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The processes associated with pollen germination were studied in vitro for two tree species, Salix caprea and Fraxinus excelsior under different nutrient conditions. The results provide evidence of changes in chemical composition of the pollen grains during germination. From the comparison of spectra of the pollen grain body and the growing pollen tube, it can be concluded that there are major chemical differences between these two morphological units. Comparison of germinated and ungerminated pollen grains reveals alterations in the metabolism. Composition of the germinating pollen grain and its morphological units depends on the plant species, but also on the nutrient conditions. The results suggest species-specific utilization of metabolite storage, and potential alterations of the pollen outer coat. Furthermore, discharge of molecules into the nutrient medium may depend on the nutrient conditions in the germination experiments. This has implications for further experiments on dynamic processes in pollen and related plant materials.
Determination of phosphate phases in sewage sludge ash-based fertilizers by Raman microspectroscopy
(2013)
The chemical form of phosphate phases in sewage sludge ash (SSA)-based fertilizers was determined by Raman microspectroscopy. Raman mapping with a lateral resolution of 5 × 5 µm² easily detected different compounds present in the fertilizers with the help of recorded reference spectra of pure substances. Quartz and aluminosilicates showed Raman bands in the range of 450-520 cm-1. Phosphates with apatite structure and magnesium triphosphate were determined at around 960 and 980 cm-1, respectively. Furthermore, calcium/magnesium pyrophosphates were detected in some samples.
The reaction mechanisms of phosphate-bearing mineral phases from sewage sludge ash-based fertilizers in soil were determined by Raman and synchrotron infrared microspectroscopy. Different reaction mechanisms in wet soil were found for calcium and magnesium (pyro-) phosphates. Calcium orthophosphates were converted over time to hydroxyapatite. Conversely, different magnesium phosphates were transformed to trimagnesium phosphate. Since the magnesium phosphates are unable to form an apatite structure, the plant-available phosphorus remains in the soil, leading to better growth results observed in agricultural pot experiments. The pyrophosphates also reacted very differently. Calcium pyrophosphate is unreactive in soil. In contrast, magnesium pyrophosphate quickly formed plant-available dimagnesium phosphate.
Sewage sludge ash (SSA) based P-fertilisers were produced by thermochemical treatment of SSA with Cl-donors at approximately 1000 °C. During this thermochemical process heavy metals are separated as heavy metal chlorides via the gas phase. Chromium cannot be separated under normal conditions. The risk of the development of toxic Cr(VI) during the thermochemical process was investigated. X-ray Absorption Spectroscopy measurements showed that SSA and thermochemically treated SSA with CaCl2, MgCl2 and NaCl contain Cr(III) compounds only. In contrast, treating SSA with elevated quantities of Na2CO3, to enhance the plant-availability of the phosphate phases of the fertiliser, developed approximately 10–15% Cr(VI). Furthermore, Raman microspectroscopy showed that using Mg-carbonate reduces the risk of a Cr(VI) development during thermochemical treatment. Additionally, leaching tests showed that only a Crwater solubility >10% is an indicator for Cr(VI) in SSA based P-fertilisers.
In the second half of the 19th century, Roman and Portland cements played an essential role as active hydraulic binder material in building construction and façade ornamentation. Size and heterogeneous phase assemblage of unhydrated cement clinker remnants in historical cement stone differ significantly from those of remnants occurring in modern Portland cement clinker burnt in rotary kilns due to limitations of the production technology available in the 19th century (e.g., comminution and homogeneity of the feedstock, burning temperature and regime in the intermittently operated shaft kilns, grinding machinery). In the common analytical approach, thin sections and fracture surfaces of historical Roman and Portland cement mortars are characterised regarding their mineralogical composition and microstructure using optical and electron microscopic imaging techniques. Raman microspectroscopy can be additionally employed for petrographic examination, overcoming some limitations of the methods used so far. The determination of the phase content of residual cement clinker grains in the hydrated matrix allows for the differentiation of Roman and Portland cement binders. As marker phases, we propose the calcium aluminates CA, C12A7, C2AS and C3A – besides the commonly used calcium silicates C2S and C3S – because of their different formation temperatures and stability fields. This study focuses on the identification of different calcium aluminate and aluminoferrite phases in clinker remnants in samples of cast ornaments of three buildings in Switzerland raised between 1875 and 1893; the obtained Raman spectra are compared with fingerprint spectra of the corresponding pure, synthesised clinker phases collected with the same instrument for an unambiguous data interpretation. In addition to these phases, mainly minerals showing no hydraulic activity, such as, wollastonite CS, rankinite C3S2, free lime, portlandite, iron oxides, garnets, augite, albite and feldspathoids have been identified in the sampled historical cement stones by Raman microspectroscopy. As there is a strong relationship between coexisting clinker phases and the chemical composition of the raw meal as well as the burning and cooling history during clinkering, the results can help in understanding the physical and mechanical characteristics of historical cement mortars. This knowledge is fundamental for the choice and the formulation of appropriate repair materials with tailored properties employed in the field of restoration and preservation of the architectural heritage of the 19th and early 20th centuries.
The investigation of the microstructure in functional, polycrystalline thin films is an important contribution to the enhanced understanding of structure–property relationships in corresponding devices. Linear and planar defects within individual grains may affect substantially the performance of the device. These defects are closely related to strain distributions. The present work compares electron and X-ray diffraction as well as Raman microspectroscopy, which provide access to microstrain distributions within individual grains. CuInSe₂ thin films or solar cells are used as a modelsystem. High-resolution electron backscatter diffraction and X-ray microdiffraction as well as Ramanmicrospectroscopy were applied for this comparison. Consistently, microstrain values were determined of the order of 10⁻⁴ by these three techniques. However,only electron backscatter diffraction, X-ray microdiffraction exhibit sensitivities appropriate for mapping local strain changes at the submicrometer level within individual grains in polycrystalline materials.
Raman microscopic imaging was just recently introduced into the analysis of residual Roman and Portland cement grains in 19th century cement stone, displaying evidence of the experimental adaptation of contemporary technological knowledge and practice to local circumstance. Beyond calcium ferrites, this study deals with ferruginous clinker phases that are atypical compared to present-day commercial conditions of manufacture, such as iron oxides, clinopyroxenes or pyroxenoids. Analog, microtexture, mineralogy and chemical composition of pulverized ferrosilicate slag used in the course of the 19th century as mineral additive in lime mortar reflect local resource utilization, recording the melting history within the furnace and the effectiveness of the reduction process of a single smelting event. In the case of the discussed example, chemical imaging by Raman microscopy allowed deducing a lime-rich, low-silica melt exposed to fairly reducing conditions because of the detection of the pyrometallurgic phases fayalite (Fe2SiO4), kirschsteinite (CaFeSiO4) and calcioolivine (Ca2SiO4) in zoned olivine laths and (with the melilite gehlenite, Ca2Al2SiO7) in the Interstitial matrix, cross-cut by dendritic wuestite (FeO). The presented analytical approach faces the high spatial complexity of such mortar samples by microspectroscopic imaging with micrometer lateral resolution and their chemical complexity by extracting the rich chemical information content from Raman spectra. Intensity maps of marker bands provide spatial phase distributions. Furthermore, maps of peak positions can give access to the dissemination of spectroscopically similar phases of solid solution series (e.g., olivine and calcium Aluminate ferrite) as well as varying amounts of incorporated foreign cations (e.g., in hematite).
Measuring the Burning Temperatures of Anhydrite Micrograins in a High-Fired Medieval Gypsum Mortar
(2017)
Typical feature of high-fired medieval gypsum mortars is a compact microstructure of squat gypsum crystals containing firing products as remains of the calcination process. So far, the burning history of the binder is estimated based on morphological characteristics of the latter. A novel Raman microspectroscopy approach provides access to the calcination temperatures of individual anhydrite grains based on quantifiable spectroscopic changes appearing due to gradual variations of crystallinity, as independently confirmed by X-ray diffraction analysis of anhydrites synthesised at temperatures between 500°C and 900°C. The approach was successfully applied to the high-fired gypsum mortar of a South Tyrolean stucco sculpture of a pieta dated around 1420. Microparticles of burned anhydrite II with firing temperatures scattered around 650°C and clusters of thermally damaged natural anhydrite II crystals from the raw material were identified and imaged.
The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Not hydrated clusters of firing products preserved in the binder matrix are a typical feature of such mortars. A novel Raman microspectroscopic approach, providing access to the burning history of individual anhydrite grains, was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for tracing and visualising pyrometamorphic reactions in natural impurities of the kiln run. In the discussed examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 to periclase MgO and lime CaO yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Hydration of periclase in the mixed gypsum paste containing sulphate ions also resulted in magnesium sulphate hydrates, here identified in the form of hexahydrite MgSO4·6H2O. Lower burning temperatures left the accessory minerals in their pristine form, but can be traced by measuring the spectra of individual anhydrite crystals in grains of firing products and evaluating Raman band widths. Throughout the present study, calcination temperatures ranging from approx. 600°C to 900°C were determined.
Raman band widths of anhydrite II reveal the burning history of high-fired medieval gypsum mortars
(2019)
The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Because of the absence of medieval textbooks, the observation of high-temperature, low-pressure mineral transformations and the correlation of phases coexisting in not hydrated binder relicts in the gypsum matrix to the mineralogy of the raw material and the burning conditions constitute the only source to the historical technological know-how.
The CaSO4–H2O system consists of five crystalline phases, which can be discriminated by structural analysis methods, such as Raman spectroscopy, due to obvious differences in their spectroscopic data: gypsum (CaSO4 ⋅ 2 H2O), bassanite (hemihydrate, CaSO4 ⋅ ½ H2O), anhydrite III (CaSO4), anhydrite II (CaSO4), and anhydrite I (CaSO4). Only recently, it was possible to demonstrate that small spectroscopic variations exist also within the relatively large stability range of anhydrite II from approx. 180°C to 1180°C: all Raman bands narrow with increasing burning temperature applied in the synthesis from gypsum powder. The determination of band widths of down to 3 cm-1 and differences between them of a few tenths of a wavenumber is not a trivial task. Thus, this contribution discusses peak fitting and strategies for correction of instrument-dependent band broadening.
Raman maps of polished thin sections of gypsum mortars provide access to the burning histories of individual remnant thermal anhydrite grains and enable the discrimination of natural anhydrite originating from the gypsum deposit. This novel analytical method was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for following pyrometamorphic reactions in natural impurities of the raw material. In the presented examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Lower burning temperatures, which leave the accessory minerals in their pristine form, can be traced by measuring the spectra of anhydrite crystalites in grains of firing products and evaluating Raman band widths. Throughout the applications of this analytical method so far, calcination temperatures ranging from approx. 600°C to 900°C were determined.