Mars’ transition from an early “warm and wet” to a “cold and dry” environment left fingerprints on the geological record of fluvial activity on Mars. The morphological and mineralogical observations of aqueous activity provided varying constraints on the condition and duration of liquid water on the Martian surface. In this study, we surveyed the mineralogy of Martian alluvial fans and deltas and investigated the hydrated silica-bearing deposits associated with some of these landforms. Using CRISM data, we identified 35 locations across Mars with hydrated silica in proximity to fans/deltas, where the spectral characteristics are consistent with immature or dehydrated opal-A. In a few stepped fans/deltas, we find hydrated silica occurs within the bulk fan deposits and form sedimentary layers correlated with elevation. Meanwhile, in the older fans/deltas, silica mostly occurs at distal locations, and the relation to primary sedimentary deposits is more complex. We propose that the hydrated silica-bearing deposits in stepped fans/deltas likely formed authigenically from Martian surface waters, mainly during the Late Hesperian and Early Amazonian. These silica-bearing deposits could be a tracer for the temperature or duration of water involved in the formation of these deposits, given more precise and detailed observations of the sedimentary context, accessory minerals, the concentration of hydrated silica, and sediment-to-water ratio. Therefore, we consider that silica-bearing deposits should be among the most critical samples to investigate for future Mars missions, which are accessible in the landing sites of Mars 2020 and ExoMars 2022 missions.

Kaolinite is an Al-rich phyllosilicate commonly observed on Earth as a product of the chemical weathering of aluminosilicates. It has also been detected on the martian surface by orbital remote sensing observations. While the determination of the geological processes of formation of terrestrial kaolinite (i.e., hydrothermal activity, continental surface weathering, diagenesis) involves the coupling of field observation and multiple laboratory measurements, only geomorphology and associated minerals are generally available to determine their geological origin on Mars. Kaolinite crystallinity depends on many physicochemical parameters reflecting its conditions of crystallization. To determine if the near-infrared (NIR) spectral signature of kaolinite enables estimation of its crystallinity and furthermore if this method can be used to identify the geological processes involved in kaolinite formation, we carried out an in-depth analysis of NIR spectra of reference terrestrial kaolinites that formed in various geological contexts. We calculated second and third derivatives for each spectrum to highlight subtle variations in the spectral properties of kaolinite. This allowed the identification of 27 spectral contributions for the 4500 and 7000 cm−1 Al-OH-related regions of absorption bands. The position shifts and shape variations of these spectral contributions were intimately linked to variations of crystallinity, which was qualitatively estimated using Hinckley and Liétard XRD (dis)order indices. The results obtained show that the NIR signature of kaolinite is influenced by the stacking disorder of layers that has some influence on the vibrations of the interfoliar and inner Al-OH groups. Our study also confirms that: (1) well-ordered kaolinites are not restricted to hydrothermal deposits; (2) kaolinites from a similar sedimentary or pedogenetic context often display contrasting degrees of crystalline order; and (3) poorly ordered kaolinites are more likely to have a sedimentary or pedogenetic origin. Finally, this work highlights that obtaining spectra with sufficient spectral resolution could help to estimate the crystallinity of kaolinite and, in the best cases, its geological origin, both on Earth and Mars, especially with in situ NIR measurements.

The Trikorfo area (Thassos Island, Rhodope massif, Northern Greece) represents a unique mineralogical locality with Mn-rich minerals including kyanite, andalusite, garnet and epidote. Their vivid colors and large crystal size make them good indicators of gem-quality materials, although crystals found up to now are too fractured to be considered as marketable gems. The dominant lithology is represented by a garnet–kyanite–biotite–hematite–plagioclase ± staurolite ± sillimanite paragneiss. Thermodynamic Perple_X modeling indicates conditions of ca. 630–710 °C and 7.8–10.4 kbars. Post-metamorphic metasomatic silicate and calc-silicate (Mn-rich)-minerals are found within (i) green-red horizons with a mineralogical zonation from diopside, hornblende, epidote and grossular, (ii) mica schists containing spessartine, kyanite, andalusite and piemontite, and (iii) weakly deformed quartz-feldspar coarse-grained veins with kyanite at the interface with the metamorphic gneiss. The transition towards brittle conditions is shown by Alpine-type tension gashes, including spessartine–epidote–clinochlore–hornblende-quartz veins, cross-cutting the metamorphic foliation. Kyanite is of particular interest because it is present in the metamorphic paragenesis and locally in metasomatic assemblages with a large variety of colors (zoned blue to green/yellow-transparent and orange). Element analyses and UV-near infrared spectroscopy analyses indicate that the variation in color is due to a combination of Ti⁴⁺–Fe²⁺, Fe³⁺ and Mn³⁺ substitutions with Al³⁺. Structural and mineralogical observations point to a two-stage evolution of the Trikorfo area, where post-metamorphic hydrothermal fluid circulation lead locally to metasomatic reactions from ductile to brittle conditions during Miocene exhumation of the high-grade host-rocks. The large variety of mineral compositions and assemblages points to a local control of the mineralogy and fO₂ conditions during metasomatic reactions and interactions between hydrothermal active fluids and surrounding rocks.

The value of gem opals is compromised by their potential susceptibility to “crazing”, a phenomenon observed either in the form of whitening or cracking. To understand the latter, 26 opal samples were investigated and separated into 2 groups based on handling: “water-stored” opal samples, which are stored in water after extraction, and “air-stored” opal samples, which are stored in air for more than a year. To induce cracking, samples were thermally treated by staged heating and characterized using optical microscopy and Raman spectroscopy before and after cracking. For water-stored opals, cracking was initiated with moderate heating up to 150 °C, while for air-stored opals, higher temperatures, circa 300 °C, were required. In water-stored opals that cracked, polarized light microscopy revealed stress fields remaining around the cracks, and a red shift in the Raman bands suggested tensile stresses. These stresses were not observed in air-stored samples that cracked. Based on these observations, for air-stored samples, cracking was ascribed to super-heated water-induced decrepitation. By contrast, for water-stored samples, cracking was linked to drying shrinkage, which correlates with the anecdotal reports from the gem trade. We thus identify the physical origin of cracking, and by comparing it to current knowledge, we determine the factors leading to cracking.