Co/sub 2/SiO/sub 4/ Mn/sub 2/siO/sub 4/ has two transitions in C/sup o//sub p/, a sharp lambda-type transition at 47.38 +/- 0.05 K and a smaller ''shoulder'' in C/sup o//sub p/ centered near 12 K. The upper transition corresponds to the paramagnetic (disordered) to collinear antiferromagnetic ordering of the Mn/sup 2 +/ moments, whereas the shoulder near 12 K corresponds to the change from the collinear to a canted spin structure. Our calorimetric values for the antiferromagnetic-paramagnetic transition temperature (Neel Temperature) are in excellent agreement with those obtained by powder magnetic susceptibility measurements, 49 +/- 2 K and 50 +/- 5 K for Co/sub 2/SiO/sub 4/ and Mn/sub 2/SiO/sub 4/ respectively. The thermal Debye temperature, THETA/sup C//sub D/, of Mg/sub 2/SiO/sub 4/ calculated from our C/sup o//sub p/ measurements between 6.3 and 13.8 K is 768 +/- 15 K and agrees well with the elastic value THETA/sup E//sub D/ of 758 K based on the mean sound velocity calculated from the room temperature elastic stiffness constants (c/sub ij/) of Graham and Barsch. At 298.15 K (25/sup 0/C) the molar heat capacities are 118.6, 128.7, and 133.4 J/(mol . K) and the molar entropies are 94.11 +/- 0.10, 155.9 +/0 0.4 and 142.6 +/- 0.2 J/(mol . K) respectively for Mg/sub 2/SiO/sub 4/, Mn=2SiO/sub 4/, and Co/sub 2/SiO/sub 4/.

• OSTI ID: 6873605
• Journal Information: Am. Mineral.; (United States), Vol. 67:5-6
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
COBALT SILICATES
ENTROPY
SPECIFIC HEAT
MAGNESIUM SILICATES
MANGANESE SILICATES
OLIVINE
LOW TEMPERATURE
MEDIUM TEMPERATURE
ULTRALOW TEMPERATURE
VERY LOW TEMPERATURE
ALKALINE EARTH METAL COMPOUNDS
COBALT COMPOUNDS
IRON COMPOUNDS
IRON SILICATES
MAGNESIUM COMPOUNDS
MANGANESE COMPOUNDS
MINERALS
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPOUNDS
152002* - Geothermal Data & Theory- Properties of Minerals & Rocks