Solid Earth, 7, 1509–1519, 2016
www.solid-earth.net/7/1509/2016/
doi:10.5194/se-7-1509-2016
© Author(s) 2016. CC Attribution 3.0 License.
Feathery and network-like filamentous textures as indicators for the
re-crystallization of quartz from a metastable silica precursor at the
Rusey Fault Zone, Cornwall, UK
Tim I. Yilmaz1,2, Florian Duschl3, and Danilo Di Genova2,4
1Technical University Munich, Tectonics and Material Fabrics Section, Arcisstr. 21, 80333 Munich, Germany
2Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität, Theresienstr. 41/III,
80333 Munich, Germany
3Geoscience Center of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany
4School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK
Correspondence to: Tim I. Yilmaz (tim.yilmaz@min.uni-muenchen.de)
Received: 31 March 2016 – Published in Solid Earth Discuss.: 5 April 2016
Revised: 14 October 2016 – Accepted: 19 October 2016 – Published: 7 November 2016
Abstract. Hydrothermal quartz crystals, which occur in
the Rusey Fault Zone (Cornwall, UK), show feathery tex-
tures and network-like filamentous textures. Optical hot-
cathodoluminescence (CL) analysis and laser ablation in-
ductively coupled plasma mass spectrometry (LA-ICP-MS)
investigations on quartz samples revealed that positions
exhibiting feathery textures (violet luminescence) contain
higher amounts of Al and Li than quartz positions without
feathery textures (blue luminescence), while concentrations
of Al and Li are significantly lower in feathery textures. Both
Al and Li correlate negatively with Si. Raman spectroscopy
investigations revealed the presence of a weak peak at 507–
509 cm−1 in quartz affected by feathery textures, which we
attribute to the presence of ≤ 5 % moganite, a microcrys-
talline silica polymorph, intergrown with chalcedony. The
combined occurrence of feathery textures and network-like
filamentous textures in quartz samples from the Rusey Fault
Zone points to the presence of a metastable silica precursor
(i.e., amorphous silica or silica gel) before or during the crys-
tallization.
1 Introduction
In the Rusey Fault Zone, hydrothermal quartz precipitated
which shows (i) feathery textures and (ii) network-like fil-
amentous textures. These microstructures occur in quartz
coatings of so-called cockade-like quartz coatings (Fren-
zel and Woodcock, 2014). Feathery textures generally ap-
pear in blocky to subhedral quartz grains (Gebre-Mariam et
al., 1993; Moncada et al., 2012; Henry et al., 2014), while
network-like filamentous textures in general occur in micro-
crystalline chalcedony (Duhig et al., 1992; Grenne and Slack,
2003; Little et al., 2004). Both quartz and chalcedony pre-
cipitated under hydrothermal conditions. Feathery textures,
microstructures commonly occurring in many hydrothermal
quartz deposits, were first reported in quartz veins in King-
man, Arizona (Adams, 1920). Two models have been pro-
posed to explain the origin of these textures: (i) epitaxial
overgrowth of small quartz crystals on large existing quartz
crystals (Dong et al., 1995) and (ii) re-crystallization from
former fibrous, water-rich chalcedony (Sander and Black,
1988). Recently, Marinova et al. (2014) reported that feathery
textures are generally accepted as being a re-crystallization
product from chalcedony in the context of having a gel pre-
cursor.
Despite the recent advantages in studying these textures,
our understanding of the origin of this texture is still incom-
plete, mostly because few data have been published. Here
we report the obtained results from a multidisciplinary ap-
proach based on optical hot-cathodoluminescence (CL) anal-
ysis, laser ablation inductively coupled plasma mass spec-
trometry (LA-ICP-MS), and Raman spectroscopy investiga-
Published by Copernicus Publications on behalf of the European Geosciences Union.
1510 T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators
Figure 1. (a) Geological sketch map of Cornwall showing Devonian to pre-Devonian mica and amphibolite schists, Devonian to Carbonifer-
ous basins (indicated by short dashed lines), and Upper Carboniferous to Lower Permian granitic intrusions. Furthermore two structures, the
Rusey Fault (RF) and the Sticklepath-Lustleigh Fault (SP), are indicated. The dashed rectangle indicates the position of Fig. 1b. Modified
after Leveridge and Hartley (2006). (b) Geological and structural map of the Rusey Fault Zone area showing the Boscastle Formation to the
south and Crackington Formation to the north of the Rusey Fault Zone. Most faults have an NW–SE to WNW–ESE trend; only a few faults
have a NE–SW to ENE–WSW orientation. The position of the Rusey Fault Zone outcrop is indicated by the black arrow. Modified after
British Geological Survey 1 : 50 000 geological map (British Geological Survey, 2013).
tions on quartz crystals in order to obtain chemical insight
into these microstructures.
2 Geological setting
The E–W-trending Rusey Fault is ∼ 100 km long and sit-
uated in the Carboniferous Culm Basin of Cornwall, UK
(Fig. 1a). The studied outcrop (“Rusey Fault Zone”) is po-
sitioned on the northern coast of Cornwall in metasediments
of the Variscan foreland (Fig. 1b) at the contact between the
Crackington and Boscastle formations of the Culm Basin
(Shackleton et al., 1982). The Crackington Formation is
made up of cycles of fine sediments (sandstones, siltstones,
and mudstones) interpreted as parts of the lower Bouma se-
quences. The Boscastle Formation is made up of grey to dark
grey and black slates with a relatively strong, slightly dip-
ping cleavage that has been interpreted as segments of the
upper Bouma sequences (Isaac and Thomas, 1998). Both
formations have been considered as lateral counterparts of
the Namurian in the Culm Basin (Thompson and Cosgrove,
1996). The British Geological Survey 1 : 50 000 geological
map (British Geological Survey, 2013) (Fig. 1b) suggests that
further fault-bounded lithological units of transitional nature
between the Crackington Formation and the Boscastle For-
mation do exist at the studied outcrop on the coast.
The Rusey Fault is an important structure in Cornwall and
has most likely had a long and complex history. Andrews et
al. (1996) suggest that it originally started as an extensional
fault during the development of the Culm Basin. It afterwards
may have been reactivated as a post-peak metamorphic back-
thrust and became again an extensional fault at the end of
the Variscan orogeny (Thompson and Cosgrove, 1996). Il-
lite crystallinity studies (Primmer, 1985) suggest a tempera-
ture profile for the area between Tintagel and Bude, which
indicate that the Boscastle Formation at the Rusey Fault
Zone area reached epizonal metamorphic temperatures of
∼ 300 ◦C, whereas the Crackington Formation only reached
anchizonal temperatures of ∼ 200 ◦C. Oxygen isotope inves-
tigations (Primmer, 1985) imply temperatures between 161
to 197 ◦C north of the fault and temperatures ≥ 287 ◦C south
of the fault. Vitrinite reflectance data of the Boscastle For-
mation indicate a progressive increase in temperature as the
Rusey Fault Zone is approached, suggesting that peak tem-
perature close to the Rusey Fault Zone was ∼ 70 ◦C higher
than the metamorphic peak temperatures of the Boscastle
Formation, thus implying flow of great amounts of hot flu-
ids (∼ 370 ◦C) through the Rusey Fault Zone (Andrews et
al., 1996).
3 Methods
3.1 Cathodoluminescence (CL) microscopy
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 de-
fects 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 addi-
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T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators 1511
Figure 2. Photomicrographs of a quartz coating surrounding a wall rock fragment from the Rusey quartz zone: (a) X pol; (b) CL. (a) A wall
rock fragment is surrounded by quartz, which increases in size from the fragment toward the left. The large comb quartz shows zones with
densely distributed fluid inclusions (black arrow). (b) The CL image reveals that the comb quartz is made up by a core with partly euhedral
faces. That core shows initial blue luminescence colors, a patchy area with violet luminescence is representing feathery textures, and yellow
CL represents fluid inclusion trails (exposure time: 18.3 s) (non-oriented sample TY39X2).
Figure 3. Photomicrograph of blocky anhedral to subhedral quartz
crystals of the quartz coating from the quartz zone (Rusey Fault)
showing feathery textures as indicated by the white arrow. The red
arrow indicates feathery textures appearing within a zone in which
a high amount of fluid inclusions is situated (non-oriented sample
TY33X4; X pol).
tion to uranyl groups. CL can be used to detect quartz and to
reveal the processes of crystal growth, deformation, recrys-
tallization, and alteration (Götze et al., 2001). This method
can reveal zoning features within (hydrothermal) quartz crys-
tals and can help to identify various quartz generations (Ram-
seyer 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 representa-
tive samples were coated with a carbon layer for CL imag-
ing. 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).
3.2 LA-ICP-MS
Trace element compositions of SiO2 minerals were deter-
mined from in situ investigation of a 200 µm thick fluid wafer
using a PerkinElmer Sciex Elan DRC2 ICP–MS. The appara-
tus was combined with a Lambda Physik Compex 110 ArF-
Excimer laser ablation element working at 193 nm contain-
ing a low-volume sample chamber and an optical imaging
system; Ar was used as the carrier gas. Before and after ev-
ery 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 cal-
ibrated using the National Institute of Standards and Technol-
ogy (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×σ√
n
, and the error is below 5 %.
3.3 Raman spectroscopy
In this study we used Raman spectroscopy to study the spec-
tral fingerprint of quartz and feathery textures in our samples.
In comparison with other spectroscopic techniques such as
infrared spectroscopy and X-ray fluorescence, Raman spec-
troscopy offers several advantages. Indeed, this technique is
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1512 T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators
Figure 4. Photomicrographs of blocky anhedral to subhedral quartz from the quartz coating in the Rusey samples. (a) Network-like filamen-
tous and/or dendritic textures are made up by dark pigments, most likely micrometer-sized iron oxides, which are restricted by a irregular and
diffuse boundary (indicated by the black arrow); the rectangle indicates the position of (c); II pol. (b) Quartz is made up by blocky anhedral
to subhedral quartz; X pol. (c) A zoom into the dense distribution of the particles reveals that the inclusions form a network-like or dendritic
texture (black arrow); II pol. Oriented sample TY31C1.
non-destructive, requires little sample preparation, and al-
lows for high-resolution investigation (in the order of mi-
crons). 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 con-
focal hole of 100 µm, and a slit of 100 µm. The exposure time
was 10 s (5 times). Backscattered Raman radiation was col-
lected 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 correc-
tion was applied to the acquired spectra because the Raman
signal showed no fluorescence background.
4 Results and discussion
The feathery textures in our samples are frequently ar-
ranged on the intragranular growth zoning of both anhedral
to subhedral quartz grains and locally comb-shaped crystals
(Fig. 2a, b). They are characterized by 5–20 µm sized sub-
grains, which appear as splintery or feathery patterns under
a standard petrological microscope with crossed polarizers
due to slight optical differences in maximum extinction posi-
tions (Fig. 3). Subgrains of the feathery textures are arranged
along the c axes in a sub-parallel arrangement to each other,
forming filamentous bundles; the subgrain long-axes orien-
tation within these bundles is perpendicular to the c axis of
the quartz core, a growth feature typically observed in chal-
cedony (Braitsch, 1957; Flörke et al., 1991). Locally they are
restricted to growth zones and are accompanied by a high
amount of fluid inclusions (Fig. 3).
Feathery textures are also accompanied by fluid inclusions
(FI) that can be grouped into two types: (a) primary aqueous
biphase FI within feathery textures (intragranular) (Figs. 2a,
3) and (b) secondary aqueous biphase FI that trace healed mi-
crofractures (intergranular). These microfractures affect both
feathery textures and otherwise FI-free crystal cores. Type-
a inclusions occur along boundaries between subgrains or
filamentous bundles. The shape of type-a inclusions is usu-
ally irregular or tubular-elongated with acicular ends, many
of them are interconnected by minute channel-like features;
type-b inclusions typically show irregular shapes. The size of
fluid inclusions range from < 5 to 10 µm.
Network-like filamentous textures are exhibited locally in
quartz of the quartz coatings surrounding wall rock frag-
ments (Fig. 4a–c). They are localized within a zone confined
by a smooth irregular outline (Fig. 4a) and are characterized
by fine (< 5 µm) irregular-shaped pigment inclusions proba-
bly composed of iron oxides (Fig. 4c). The pigment inclu-
sions are not restricted by grain boundaries and occur within
quartz exhibiting and not exhibiting feathery textures. These
textures are similar to those described by Duhig et al. (1992)
in chalcedony. The similarity to gel polymerization textures
(Brinker and Scherer, 1985; Shih et al., 1989; Scherer, 1999)
might indicate the relics of a polymerized material before
overgrowth by blocky to subhedral and partly euhedral hy-
drothermal quartz.
4.1 Cathodoluminescence (CL)
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
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T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators 1513
Figure 5. Photomicrographs (a, c) and CL images (b, d) of anhedral to subhedral quartz grains exhibiting feathery textures. (a) Splintery or
fibrous appearance of feathery textures, as indicated by the white arrow. The outer dashed line indicates the grain boundaries of one quartz
grain, and the inner dashed line indicates a core with no feathery textures; X pol. (b) CL in which the patchy area of the feathery textures is
shown by violet to reddish-brown colored with bright blue patches; the core is shown by intense blue luminescence. The patchy violet area
makes up∼ 70–80 % of the image (exposure time: 22.2 s). (c) Photomicrograph of subhedral quartz grains locally showing feathery textures.
The white dashed lines indicate intergranular zoning features (revealed in CL mode within d) X pol. (d) CL reveals red zoning features,
which are indicated by black arrows. White arrows indicate growth zoning features within blue cores appearing in various shades of blue.
The patchy violet area representing positions of feathery textures makes up ∼ 60–75 % of the image (exposure time: 18.3 s) (non-oriented
sample TY39X2).
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 in-
tensity after ∼ 60 s exhibiting an orange to red hue. The es-
timated 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 hy-
drothermal quartz, which are locally highlighted by oscilla-
tory 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).
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1514 T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators
Figure 6. (a) Fluid wafer scan of angular to sub-rounded and quartz-coated gouge fragments from the quartz zone of the Rusey Fault Zone.
The red line shows the analyzed LA-ICP-MS array. The white rectangle indicates the position represented in Fig. 6b, in which quartz grains
with and without feathery textures have been investigated. (b) Measured LA-ICP-MS spots are indicated by red dots and spots on a single
quartz grain for detailed analysis are marked by green dots (18 to 21). Oriented sample TY39X1, II pol.
Figure 7. To illustrate fractionation trends between Si and substituting elements such as Al and Li the concentration of respective elements
from all 33 measured spots were compared. The SiO2 vs. Al2O3 plot on the left reveals a negative correlation between Si and Al concentration
in quartz in general, which may reflect a replacement of Si by Al and a monovalent cation. This observation is supported by the Li2O vs.
Al2O3 plot on the right, which proves a positive correlation between Li and Al.
4.2 LA-ICP-MS
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 re-
ported 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, particu-
larly in hydrothermal quartz (Rusk et al., 2011). Other com-
monly occurring elements including Ca, Cr, Cu, Mg, Mn, Pb,
Rb, and U are suggested to be input from fluid or solid inclu-
sions, 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 correla-
tion, and both elements correlate negatively with Si (Fig. 7).
The intensities of Sb vary slightly but essentially remain sta-
ble over the entire array; a weak negative correlation of Sb
with Si could be observed. Ti shows relatively low concen-
trations 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 feath-
ery textures, but measured concentrations are in general low.
Though the peaks of these elements correlate locally, no cor-
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T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators 1515
Figure 8. Detail of a single quartz grain exhibiting a clear quartz core without feathery textures (C) surrounded by feathery textures (FT).
Both areas were analyzed using LA-ICP-MS in order to directly compare quartz composition on a single grain. A solid white line outlines
the quartz grain, while the dotted white lines delimit core and feathery texture sections. Spots 18 and 21 completely hit feathery textures,
spots 19 and 20 affected feathery texture-free quartz only. Anomalous quartz interference colors are due to the fact that the fluid wafer has a
thickness of about 200 µm. Oriented sample TY39X1, X pol.
Figure 9. Three concentration plots illustrating the distribution of
Si (a), Al (b) and Li (c) across the area shown in Fig. 8. Both Al
and Li show a positive correlation and do correlate negatively with
Si. Obviously the clear quartz core (spots 19 and 20) contains more
Al /Li than quartz with feathery textures (spots 18 and 21), while
Si at the same time shows lower concentrations.
relation 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, us-
ing LA-ICP-MS, supports the previously observed correla-
tion 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 incorpo-
rated significantly lower amounts of Al and Li (Fig. 9). Other
elements typically found in quartz show no significant char-
acteristics. A compilation of major and trace element con-
centrations is given in Table S1 in the Supplement.
4.3 Raman spectroscopy
Samples from the quartz zone of the Rusey Fault, specifi-
cally 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 signa-
tures with respect to the quartz spectra. Over three differ-
ent thin sections we conducted ∼ 70 measurements in quartz
grains exhibiting feathery textures. Initially the measure-
ments 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 Ra-
man 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 mea-
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1516 T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators
Figure 10. Raman bands of SiO2 samples from the Rusey Fault Zone: representative Raman spectrum from feathery textures from a quartz
coating in the Rusey Fault Zone showing a peak at ∼ 507 cm−1 (dark Raman spectrum); Raman spectra from euhedral quartz from a quartz
coating in the Rusey Fault Zone showing no peak at ∼ 507 cm−1 (grey spectra).
surements acquiring Raman spectra using a grating with
2400 lines mm−1. In Fig. 8 we report a selection of the Ra-
man 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. Impor-
tantly, 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. Further-
more 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 per-
formed on SiO2 samples from hydrothermal deposits, cherts,
and flints have demonstrated that moganite in microcrys-
talline quartz/chalcedony can be detected (Kingma and Hem-
ley, 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 ref-
erences therein) showed that moganite band positions shift
to higher wavenumbers with decreasing wt % moganite con-
tent 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 vari-
ation 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 stan-
dard procedure). This latter parameter was calculated start-
ing from the well-known position of the main silicon band
(520.7 cm−1). When comparing Raman features from differ-
ent 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 sam-
ples with those obtained from literature (e.g., Götze et al.,
1998; Pop et al., 2004; Schmidt et al., 2013) allows us to
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T. I. Yilmaz et al.: Feathery and network-like filamentous textures as indicators 1517
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 maxi-
mum of 5 wt % of moganite intergrown with chalcedony can
be estimated.
5 Conclusions
Based on the results and discussion presented above we draw
the following conclusions: (1) laser ablation measurements
indicate that the irregular distribution of elements such as
Ca, Mg, Na, and K within the observed quartz types may
be caused by fluid and solid inclusions. Short-lived blue CL
is related to the substitution of Si with Al (+Li) in the crystal
lattice (Ramseyer and Mullis, 1990). Red to violet CL col-
ors within feathery textures are most likely caused by point
defects (non-bridging oxygen hole centers) (Marfunin, 1979;
Kalceff and Phillips, 1995; Götze et al., 2009) and not by
incorporation of interstitial cations (Ramseyer et al., 1988;
Ramseyer and Mullis, 1990; Götze et al., 2005). The forma-
tion of primary fluid inclusions was likely favored by the re-
crystallization (i.e., dissolution and precipitation) of a silica
polymorph (moganite and/or chalcedony) to quartz (Gold-
stein and Rossi, 2002), as (CL−) microscopy of microfab-
rics suggests. The Ti values, which are sometimes below
the detection limit, indicate re-crystallization temperatures
below 400 ◦C (Götte and Ramseyer, 2012). Relatively low
re-crystallization temperatures (< 300 ◦C) are also confirmed
by the presence of intense yellow CL along feathery texture
subgrains and microcracks (Ioannou et al., 2004; Rusk et
al., 2008). Though LA-ICP-MS analysis did not reveal es-
sential differences in composition between observed quartz
types, the obvious correlation in Al /Li and Si concentrations
proves a different incorporation mechanism of respective el-
ements that cannot be explained by varying P/T conditions,
since both quartz types formed simultaneously. Either the
re-crystallization of feathery textures may have caused a de-
pletion of Al and Li within the crystal lattice, or the quartz
precursor did contain lower amounts of respective elements
(growth-direction controlled incorporation) (Ramseyer and
Mullis, 1990). Yellow CL in the Rusey samples indicates
high concentrations of lattice defects, probably generated by
the rapid crystallization of a non-crystalline precursor (Götze
et al., 1999). (2) Raman spectroscopy revealed the occur-
rence of a weak peak at 507–509 cm−1 (Fig. 10), which we
attribute to the presence of ≤ 5 wt % of moganite intergrown
with chalcedony within the feathery textures. (3) The pres-
ence of locally occurring network-like filamentous textures
(Fig. 4a–c), which have appearances similar to polymeriza-
tion structures (Scherer, 1999), may indicate a polymeriza-
tion stage of a possible silica gel phase (Duhig et al., 1992).
(4) The presence of (a) feathery textures and (b) network-like
textures indicate the re-crystallization from (a) a microcrys-
talline silica polymorph (moganite and/or chalcedony) and
(b) a metastable SiO2 phase (e.g., amorphous silica or sil-
ica gel) (Oehler, 1976; Sander and Black, 1988; Duhig et al.,
1992; Marinova et al., 2014). As this study is based on anal-
yses of natural samples, hydrothermal crystallization experi-
ments should be carried out to investigate growth conditions
of feathery and network-like filamentous textures.
The Supplement related to this article is available online
at doi:10.5194/se-7-1509-2016-supplement.
Acknowledgements. We are grateful to Jörn H. Kruhl for plenty of
helpful discussions and Alfons van den Kerkhof for his support dur-
ing CL measurements and interpretation, as well as to Klaus Simon
for LA-ICP-MS measurements and his expertise. We also thank
Ottomar Krentz for field work support, Melanie Kaliwoda and
Rupert Hochleitner for Raman measurements at the Mineralogical
State Collection Munich (SNSB), Klaus Mayer for sample cutting
and preparation, and Moritz Wiegand (Bureau Mirko Borsche)
for his support in graphic design. A detailed and thoughtful
review by Brian Rusk and two anonymous reviewers strongly
improved the manuscript. This study was financially supported
by the German Academic Exchange Service (DAAD) within the
Australia–Germany Joint Research Cooperation Scheme (project
56267246). Tim Ibrahim Yilmaz gratefully acknowledges financial
support by the Leonhard Lorenz Foundation (grant no. 826/12)
and the TUM Graduate School (TUM GS). Please contact the
corresponding author for access to the raw Raman spectra as well
as to the LA-ICP-MS data that this study is based upon.
This work was supported by the German Research
Foundation (DFG) and the Technische Universität
München within the funding programme
Open Access Publishing.
Edited by: F. Rossetti
Reviewed by: B. Rusk and two anonymous referees
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