Extended Abstract Samples of gem varieties of quartz from the Evros region in north-eastern Greece were spectroscopically analysed to determine their micro- to cryptocrystalline mineralogical properties using a Renishaw inVia micro-Raman microscope. The micro-Raman instrument provides point-andshoot Raman capabilities with a spatial resolution of 1 μm integrated into a standard microscope. The frequency shifts were calibrated with an internal Si reference sample. Samples analysed included different types of quartz, such as chalcedony and carnelian. During the micro-Raman analysis, the samples of chalcedony and carnelian showed different ratios of quartz/moganite, indicating heterogeneous formation conditions. This observation indicates that nanocrystalline to microcrystalline moganite is present in the quartz [1]. The bands at 465 (Si-O bending vibrations), 209 and 129 cm-1 (lattice vibrations) are the characteristic bands of α-quartz [2]. The peak at 503 cm-1 corresponds to the symmetrical stretching vibrations of the Si-O bonds in moganite. The proportion of moganite in the chalcedony was I503/I465 = ~25%, while it was particularly high in the carnelian samples with I503/I465 = ~35%. Both samples show a weak band at 395 cm-1, which could indicate the presence of the Fe+3 - O vibrational mode, possibly due to the colorant substance [3]. The peaks at 2851 and 2930 cm-1, which only occur in the carnelian sample, correspond to the OH- vibrational modes. Τhe presence of moganite in SiO2 varieties has been interpreted as an indication of their formation age. According to Lee et al., [4], the presence of moganite suggests that the SiO2 varieties are younger than -100Ma. This fact is consistent with the available geochronological data for the formations hosting the studied SiO2 varieties, which indicate an Oligocene age [5,6]. This is evidenced by the transformation of thermodynamically unstable moganite into quartz, or by the dissolution process of moganite during silica-water interactions at ambient pressure [6-9]. Moganite also exhibits a relatively intense band at 463 cm–1 that overlaps with the main 465 cm–1 mode of quartz at 465 cm–1 while the 503 cm–1 peak of moganite shows no interference [10]. Moganite is a low-temperature precipitate that coexists with fine-grained quartz [4]. In Schmidt et al. [9], the peak at 503 cm-1, of moganite is described as a temperature dependent vibration band that tends to be shifted to lower Raman shifts at higher temperatures. The moganite-specific Raman band was found exactly at a wavenumber of 503 cm-1 in the samples analysed in this study. Moganite formation is associated with the presence of silanol in hydrothermal fluids. This is evident from our Raman spectroscopy results, which show a proportion of quartz/moganite. In addition, Caggiani et al., [11] suggest that a peak at 504 cm⁻¹ also indicates the presence of silanol. Furthermore, the occurrence of peaks at wavenumbers 2851 and 2930 cm-1 suggests the presence of OH- within the quartz (var. carnelian) crystal structure. These observations not only confirm the presence of moganite by the Si-O peak at 503 cm-1, but also indicate the presence of Si-OH- resulting from the hydrothermal process, which requires silica-rich fluids known as silanol [12]. Lee et al., [4] found that carnelian is formed mainly from a mixture of higher-temperature hydrothermal fluids and shallower, lowertemperature, meteoric water. These were formed in Tertiary magmatic-hydrothermal environments throughout Greece, more specifically in hydrothermally altered igneous rocks that host several silica varieties, most of which yield gem- or semi-precious quality material [13]. The results presented here correspond to the same statement confirmed by micro-Raman analysis. In addition, stability studies of moganite during thermal treatment show that moganite disappears at high temperatures [11]. Similarly, moganite is observed as an intermediate phase in the diagenesis of permineralised fossil wood [14], suggesting a hydrothermal influence on the rock at low temperatures. Based on all the above, moganite could indicate a low-temperature precipitating mineral coexisting in relation to α- quartz.
Moganite occurrence in quartz varieties (Chalcedony and Carnelian) from Evros Region, NE Greece
Konstantinos Mavrogonatos;
2024-01-01
Abstract
Extended Abstract Samples of gem varieties of quartz from the Evros region in north-eastern Greece were spectroscopically analysed to determine their micro- to cryptocrystalline mineralogical properties using a Renishaw inVia micro-Raman microscope. The micro-Raman instrument provides point-andshoot Raman capabilities with a spatial resolution of 1 μm integrated into a standard microscope. The frequency shifts were calibrated with an internal Si reference sample. Samples analysed included different types of quartz, such as chalcedony and carnelian. During the micro-Raman analysis, the samples of chalcedony and carnelian showed different ratios of quartz/moganite, indicating heterogeneous formation conditions. This observation indicates that nanocrystalline to microcrystalline moganite is present in the quartz [1]. The bands at 465 (Si-O bending vibrations), 209 and 129 cm-1 (lattice vibrations) are the characteristic bands of α-quartz [2]. The peak at 503 cm-1 corresponds to the symmetrical stretching vibrations of the Si-O bonds in moganite. The proportion of moganite in the chalcedony was I503/I465 = ~25%, while it was particularly high in the carnelian samples with I503/I465 = ~35%. Both samples show a weak band at 395 cm-1, which could indicate the presence of the Fe+3 - O vibrational mode, possibly due to the colorant substance [3]. The peaks at 2851 and 2930 cm-1, which only occur in the carnelian sample, correspond to the OH- vibrational modes. Τhe presence of moganite in SiO2 varieties has been interpreted as an indication of their formation age. According to Lee et al., [4], the presence of moganite suggests that the SiO2 varieties are younger than -100Ma. This fact is consistent with the available geochronological data for the formations hosting the studied SiO2 varieties, which indicate an Oligocene age [5,6]. This is evidenced by the transformation of thermodynamically unstable moganite into quartz, or by the dissolution process of moganite during silica-water interactions at ambient pressure [6-9]. Moganite also exhibits a relatively intense band at 463 cm–1 that overlaps with the main 465 cm–1 mode of quartz at 465 cm–1 while the 503 cm–1 peak of moganite shows no interference [10]. Moganite is a low-temperature precipitate that coexists with fine-grained quartz [4]. In Schmidt et al. [9], the peak at 503 cm-1, of moganite is described as a temperature dependent vibration band that tends to be shifted to lower Raman shifts at higher temperatures. The moganite-specific Raman band was found exactly at a wavenumber of 503 cm-1 in the samples analysed in this study. Moganite formation is associated with the presence of silanol in hydrothermal fluids. This is evident from our Raman spectroscopy results, which show a proportion of quartz/moganite. In addition, Caggiani et al., [11] suggest that a peak at 504 cm⁻¹ also indicates the presence of silanol. Furthermore, the occurrence of peaks at wavenumbers 2851 and 2930 cm-1 suggests the presence of OH- within the quartz (var. carnelian) crystal structure. These observations not only confirm the presence of moganite by the Si-O peak at 503 cm-1, but also indicate the presence of Si-OH- resulting from the hydrothermal process, which requires silica-rich fluids known as silanol [12]. Lee et al., [4] found that carnelian is formed mainly from a mixture of higher-temperature hydrothermal fluids and shallower, lowertemperature, meteoric water. These were formed in Tertiary magmatic-hydrothermal environments throughout Greece, more specifically in hydrothermally altered igneous rocks that host several silica varieties, most of which yield gem- or semi-precious quality material [13]. The results presented here correspond to the same statement confirmed by micro-Raman analysis. In addition, stability studies of moganite during thermal treatment show that moganite disappears at high temperatures [11]. Similarly, moganite is observed as an intermediate phase in the diagenesis of permineralised fossil wood [14], suggesting a hydrothermal influence on the rock at low temperatures. Based on all the above, moganite could indicate a low-temperature precipitating mineral coexisting in relation to α- quartz.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
