CL is generally applied in a qualitative descriptive manner to classify and distinguish different minerals by their emission colors (Götze, 2002). CL emission is affected by lattice defects such as electron defects on broken bonds, vacancies, or radiation-induced defects and is related to trace activator ions such as Fe3+, Cr3+, Al3+, Mn2+, Pb2+, Cu2+, Sn2+ in addition to uranyl groups. CL can be used to detect quartz and to reveal the processes of crystal growth, deformation, recrystallization, and alteration (Götze et al., 2001). This method can reveal zoning features within (hydrothermal) quartz crystals and can help to identify various quartz generations (Ramseyer et al., 1988; Ramseyer and Mullis, 1990).

To visualize the internal structures and the growth and alteration features, and to detect different quartz types and generations, optical hot CL microscopy investigations were conducted using a CL microscope (HC3-LM Simon-Neuser; Neuser et al., 1995) coupled with a Kappa PS 40C-285 (DX) camera system with a resolution of 1.5 MP attached to an Olympus BH-2 microscope at the University of Göttingen. An electron gun operated at 14 keV under a high vacuum of 10-4 bar with a filament current running at 0.18 mA was used in which the electron beam diameter was ∼ 0.4 cm. The uncovered and highly polished thin sections of representative samples were coated with a carbon layer for CL imaging. The exposure times were 4.9–22.2 s for the 5× objective, 12.6–22.2 s for the 10× objective, and 37–46.5 s for the 20× objective. The detailed description of this method is given by Marshall (1988).

Trace element compositions of SiO2 minerals were determined from in situ investigation of a 200 µm thick fluid wafer using a PerkinElmer Sciex Elan DRC2 ICP–MS. The apparatus was combined with a Lambda Physik Compex 110 ArF-Excimer laser ablation element working at 193 nm containing a low-volume sample chamber and an optical imaging system; Ar was used as the carrier gas. Before and after every laser ablation spot measurement, the background setting was recorded for 20 s. Each of the 33 laser ablation spots was measured for 60 s. The laser spot diameter was set to 23 µm, and the laser pulse repetition rate was 8 Hz. All data were calibrated using the National Institute of Standards and Technology (NIST) external standard 610 (Pearce et al., 1997) and Si was used as a standard. ICP–MS data were processed using Iolite, the precision was calculated according to the formula 2×σn, and the error is below 5 %.

In this study we used Raman spectroscopy to study the spectral fingerprint of quartz and feathery textures in our samples. In comparison with other spectroscopic techniques such as infrared spectroscopy and X-ray fluorescence, Raman spectroscopy offers several advantages. Indeed, this technique is non-destructive, requires little sample preparation, and allows for high-resolution investigation (in the order of microns). We acquired the Raman spectra using a micro-Raman spectrometer (HORIBA; XploRa-Raman-System) equipped with a green laser (Ar ion, 532 nm). The beam provided an energy of ∼ 9.5 mW focused within a 50× objective on a spot of ∼ 1 µm. In order to achieve the highest spectral resolution (fundamental in this study to investigate both small shifts in wavelength and the development of shoulders), we used the maximum grating groove density (2400 lines mm-1), a confocal hole of 100 µm, and a slit of 100 µm. The exposure time was 10 s (5 times). Backscattered Raman radiation was collected from 50 to 700 cm-1. The elastically scattered photons were suppressed using a sharp edge filter. The system was calibrated using a silicon standard. No background correction was applied to the acquired spectra because the Raman signal showed no fluorescence background.

The primary CL signal of quartz crystals from coatings exhibiting feathery textures is generally characterized by an intense blue (initial blue) core (∼ 20–40 %), which is surrounded by a violet to weak red to reddish-brown patchy area. This short-lived blue emission is usual for quartz growth in a hydrothermal environment (Ramseyer and Mullis, 1990; Perny et al., 1992; Götze et al., 2001). The blue core of the quartz grain shows locally euhedral faces (Fig. 2b) whereas the patchy area represents feathery textures made up by quartz fibers (Fig. 2a, b). Note that CL colors strongly depend on the duration and intensity of electron radiation (Ramseyer et al., 1988). In particular, within hydrothermal quartz, short-lived blue luminescence may disappear after several seconds of radiation (Fig. 5a–d). In our samples this transient blue CL lasts for ∼ 40 s under electron radiation. Besides, transient yellow CL significantly decreases in intensity after ∼ 60 s exhibiting an orange to red hue. The estimated percentage obtained by image analysis (ImageJ) of quartz exhibiting feathery textures is ∼ 60 to 85 % (Fig. 5b, d).

The patchy appearance of the feathery textures represents observed filamentous bundles under crossed polarized light. Grains with feathery textures locally show blue cores of hydrothermal quartz, which are locally highlighted by oscillatory growth zonings. These growth zonings appear in various shades of blue (Fig. 5d). Furthermore, red zoning features were observed, which are located within the feathery textures and in the blue cores (Fig. 5d). As they locally do affect both core and feathery textures within the same crystal, these red zoning features demonstrate the simultaneous growth of both quartz types. The quartz grains of the coating are traversed by fine irregular networks exhibiting bright yellow, transient CL (Fig. 5b, d).

CL of the network like filamentous textures do not show any difference from euhedral quartz and quartz cores in feathery textures (blue CL).

A traverse of laser ablation spots within one fluid wafer (Fig. 6) was defined to examine the chemistry of the quartz grains in general, and more specifically to reveal differences in composition between quartz without feathery textures and quartz with locally well-developed feathery textures. As reported by several studies (Rusk et al., 2008; Flem and Müller, 2012; Rusk, 2012), the main substituent for Si4+ in quartz is Al3+ with Li+ or Na+ balancing the missing positive charge in the crystal lattice. Moreover, various other trace elements such as B, Ge, Fe, H, K, Na, P, and Ti tend to be incorporated as lattice-bound impurities. Sb may also play a role, particularly in hydrothermal quartz (Rusk et al., 2011). Other commonly occurring elements including Ca, Cr, Cu, Mg, Mn, Pb, Rb, and U are suggested to be input from fluid or solid inclusions, which may occasionally influence mass spectrometric analysis (Müller et al., 2003; Flem and Müller, 2012).

Within the array of laser ablation spots shown in Fig. 6, the Si content showed slight variation throughout the measured part of the fluid wafer with SiO2 contents varying between 99.06 and 99.7 wt %. Al and Li show a clear positive correlation, and both elements correlate negatively with Si (Fig. 7). The intensities of Sb vary slightly but essentially remain stable over the entire array; a weak negative correlation of Sb with Si could be observed. Ti shows relatively low concentrations in general (0.0002 to 0.0042 wt % TiO2) and only a weak correlation with Si could be detected. Elements such as Ca, Na, Mg, and K are abundant within cores and feathery textures, but measured concentrations are in general low. Though the peaks of these elements correlate locally, no correlation between named elements and Al and/or Li could be detected.

The detailed analysis of single quartz grains exhibiting both feathery textures and a feathery-texture-free core, using LA-ICP-MS, supports the previously observed correlation between Al, Li, and Si, where Al and Li probably replace Si in the crystal lattice (an example is shown in Fig. 6b, spots 18 to 21, and Fig. 8). This replacement mostly affects the feathery-texture-free cores, as high Al and Li concentrations prove, while quartz with feathery textures obviously incorporated significantly lower amounts of Al and Li (Fig. 9). Other elements typically found in quartz show no significant characteristics. A compilation of major and trace element concentrations is given in Table S1 in the Supplement.

Samples from the quartz zone of the Rusey Fault, specifically the quartz coatings surrounding wall rock and gouge fragments, were analyzed in order to examine the origin of the feathery textures. The obtained Raman spectra collected from the feathery textures exhibit different spectral signatures with respect to the quartz spectra. Over three different thin sections we conducted ∼ 70 measurements in quartz grains exhibiting feathery textures. Initially the measurements were performed using a grating with 1200 lines mm-1. Results were not unequivocally positive or negative, so no significant difference (i.e., development and/or shift in Raman peaks) could be determined between the quartz and the moganite spectra in feathery textures. For this reason we decided to maximize the spectral resolution of our measurements acquiring Raman spectra using a grating with 2400 lines mm-1. In Fig. 8 we report a selection of the Raman spectra from quartz and feathery textures acquired over the samples. The main quartz band systematically shifts from ∼ 464 to ∼ 467 cm-1 in the region where feathery textures were observed. Moreover, two other bands located at the low-wavenumber region show a shift in the same direction (i.e., to higher wavenumbers). In particular, the bands located at ∼ 128 and ∼ 203 cm-1 shifted to 130 and 208 cm-1. Importantly, a new peak at ∼ 507–509 cm-1 was detected within the feathery textures regions and not within the crystal cores (Fig. 10). The development of this peak in the spectra of the feathery textures region was not possible to absolutely discern when using the 1200 lines mm-1 grating because it was incorporated in the tail of the main quartz peak. Furthermore we performed our measurements using polarizers on quartz crystals with different c axis orientations to explore the changes of Raman features due to the crystal orientation (Fig. S1 in the Supplement). This was necessary to rule out the Raman peak at ∼ 507–509 cm-1 in feathery textures not being related to changes of the c axis orientation.

The origin of feathery textures may be explained with the re-crystallization of former fibrous, water-rich, chalcedony. Chalcedony is microcrystalline silica composed of nano-scale intergrowths of α-quartz and moganite (Heaney, 1993). Within chalcedony and other microcrystalline silica varieties between 5 and 20 wt % of moganite may crystallize (Heaney and Post, 1992). Raman spectroscopy investigations performed on SiO2 samples from hydrothermal deposits, cherts, and flints have demonstrated that moganite in microcrystalline quartz/chalcedony can be detected (Kingma and Hemley, 1994; Hopkinson et al., 1999; Götze et al., 1998, 1999; Rodgers and Cressey, 2001; Rodgers and Hampton 2003; Pop et al., 2004; Rodgers et al., 2004; Heaney et al., 2007; Schmidt et al., 2012, 2013; Sitarz et al., 2014). Results show that the main Raman bands are located at 462–465 and 500–503 cm-1 for α-quartz and chalcedony with different wt % of moganite, respectively. Indeed, Schmidt et al. (2013 and references therein) showed that moganite band positions shift to higher wavenumbers with decreasing wt % moganite content dispersed in a chalcedony sample (Fig. 3a in Schmidt et al., 2013). Based on these results we attribute the peak ∼ 507 cm-1 to be the moganite peak. Moreover, a possible calibration issue can explain the slight discrepancy between our data and some literature results. Indeed, a possible variation of Raman peaks position can be associated with the non-linearity of the used spectrometer. The linearity during our calibration procedure was defined by the laser position (zero position) and the Koeff value (LabSpec 5 software standard procedure). This latter parameter was calculated starting from the well-known position of the main silicon band (520.7 cm-1). When comparing Raman features from different spectrometers, particular attention has to be paid to other Raman systems and software that use different approaches to correct for nonlinearity. In this study, the spectral region of interest (i.e., quartz and moganite bands) is located at higher wavenumber than the calibration one (0–520.7 cm-1). Therefore, the small discrepancy (a systematic shift to higher frequency of both quartz and moganite Raman peak position) we observed can be ascribed to some non-linear effect due to the calibration procedures.

A comparison of the spectra obtained from Rusey samples with those obtained from literature (e.g., Götze et al., 1998; Pop et al., 2004; Schmidt et al., 2013) allows us to conclude that within feathery textures a re-crystallization of chalcedony took place in our samples. Moreover, according to the calibration of Götze et al. (1998) based on the Raman band integral ratio of the quartz and moganite bands, a maximum of 5 wt % of moganite intergrown with chalcedony can be estimated.