Human dental enamel is a uniquely hard and tough biological realization of the inherently brittle apatite crystal. In sound enamel both structure and chemistry are organically fine-tuned, while developmental disorders - such as hypomineralization - often lead to severe enamel alterations resulting in weaker mechanical performance and shorter lifetime as in case of molar-incisor hypomineralization (MIH) which has a global prevalence of 13.5%. However, causes of the development of hypomineralization disorder remain unclear up to date. In the present paper, a new form of dental hypomineralization is reported for the first time. Structural and chemical alterations of dental enamel exhibiting a special ribbon-like hypomineralization (RLH) are compared to the characteristics of sound and MIH-affected teeth. Microporosity, mineral density and characteristic 3D macroscopic shape of the RLH affected volumes were captured by micro-CT analysis. On the submicron-scale, nanoporosity, unusual morphologies and reduced size of apatite nanocrystals have been revealed. These RLH specific features are coupled with a particular chemical fingerprint in Raman spectroscopy, which allows its clear separation from MIH affected and sound enamel. Based on these nanostructural features we conclude different failure mechanisms in the biochemical control during crystallization of the RLH and MIH enamel. STATEMENT OF SIGNIFICANCE: Molar-incisor hypomineralization (MIH) is a developmental enamel disorder with up to 14% prevalence, characterized by reduced mineral content, hypersensitivity, and post-eruptive enamel breakdown. The causes of the development of this disorder remain unclear up to date. In this study, we examine molars with hypomineralization in a ribbon-like macroscopic morphology. These molars show increased porosity, organic content, and reduced mineral content, while carbonate levels remain unchanged in both affected and unaffected areas, unlike MIH-affected enamel. Ribbon-like hypomineralized (RLH) enamel also features unusual apatite nanocrystal morphologies, including isometric crystals in the prism boundary zone and bean-shaped crystals with a soft central zone. Nanoscale analysis reveals RLH as a developmental anomaly distinct from MIH, offering novel insights into the biochemical controls of hypomineralization.

This contribution provides a petrographic and Raman investigation of fluid inclusions found in chromitites collected in the ophiolites of Santa Elena (Costa Rica), Bracco (Italy), Otrhys and Vourinos (Greece), and Troodos (Cyprus). Most of the analyzed chromites are classified as high-Cr, with the exception of those from Bracco and some of the Othrys complexes that are high-Al. Although the investigation of fluid inclusions in chromitites is very challenging due to the poor transparency of the host chromite, the studied samples contain numerous fluid inclusions. The fluid inclusions look to be more abundant in the high-Cr chromitites, related to a subduction zone environment, compared to the high-Al chromitites generated in a mid-ocean ridge. This is in agreement with the petrogenetic model for the formation of podiform chromitites that implies the presence of a metasomatic event caused by hydrous fluids that reacted pervasively with variable depleted mantle tectonites, especially in the subduction zone setting. The fluid inclusions, between 1 and 15 µm in size, show negative crystal or irregular angular shapes. They occur when enclosed in chromite crystals that have not been affected by low-temperature processes. The fluid inclusions consist of liquid (L), vapour(V~30–50 area%) and L + V (V~40–60 area% rarely 10–80 area%). The fluid inclusions may contain only vapour and a vapour and a solid phase, too. The Raman spectra reveal the presence of CH₄ in certain fluid inclusions. Considering the high number of fluid inclusions that potentially contain CH₄, we suggest that the fluid inclusions in the chromite crystals and their leaching can be a possible source in order to explain the high amount of CH₄ detected in some podiform chromitites, previously attributed to the Sabatier reaction. The mode of the occurrences of the studied CH₄ bearing fluid inclusions, i.e., entrapped in unaltered chromite crystals formed at a magmatic temperature, suggest their abiotic origin from mantle-derived fluids, rather than those related to the low-temperature serpentinization processes. The investigation of fluid inclusions, although it is difficult and challenging or even impossible when the chromite is too opaque, can be applicable to other chromitites worldwide to verify the presence of H₂O, CH₄ or other gases. This information will greatly improve our understanding of the nature of the fluid phases during the formation of podiform chromitites.

Enamel prism is the main microstructural unit of mammalian enamel which composed of hundreds of bioapatite nanocrystals. Prism structure plays a key role in the excellent mechanical performance of dental enamel during millions of chewing cycles without significant remodeling. Thus, quantitative understanding of prism architecture is of utmost importance for biomechanical materials design. To characterize enamel prism orientation quantitatively, a novel image processing method has been developed. Our method is based on scanning electron microscopy images of etched enamel surface and consists of an ellipse fitting procedure, which provides a numerical approximation of prism shape and orientation in the studied cross section. The obtained analytical data allow to construct color coded orientation maps, which provide quick and useful insight into the microstructure of enamel. Besides striking visualization, orientation maps allow to extract and plot the rich information on the azimuthal and inclination angles of the prisms as function of location. Numerical data on prism arrangement can be analyzed using statistical tools over large areas, which paves the way towards quantifying comparative investigation of prism arrangement either in dentistry research or evolution biology. The application of the method is demonstrated for a distal–mesial cross‐section of sound human tooth enamel.Highlights: Scanning electron microscopy images of etched enamel surface are analyzed using ellipse fitting. Geometrical parameters of the fitted ellipses provide numerical data of thousands of prisms. Prism arrangement is visualized on color coded orientation maps and analyzed using statistical tools.

Fluid and melt inclusions trapped in mantle xenoliths provide direct insights into the metasomatic agent in the lithospheric mantle, including its volatile content. We conducted 3D Raman mapping on fluid and melt inclusions in modally metasomatized mantle xenoliths from the Styrian Basin (W‐Carpathian Pannonian Region) to explore how this method can be utilized to study the role of fluids and melts in the upper mantle. 3D Raman mapping revealed complex phase assemblages of coexisting fluid and solid phases in the inclusions. Fluid phases are CO₂ (49.2–98.4 mol%, 19.1–61.0 vol%) and H₂O (1.5–50.8 mol%, 8.6–35.3 vol%). This H₂O concentration range is considerably higher than in most mantle fluids (∼10–15 mol%). Solid phases are silicates, carbonates, sulfides, and sulfates present in varying volume% (vol%). 3D Raman mapping shows that liquid H₂O wets other phases in the mapped fluid inclusions and may be preferentially lost compared to the CO₂‐rich phase during inclusion decrepitation. The accuracy of CO₂‐H₂O mol ratios from Raman 3D mapping in fluid inclusions can be affected by variable Raman cross‐sections of trapped phases. Therefore, thermodynamic modeling is recommended to validate measured CO₂‐H₂O mol ratios. 3D Raman mapping may underestimate low Raman scatterers like silicate glass in fluid inclusions, but their volumes can be corrected based on FIB‐SEM analyses. Thermodynamic modeling suggests that the fluid compositions in the Raman‐mapped fluid inclusions may reflect non‐equilibrium entrapment, whereas those in the melt inclusions reflect equilibrium entrapment in this mantle portion. The discovered C‐O‐H fluids provide new information on fluid‐rock reactions in the lithospheric mantle.