Laboratory exploration of mineral precipitates from Europa's subsurface ocean

The precipitation of hydrated phases from a chondrite-like Na–Mg–Ca–SO4–Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid- (360 K h−1, T = 250–80 K, t = 3 h) and ultra-slow-freezing (0.3 K day−1, T = 273–245 K, t = 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2–H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, ΔT = 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.

Precipitation Behavior of Mg-7Gd-3Y-2Zn-0.5Zr Alloy during Isothermal Aging

Precipitate phases in an Mg–7Gd–3Y–2Zn–0.5Zr alloy aged isothermally at 240 °C were examined using high-resolution transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM. The two types of precipitation sequence that involve Mg–Gd and long period stacking ordered (LPSO) type were found. The LPSO type sequence consisted of the precipitation of γ′′, γ′, 14H-LPSO/18R-LPSO. The Mg–Gd type precipitation sequence involved the formation of β′(b.c.o.) and β1(f.c.c.). The sequence, morphology, distribution, and crystal structure of these precipitates formed during isothermal aging were investigated. The results indicated that the priority precipitation of Mg–Gd and LPSO type sequences during aging can be affected by Nd, which has a higher diffusion coefficient than Gd and Y. The dislocation structures and strengthening mechanism were also discussed.

Monitoring mineral precipitation sequence of Lake Magadi soda lake: A multi-technical approach

<p>Lake Magadi is a saline soda lake in East African Rift Valley, occupying the axial trough of Southern Kenyan Rift. Its fed by perennial saline hot/warm springs, which evolve into the soda and saline chemistry of the lake. The main processes thought to cause the enrichment of the lake in Na<sup>+</sup>, CO<sub>3</sub><sup>2-</sup>, Cl<sup>-</sup>, HCO<sub>3</sub><sup>-</sup> and SO<sub>4</sub><sup>2-</sup> are evaporative concentration, mineral precipitation and fractional dissolution [1]. Lake Magadi is considered an analogous environment to the early Earth [2]. The high pH, silica and carbonate content of Lake Magadi allows the formation of silica and carbonate induced self-assembled mineral structures [3,4]. Revealing the mineral precipitation sequence of Lake Magadi have implications in understanding the geochemistry of evaporative rift settings and soda oceans. We have experimentally investigated the mineral precipitation sequence during evaporation at 25 °C. The sequence of mineral precipitation was recorded by using in-situ video microscopy. The mineral patterns observed in video microscopies were identified by spectroscopic, diffraction and electron microscopy techniques. The mineralogy and elemental composition of the precipitates were determined by using Raman spectroscopy, powder X-ray diffractions and scanning electron microscopy coupled with energy dispersive X-ray analyser. The results of the ex-situ analyses were compared with the in-situ X-ray diffraction. In-situ X-ray diffractions were performed on acoustically levitated droplets in the μSpot beamline at BESSY II synchrotron (Berlin, Germany). Finally, thermodynamic evaporation simulation was performed by using PHREEQC code with Pitzer database. Ex-situ and in-situ experiments revealed that mineral precipitation begins with trona, followed by halite and finally thermonatrite. In PHREEQC simulations, natron was observed instead of thermonatrite, suggesting the role of kinetics in the mineral assemblages. This multi-technical approach of in-situ monitoring and ex-situ characterization is a powerful approach to unveil mineral precipitation patterns and the resulting geochemical evolution in evaporative rift settings.</p><p><strong>Acknowledgments</strong><strong>: </strong>We acknowledge funding from the European Research Council under grant agreement no. 340863, from the Ministerio de Economía y Competitividad of Spain through the project CGL2016-78971-P and Junta de Andalucía for financing the project P18-FR-5008. M.G. acknowledges Grant No. BES-2017-081105 of the Ministerio de Ciencia, Innovacion y Universidades of the Spanish government.</p><p><strong>References:</strong></p><p>[1] Eugster, H.P. (1970). Chemistry and origin of the brines of Lake Magadi, Kenya. Mineralogical Society of America Special Papers, 3, 213–235.</p><p>[2] Kempe, S.; Degens, E.T. (1985). An early soda ocean?. Chem. Geol.  53, 95–108</p><p>[3] Getenet, M.; García-Ruiz, J.M.; Verdugo-Escamilla, C.; Guerra-Tschuschke, I (2020). Mineral Vesicles and Chemical Gardens from Carbonate-Rich Alkaline Brines of Lake Magadi, Kenya, Crystals, 10, 467.</p><p>[4] García-Ruiz J.M., van Zuilen M.A., Bach W. (2020) Mineral self-organization on a lifeless planet. Phys Life Rev, 34–35,62–82</p>

The Sb-Au-district Brandholz/Goldkronach (Fichtelgebirge, Germany): mineralogical indications for the evolution of hydrothermal Sb-mineralization.

<p>The Brandholz/Goldkronach district is situated in the southeastern part of Germany in the Bavarian Fichtelgebirge. Previous literature of the mineralogy of the district is rather descriptive and modern geochemical analysis are entirely missing. In this contribution, we combine petrography, bulk rock-geochemical analysis, SEM-MLA as well as EPMA to infer on precipitation mechanism and ore-forming processes. The quartz-polymetallic-sulfide veins are hosted in Ordovician shists, called “Phycodenschiefer”, which were intruded by upper Devonian meta-basalts. Antimony-sulfides are the main ore mineralization inside of the quartz-veins, accompanied by minor auriferous arsenopyrite and pyrite. Petrographic observations suggest a precipitation of an early stibnite phase (stage I). Sb-Pb-sulfides/sulfosalts (stage II) precipitated in fractures and fissures of stage I stibnite with a slightly change to Pb-rich Sb-phases. The antimony-mineralization event evolved from stibnite (Sb<sub>2</sub>S<sub>3</sub>), over fülöppite (Pb<sub>3</sub>Sb<sub>8</sub>S<sub>15</sub>), zinkenite (Pb<sub>9</sub>Sb<sub>22</sub>S<sub>42</sub>), plagionite (Pb<sub>5</sub>Sb<sub>8</sub>S<sub>17</sub>) to boulangerite (Pb<sub>5</sub>Sb<sub>4</sub>S<sub>11</sub>). Chemical analyses corroborate the petrographic observations and indicate a change in the hydrothermal environment from a Sb- to Sb-Pb dominated system with a distinct geochemical change from Pb-free to Pb-containing Sb-phases. A characterization of the precipitation sequence can be used to improve the understanding of the hydrothermal evolution of the whole Sb-Au-ore system in Goldkronach.</p>