Abstract. Raman spectroscopy combined with electron microprobe analysis as well as Mössbauer spectroscopy was applied to a series of 18 samples along the phlogopite (KMg3AlSi3O10(OH)2)–annite (KFe32+AlSi3O10(OH)2) join to establish a truly non-destructive method for crystallochemical characterization of biotite (A1M3T4O10X2, M3 = M1M2M2). The Raman scattering arising from the framework (15–1215 cm−1) and OH-stretching phonon modes (3000–3900 cm−1) was used to build up correlation trends between the Raman spectral features and crystal chemistry of biotite. We show that (a) the contents of MMg, MFe2+, and MFe3+ contents can be quantified with a relative error of ∼ 6 %, ∼ 6 %, and ∼ 8 %, respectively, by combining the integrated intensities of the OH-stretching peaks assigned to various M1M2M2 local configurations with the wavenumber of the MO6 vibrational mode near 190 cm−1; (b) the MTi content can be estimated from the peak position and FWHM (full width at half maximum) of the second strongest TO4-ring mode at ∼ 680 cm−1, with a precision of 22 %; (c) the content of TSi can be estimated from the position of the second peak related to TO4-ring vibrations near 650 cm−1; (d) for phlogopite the TAl content can indirectly be calculated by knowing the amount of TSi, whereas for annite it is hindered by the plausible presence of TFe3+; (e) the AK content can be quantified by the position of the peak generated by T-Ob-T bond-stretching-and-bending vibration at ∼ 730 cm−1; and (f) interlayer-deficient biotites and F-rich phlogopite can be identified via their unique OH-stretching Raman peaks around 3570 cm−1 and 3695 cm−1, respectively. Our results show a potential tool for non-destructive quantitative estimations of the major (Mg, Fe, Si, Al, K) and minor (Ti) elements of the crystal chemistry of the biotite mineral group by using a non-destructive technique such as Raman spectroscopy, although its sensitivity is generally lower than that of electron microprobe analysis and therefore cannot detect trace elements. This is fundamental within the framework of cultural heritage where samples cannot be powdered or disassembled.

The late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.

<p>Talc and serpentine-group minerals are Mg-dominant trioctahedral layered silicates that are common in igneous and metamorphic rocks and can be found in a wide range of geological conditions. Hence, a precise physicochemical characterization of these phyllosilicates in intact mineral assemblies, e.g. in thin sections as prepared for polarization microscopy, can provide a better insight into the processes of mineral formation, magma differentiation, and alteration. Moreover, talc and serpentines are common mineral components in a variety of cultural-heritage objects such as engraved gems and old Babylonian cylinder seals. Hence, material profiling of artefacts can help understand their origin through crystallographic and crystallochemical markers that may advance provenance studies. Since sampling of such objects is mostly prohibitive, the development of non-destructive, non-invasive, and preparation-free analytical methods is desired.</p> <p>To address this quest, a series of 18 serpentine-group minerals (nominally Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>OH<sub>4</sub>) and 10 talc samples (nominally (Mg<sub>3</sub>)Si<sub>4</sub>O<sub>10</sub>OH<sub>2</sub>) with different contents of Fe as a minor element was selected and studied by Raman spectroscopy and wavelength-dispersive electron microprobe analysis (WD-EMPA) to explore the potential of Raman spectroscopy as a truly non-destructive method for quantitative compositional characterization of these groups of phyllosilicates. The methodological approach is based on the already established quantitative relationships between the crystallochemical composition and the Raman signals of biotites (Aspiotis et al., 2022). The goal was first to verify whether the Raman scattering arising from the framework vibrations (15-1215 cm<sup>-1</sup>) and OH-bond stretching (3500-3900 cm<sup>-1</sup>) can assist in the identification of serpentine-group minerals and talc samples with various cationic compositions at the octahedral site. Secondly to establish quantitative relationships between the Raman signals (peak positions, integrated intensities, and full widths at half maximum) and the crystal chemistry of these phyllosilicates. We demonstrate that the quantification of <sup>M</sup>Mg and <sup>M</sup>(Fe<sup>2+</sup>+Mn) contents in talc from the Raman spectroscopic analysis is as accurate as from EMPA. Regarding serpentines, <sup>M</sup>Mg and <sup>M</sup>Fe<sup>2</sup>+ amounts can be determined as well with a relative precision of ~ 2 and 5%, respectively.</p>

Abstract. Raman spectroscopy has been applied to check if there are detectible material differences beneath the inscribed and non-inscribed areas of marble-based written artefacts, which could be further used to visualize lost or hardly readable text via suitable mapping. As a case study, marble segments with ∼ 2000-year-old inscribed letters from Asia Minor (western Turkey) and marble gravestones with 66 ± 14-year-old inscriptions from the cemetery of Ohlsdorf (Hamburg, Germany) have been subjected to Raman spectroscopy, as well as to complementary X-ray diffraction, wavelength-dispersive electron probe microanalysis, and Fourier-transform infrared spectroscopy, to thoroughly study the effect of different environmental conditions, grain size, and inscription age on the nature and penetration depth of marble alteration. The results demonstrate that environmental conditions rule over the type of dominant weathering changes, which are carotenoid molecular inclusions produced by lichen and amorphous carbon for marbles from Hamburg and Asia Minor, respectively. The alteration is much stronger in medium- and coarse-grained than in fine-grained marble, but it is suppressed by letter colouring. In the absence of letter colouring, the weathering-related products in both ancient and modern engraved marbles are more abundant beneath than away from the engraved areas, and the penetration depth is larger due to the enhancement of fissures and micro-cracks around the inscribed areas. We show that the Raman intensity ratio between the strongest peak of the weathering-related product (ν(C=C) ∼ 1520 cm−1 for carotenoids or the G peak ∼ 1595 cm−1 for soot-like carbon) and the strongest peak of marble (CO3 stretching near 1087 cm−1) can serve as a quantitative marker to indirectly map the lateral distribution of cracks induced during the inscribing process and hence can potentially be used to trace lost text on vanished marble inscriptions. This approach can be applied to other rock types, but further studies are required to identify the corresponding autochthonous weathering-related products.