¹Charles A. Geiger,¹Noreen M. Vielreicher,¹Edgar Dachs
American Mineralogist 106, 317-321 Link to Article [DOI: https://doi.org/10.2138/am-2021-7764CCBY]
¹Department of Chemistry and Physics of Materials, Section Materials Science and Mineralogy, Salzburg University, Jakob Haringer Strasse 2a, A-5020 Salzburg, Austria
Copyright: The Mineralogical Society of America

It is not known if the thermodynamic behavior of some minerals and their synthetic analogues are quantitatively the same. Olivine is an important rock-forming substitutional solid solution consisting of the two end-members forsterite, Mg2SiO4, and fayalite, Fe2SiO4. We undertook thefirst heat capac-ity, CP, measurements on two natural olivines between 2 and 300 K; nearly end-member fayalite and a forsterite-rich crystal Fo0.904Fa0.096. Their CP(T) behavior is compared to that of synthetic crystals of similar composition, as found in the literature. The two natural olivines are characterized by X-ray powder diffraction and 57Fe Mössbauer spectroscopy. The X-ray results show that the crystals are well crystalline. The Mössbauer hyperfine parameters, obtained from a fit with two Fe2+ quadrupole split doublets, are similar to published values measured on synthetic olivines. There are slight differences in the absorption line widths (i.e., FWHM) between the natural and synthetic crystals. CP (2 to 300 K) is measured by relaxation calorimetry. The CP results of the natural nearly end-member fayalite and published values for two different synthetic Fa100 samples are in excellent agreement. Even CP result-ing from a Schottky anomaly and a paramagnetic-antiferromagnetic phase transition with both arising from Fe2+ are similar. There are slight differences in the Néel temperature between the natural 63 K and synthetic ~65 K fayalites. This is probably related to the presence of certain minor elements (e.g., Mn2+) in the natural crystal. The third-law entropy, S°, value is 151.6 ± 1.1 J/(mol·K). CP behavior of the natural forsterite, Fo0.904Fa0.096, and a synthetic olivine, Fo90Fa10, are in excellent agreement between about 7 and 300 K. The only difference lies at T < 7 K, as the former does not show Debye T3 behavior, but, instead, a plateauing of CP values. The S° value for the natural forsterite is 99.1 ± 0.7 J/(mol·K).

¹Niclas Schneider,¹Grzegorz Musiolik,¹Jonathan E.Kollmer,¹Tobias Steinpilz,¹Maximilian Kruss,¹Felix Jungmann,¹Tunahan Demirci,¹Jens Teiser,¹Gerhard Wurm
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114307]
¹University of Duisburg-Essen, Faculty of Physics, Lotharstr. 1-21, 47057 Duisburg, Germany
Copyright Elsevier

In protoplanetary disks zones of dense particle configuration promote planet formation. Solid particles in dense clouds alter their motion through collective effects and back reaction to the gas. The effect of particle-gas feedback with an ambient solid-to-gas ratios on the stopping time of particles is investigated. In experiments on board the International Space Station we studied the evolution of a dense granular gas while interacting with air. We observed diffusion of clusters released at the onset of an experiment but also the formation of new dynamical clusters. The solid-to-gas mass ratio outside the cluster varied in the range of about 2.5–60. We find that the concept of gas drag in a viscous medium still holds, even if the medium is strongly dominated in mass by solids. However, a collective factor has to be used, depending on , i.e. the drag force is reduced by a factor 18 at the highest mass ratios. Therefore, flocks of grains in protoplanetary disks move faster and collide faster than their constituents might suggest.