¹Ted L.Roush,^1,2Luis F.A.Teodoro,³David T.Blewett,³Joshua T.S.Cahill
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114331]
¹NASA Ames Research Center, Planetary Systems Branch, MS 245-3, Moffett Field, CA 94035-0001, USA
²Bay Area Environmental Research Institute, P.O. Box 25, Moffett Field, CA 94035-0001, USA
³Planetary Exploration Group, Johns Hopkins University Applied Physics Laboratory, MS 200-W2320, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA
Copyright Elsevier

We use radiative transfer (RT) models, based upon the Hapke (1993) model, to estimate the imaginary refractive index of magnetite from laboratory reflectance measurements. We used a RT program coupled with a least-squares algorithm to fit measured reflectance data using complex refractive indices of magnetite estimated here and literature values. We included differing representations of the grain size distribution for modeling the measured reflectance of the magnetite samples. Best-fitting models were obtained when using the complex indices of refraction estimated from a specific grain size fraction to fit the same grain size of reflectance data. Magnetite complex refractive indices taken from reported literature studies resulted in the poorest fits to the measured reflectance data.

We investigated the multiple-scattering behavior of magnetite using Fresnel’s equation and complex refractive indices from literature values and our own estimates. For both we found the reflection coefficient is <1% after four reflections suggesting that multiple scattering is minimal. We also calculated the transmission via the Beer-Lambert law using the same sets of refractive indices. For both, the initial interface transmission had a comparable value near 80%. However, as the distance through the material increases the discrepancy between the two refractive indices had substantial influence. For the literature values the transmission was reduced to <1% after a distance of 8 μm at all wavelengths, whereas for the estimated values the transmission remained ≥75% at this distance. Magnetite, when viewed in a petrographic thin section (~30 μm thick), is opaque. This suggests that the optical constants estimated via the Hapke approach are not realistic. We compared the calculated Fresnel reflectance using one literature value to the measured reflectances and found that the overall spectral shape was similar to the magnetite diffuse reflectance measurements. However, the magnetite diffuse reflectance is only 30–40% of the calculated Fresnel reflectance. We speculate this may be due to the granular surfaces scattering light into a non-specular angle. Hapke-like models have been successfully applied for estimating optical constants of transparent materials. However, the present study finds that such models may not be appropriate for determining the optical constants of low-reflectance, opaque materials, as the results are not comparable to values of optical constants reported in the literature.

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.

¹C. A. Lorenz,¹M. A. Ivanova,²F. Brandstaetter,¹N. N. Kononkova,³N. G. Zinovieva
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13612]
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin St. 19, Moscow, 119991 Russia
²Museum of Natural History, A‐1014 Vien, Burgring 7, Austria
³Lomonosov Moscow State University, Leninskie Gory, Moscow, 119991 Russia
Published by arrangement with John Wiley & Sons

The Pesyanoe aubrite is an essentially polymict regolith breccia comprised by fragments of different highly magnesian pyroxenitic lithologies: albite; anorthoclase and labradorite‐bearing pyroxenites; diopside and magnesian augite pyroxenites; roedderite‐ and forsterite‐bearing pyroxenites; and impact glasses; porphyritic and melt matrix breccia fragments; FeO‐rich chondritic inclusions; and exotic oxidized clasts. The parent magma of Pesyanoe probably was carbon saturated, as suggested by pyroxenite fragments containing igneous‐textured carbon phases, possibly graphite. The composition of feldspar and trapped melt inclusions in enstatite indicates occurrence of at least three metaluminous melt sources with different (K + Na)/Al and K/(K + Na) atomic ratios on the Pesyanoe parent body and has records of K and Na loss from the melt, possibly due to evaporation from the parent body surface. The roedderite‐ and forsterite‐bearing rocks probably crystallized from a peralkaline melt. We propose that peralkaline melt could be formed from a metaluminous melt(‐s) due to gravitational segregation of djerfisherite‐bearing metal‐sulfide liquid in the lower horizon of the magma chamber and following oxidation of the magma. This should lead to enrichment of silicate melt in K2O and Na2O and increasing of (K + Na)/Al > 1, allowing forsterite and roedderite to crystallize. Rocks enriched in K and containing rare K‐bearing minerals were found among both magmatic and melt rocks. This may imply an insignificant role of regolith transport in the process of the breccia’s formation and, therefore, an origin of all of the breccia components from a local region of the Pesyanoe parent body, probably from a single complex igneous massif.