A report on the chemical and mineralogical analysis of samples’ solid inclusions.

Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO₂ (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca²⁺- and Si⁴⁺-rich, and F⁻ moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca²⁺, but also Mn²⁺, Sr²⁺, and K⁺. The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si⁴⁺ can occupy severely α-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any ^[6]Si⁴⁺ signal around −200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around −84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q⁽²⁾ species defined as Si atom with two bridging O atoms, i.e., [Si(OSi)₂(−)₂], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.

A unique specimen of fossil resin originating from the Dipterocarpaceae tree family found in Miocene brown coal deposits in Jambi Province (Sumatra, Indonesia) was investigated via microscopic observations, microhardness testing and infrared and Raman spectroscopic methods. Its form is rare in nature, being an aggregate of three varieties of resin differing in colour, transparency and internal structure. This suggests the formation of the resins at different stages. Further alteration processes, including fossilization and maturation of the resin in a swamp environment resulted in stepwise aromatization of the cyclohexane ring in steroids and cross-linking through formation of ester bonds as well as carbon–carbon bonds between steroid molecules. The various environmental and geological conditions affecting the formation processes of the resins were recorded in their physico-chemical properties. Additionally, heating conditions accelerated by volcanism were proposed as a factor determining the maturation grade of the resin.