Spectroscopic and structural investigations of blue afwillite from Ma'ale Adummim locality, Palestinian Autonomy.

Until now, only the colourless crystals of mineral afwillite, Ca3(HSiO4)2·2H2O, were known from several localities around the world. Present work focuses on blue afwillite counterparts from the Ma'ale Adummim locality in Palestine. Using the wide spectrum of analytical methods we attempted to identify the causes of this unusual colour. Structural investigation confirms the presence of two tetrahedral SiO3OH units connected by hydrogen bonds. The Raman spectrum of afwillite, obtained for the first time, shows the increased number of bands in the range of 785-970 cm-1, whose assignation was correlated with the presence of two different kinds of structural units: (SiO3OH)3- and its deprotonated counterpart (SiO4)4-. The heating process at 250 °C, in addition to the colour changes from blue to pastel green, shows the intensity reduction and disappearing of some Raman bands attributed mainly to SiO3OH units. The IR investigation confirms also the presence of that unit and provides information that the position and designation of infrared bands above ∼2300 cm-1 is related to the strength of hydrogen bonds within the structure. The stretching and bending OH vibrations of afwillite sample show the partial shift to the lower spectral frequencies after the H/D isotopic exchange in OH or H2O groups. Based on the results of the electron absorption and luminescence analyses it has been proposed that the blue colour of afwillite is caused by hole oxygen defect, most probably SiO3-.


Introduction
Mineral afwillite, ideally Ca 3 (HSiO 4 ) 2 $2H 2 O, is a low temperature hydrated calcium silicate known from several localities around the world [1e5]. For the first time, it was described in association with natrolite, calcite and apophyllite in a dolerite inclusion within kimberlite in the Dutoitspan Mine, Kimberley, South Africa [1]. Based on X-ray diffraction techniques, Megaw [6] and later Malik and Jeffery [7] determined its crystal structure. Both works confirmed the presence of O 2-, OHions, as well as neutral H 2 O molecules, in the monoclinic structure of afwillite. In turn, the structure of afwillite from the Northern Baikal region investigated by Rastsvetaeva et al. [4] was refined in a triclinic unit cell, which is characterized by a smaller c parameter and b angle in comparison to the monoclinic one determined earlier.
As hydrated calcium silicate, afwillite is one of the compounds which can be found in set cement and concrete [8e10]. According to laboratory investigation, afwillite is generally stable in Ca-rich environment and at temperatures below 200 C [3]. It loses water after heating above 300 C and can be transformed into the hightemperature mineral rankinite Ca 3 Si 2 O 7 above 1000 C [8]. Whereas, Tilley [11] suggests that afwillite being a derivative of spurrite Ca 5 (SiO 4 ) 2 (CO 3 ), because of the chemical relationship and close association of these minerals. Afwillite was also synthesized under saturated water pressure in a temperature range of 110-160 C [12].
The present work is especially focused on the characterization of mineral afwillite from Ma'ale Adummim locality (Palestinian Autonomy) by micro-and spectroscopic techniques. The main aim of the research is to determine the cause of the blue colour of afwillite crystals, which is a unique feature for this phase and was noticed for the first time in the natural specimens. Using a wide spectrum of analytical methods allowed to solve the research problem and receive substantive results. Some of them, like Raman spectrum with the detailed description of vibrational bands or the electron absorption and luminescence spectra of afwillite, were described herein for the first time and allowed to complete the spectroscopic study. Moreover, the obtained results presented in this paper for blue afwillite crystals, along with their detailed analyses, were compared and discussed with previously reported data on the colourless counterpart.

Sample preparation
Rock samples with blue afwillite were collected from the outcrop of the pyrometamorphic Hatrurim Complex in Ma'ale Adummim locality (Palestinian Autonomy) during fieldworks in 2015. The abundance amount of afwillite crystals allowed us to perform various investigations. Several dozen of afwillite crystals were extracted from rock samples. A few of them were prepared for single-crystal X-ray diffraction (SC-XRD) analyses, and the rest were embedded in epoxy and polished. The obtained mount with afwillite crystals was used for scanning electron microscopy (SEM) and electron microprobe analyses (EMPA). Crystals of afwillite were also glued to carbon discs and used in Raman spectroscopy investigation. The next portion of crystals was selected and crushed in a mortar. The powdered samples were prepared for infrared (IR), electron absorption and luminescence spectroscopy and for deuteration process. One thin section was extracted from a rock sample to recognize a mineral association.

Scanning electron microscopy (SEM) and electron microprobe analysis (EMPA)
The preliminary chemical composition, and crystal morphology of afwillite, as well as associated minerals, were examined using an analytical scanning electron microscope Phenom XL equipped with an EDS (energy-dispersive X-ray spectroscopy) detector (Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia, Poland).
Quantitative chemical analyses of afwillite were carried out on CAMECA SX100 electron-microprobe apparatus operating in WDS (wavelength dispersive X-ray spectroscopy) mode (Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Poland) at 15 kV and 10 nA, beam size~15 mm. The following lines and standards were used: CaKa, SiKa, MgKa e diopside; NaKa e albite, AlKa, KKa e orthoclase; MnKa e rhodonite; FeKa e Fe 2 O 3 .

Single crystal X-Ray diffraction (SC-XRD)
A single-crystal X-ray diffraction study of afwillite was carried out at ambient condition using a Rigaku OD Gemini-R Ultra diffractometer with MoKa radiation (l ¼ 0.71073 Å) (Institute of Mineralogy and Petrography, University of Innsbruck, Austria). The data reduction was performed with CrysAlisPro (Rigaku 2016). The crystal structure was refined with Jana2006 [13] to an agreement index R 1 ¼ 2.7%, starting from the structural model of the afwillite from type locality e Dutoitspan mine, Kimberly in South Africa, reported by Malik and Jeffery [7]. All of the atoms, except H, were described using anisotropic displacement parameters. Moreover, all H-sites were located by difference Fourier analyses. For refinement of OH bonds and water molecules, distance and angle restraints 1 (0.001) Å, and 109,47 (0.01) were used.
Further details of data collection and crystal structure refinement are reported in Table 1. Atom coordinates (x,y,z), occupancies, and equivalent isotropic displacement parameters (U iso , Å 2 ) (Table S1), as well as, anisotropic displacement parameters (Å 2 ) (Table S2) for blue afwillite crystals are reported in Supplementary Materials.

Confocal Raman (CR) and infrared (IR) spectroscopy
All spectroscopic measurements were performed at room temperature (~21 C) for natural afwillite (unheated) and at tempera-ture~250 C for heated samples. The temperature experiment allowed checking how do high-temperature influences to water molecules and hydroxyl group.
The Raman spectrum of natural (unheated) and heated afwillite made in the same crystal orientation, was recorded on a WITec alpha 300R Confocal Raman Microscope (Institute of Physics, University of Silesia, Poland) equipped with an air-cooled solid laser 532 nm, 30 mW on the sample and a CCD (closed-circuit display) camera operating at -61 C. The laser radiation was coupled to a microscope through a single-mode optical fibre with a diameter of 50 mm. An air Zeiss (LD EC Epiplan-Neofluan DICe100/0.75NA) objective was used. The scattered light was focused on multi-mode fiber (100 mm diameter) and monochromator with a 600 line/mm grating. Raman spectra were accumulated by 20 scans with an integration time of 10 s and a resolution of 3 cm -1 . The monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm À1 ). The baseline correction and cosmic ray removal were conducted using WitecProjectFour software, while the peak fitting analysis was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA).
Attenuated total Reflection-Fourier transform infrared (ATR-FTIR) spectra of unheated and heated afwillite were obtained using Nicolet iS10 Mid FT-IR Spectrometer (Thermo Scientific) fitted with an ATR device with a diamond crystal plate (Institute of Earth Sciences, Faculty of Natural Sciences,University of Silesia, Poland). The sample was placed directly on the diamond crystal prior to data acquisition. Measurement conditions were as follows: spectral range 4000-400 cm -1 , spectral resolution 8 cm -1 , beam splitter Ge/ KBr, detector DLaTGS with dynamic interferometer justification. The OMNIC 9 (Thermo Fisher Scientific Inc.) analytical software was used.
In addition, the FTIR spectra of natural and deuterated afwillite were recorded on a Thermo Scientific Nicolet iS50 FTIR spectrometer equipped with a diamond iS50-ATR accessory (Institute of Chemistry, University of Silesia, Poland) in the frequency range of 4000À400 cm À1 .

Deuteration
The deuterated afwillite was obtained by the heating of afwillite solution in heavy water, performed in a pressure reactor (150 C, 24 h) (Institute of Chemistry, University of Silesia, Poland). The solvent was removed under vacuum at temperature 60 C in a rotary evaporator (Rotavapor R-100, BÜCHI). The degree of deuterium substitution in afwillite sample was ca. 60e70%.

Electron absorption and luminescence spectroscopy
The reflectance absorption spectra of powder samples were measured with the Varian Cary 5E UVeVis-NIR spectrophotometer. Emission spectra were recorded employing FLS980 fluorescence spectrometer from Edinburgh Instrument equipped with a 450 W xenon lamp as an excitation source and a Hamamatsu 928 PMT detector. Emission monochromator was in the Czerny Turner configuration (1800 lines per mm holographic grating blazed at 300 nm, 0.2 nm resolution). Measured spectra were corrected for the sensitivity and wavelength of the experimental setup. To record some luminescence spectra and adequate decay curves a femtosecond laser (Coherent Model ''Libra'') that delivers a train of 89 fs pulses at a wavelength of 800 nm and pulse energy of 1 mJ with a repetition rate-regulated up to 1 kHz was applied as an excitation source. To attain light pulses at different wavelengths (230e2800 nm) the laser is coupled to an optical parametric amplifier (Light Conversion Model OPerA). Luminescence decay curves were recorded with a grating spectrograph (Princeton Instr. Model Acton 2500i) coupled to a streak camera (Hamamatsu Model C5680). All measurements were performed at room temperature (T ¼ 293 K).

Occurrence, physical properties and mineral association
The rock samples with blue afwillite crystals were found and collected in Ma'ale Adummim locality in the Judean Desert, 8 km east of Jerusalem in Palestinian Autonomy. This place is a part of the large pyrometamorphic Hatrurim Complex (or the Mottled Zone) distributed on both sides of the Dead Sea Transform Fault [14e20]. The uniqueness of this Complex is related to new mineral discoveries [21e28]. In these samples composed mainly, by cuspidine, gehlenite and flamite, afwillite occurs together with ettringite, thaumasite, Cr 3þ -bearing ettringite, calcite, minerals of barytehashemite solid solution and undiagnosed Ca-hydro silicate with a pearly lustre (Fig. 1). The phases listed above form lowtemperature hydrothermal assemblages, which filled cavities and fissures, and also cover the crack surface in pyrometamorphic rocks ( Fig. 1).
Afwillite from Ma'ale Adummim specimen is transparent with a vitreous lustre. The light-blue colour of crystals is the most interesting and uncommon feature of this mineral (Fig. 1a,c,e). Hitherto, only transparent and colourless afwillite crystals were described from several localities around the world [1e3]. Afwillite forms characteristic radial-spherical aggregates (3 mm in diameter) which consist of elongated tabular crystals up to 1 mm long (Fig. 1aef). Prismatic ettringite crystals with characteristic and distinct colour zonation are noted on the surfaces of these spheres (Fig. 1c). The difference in colour is connected to the gradual substitution of Al 3þ by Cr 3þ within the structure of this mineral. Spherical-aggregates are formed by intergrowing aggregates of afwillite, ettringite and minerals of the baryte-hashemite series (Fig. 1eef).

Mineral chemistry
Chemical analyses presented in Table 2 indicate that afwillite from Ma'ale Adummim locality is chemically homogenous. The amount of all impurities, like Na, K, Fe, Mg, and Mn was below the detection limit of the microprobe analyses. The empirical formula calculated on 5 cations is as follows: Ca 3.01 (HSi 1.99 The results obtained from the EMPA analyses are in good agreement with the literature data. We observed that the mass % of main constituents (SiO 2 and CaO) are very similar to afwillite chemical composition from the Kimberly mine in South Africa [1], Crestmore in California [2] and Fuka in Japan [3]. At the mentioned localities, the content of the main components ranges within 33.96-34.76 and 48.35e49.28 for SiO 2 and CaO, respectively. Moreover, the minor fluorine content was detected in the chemical composition of afwillite from Crestmore [2], as well as in afwillite from Fuka, in the composition of which, besides fluorine also B impurity is present [3].

Crystal structure of afwillite from Ma'ale Adummim locality
The crystal structure of blue afwillite from Ma'ale Adummim was refined in a monoclinic unit cell with the following parameters The crystal structure shown in Fig. 2 is composed of the double chains of calcium polyhedra, which are connected through the corners and edges with isolated silicon tetrahedra. Structural investigation allows us to recognize three types of seven coordinated calcium polyhedra and two types of silicon tetrahedra. Moreover, O 2-, OHand H 2 O groups were distinguished within the crystal structure. According to bond valence calculation, we identified O2 and O4 atom as OH -, O5-O8 as O 2and O9-O10 as H 2 O groups. Like Megaw [6], and also Malik and Jeffery [7], we determined O1 and O3 as oxygen, not as a hydroxyl group.
The interatomic distances between cation-oxygen are given in Table 3. Most of the Ca-O distances are in the range 2.316(6)-2.556(5) Å, but in Ca2 and Ca3 atoms, there are four significantly longer bonds (two in each) which have changing distances from 2.778(5) Å to 2.871(5) Å, and from 2.704(7) Å to 2.737(5) Å, respectively. The similar remark was noted by Malik and Jeffery [7] in previously reported structural data.
Each silicon atom in afwillite structure is surrounded by three oxygen and one hydroxyl group, which form together tetrahedral arrangement as SiO 3 OH unit. These two types of silicon tetrahedra do not share any corners or edges but are interconnected by hydrogen bonds.
The Si1O 4 tetrahedron shares one edge with Ca1O 7 polyhedron between O7-O8 oxygen atoms, while, the Si2O 4 shares two edges with Ca2 and Ca3 polyhedra between O5-O6 and O2-O5, respectively. The length of shared edges (equivalent of the distance between O-O) between Ca-polyhedron and Si-tetrahedron are in the range from 2.590 Å to 2.648 Å and are in good agreement with the same distances in earlier reported structures [6,7]. The average length of unshared edges in tetrahedron is equal to 2.676 Å for Si1O 4 and 2.685 Å for Si2O 4 . The Si-O distances in afwillite structure vary from 1.600(4) to 1.681(7) Å (Table 3), in turn, the tetrahedral O-Si-O angles are in ranges 106.5e116.3 and 103.4e112.8 for Si1O 4 and Si2O 4 tetrahedron, respectively. Table 1 Parameters for X-ray data collection and crystal-structure refinement for afwillite.

Crystal data
Refined chemical formula sum Ca 3 Si 2 O 10 H 6 Crystal system Monoclinic Space group The evident differences in terms of bond lengths and angles, as well as the acid character of the Si tetrahedra and the presence of long Si-OH distances between Si1-O4 and Si2-O2 equal to 1.681 (7) Å and 1.675(5) Å, causes a considerable distortion of both tetrahedra in the crystal structure of afwillite. The values of tetrahedral mean quadratic elongation (l) and bond angle variance (s 2 ), calculated according to Robinson et al. [29], are l ¼ 1.0033 and s 2 ¼ 14.12 in Si1O 4 , and l ¼ 1.0031 and s 2 ¼ 12.22 in Si2O 4 .
A hydrogen bonding arrangement has a significant contribution to the stability of the afwillite structure. The geometry of hydrogen bonds determined during the structural investigation of blue afwillite crystals, as well as characteristic donor-acceptor distances and angles are summarized in Table 4. According to the d(O-O) distances [30], we may favour four strong hydrogen bonds, in which these distances are in the range 2.5-2.7 Å. They are related for configuration between the hydroxyl group and oxygen (O2eH2/O1, O4eH4/O3) or water molecules and oxygen (O9eH9b$$$O3 and O10eH10b$$$O1). The weak hydrogen bonds with d(O-O) > 2.70 Å are assigned to O9eH9a$$$O10 and O10eH10a$$$O4 configurations.  In Fig. 3 the schematic diagram of hydrogen bonds in afwillite is shown. This bonding network is comparable with that proposed by Malik and Jeffery [7] and confirms also the nature of water molecules. Moreover, it shows the connection between two water molecules and their linkage with questionable O1 and O3 atoms.  (Table 4).
Microprobe analyses also confirm a presence of two hydroxyl groups in the crystal chemical formula, the one per each tetrahedron, and two water molecules. Therefore, chemical composition corresponds to the structural, refined composition. It means that the crystal structure should contain 6 hydrogen atoms and 10 oxygen atoms, consequently two OH group and two H 2 O molecules take place and the rest of oxygen atoms belonging to the Si tetrahedra. Determination of the position and number of hydrogen atoms is very difficult during the structural investigation, therefore based on electron density distribution we can try to confirm their presence. Rasvetsaeva et al. [4] refined the structure of afwillite from the Northern Baikal region in the triclinic unit cell. They specified in the structure Si tetrahedra containing two OH groups, but only half of them. In other tetrahedra terminal oxygen vertices were involved in donor-acceptor bonds with Н 2 О and ОН groups [4]. They proposed the crystal chemical formula with [SiO 2 (OH) 2 ] and SiO 4 including both types of tetrahedra. Our data refined in the monoclinic system are different, therefore, like other researchers, we classify O1 and O3 as O 2and only O2 and O4 as hydroxyl groups.

Raman and infrared spectroscopy data
The structural investigation revealed two types of silicon tetrahedra in the crystal structure of blue afwillite. According to the theoretical predictions, the ideal tetrahedral coordination (SiO 4 ) 4with T d point group symmetry has nine normal vibrations, which are characterized by four fundamental modes: n 1 (A) e symmetric stretching vibration, n 2 (E) e doubly degenerate symmetric bending vibration, n 3 (F 2 ) e triply degenerate asymmetric stretching vibration and n 4 (F 2 ) e also triply degenerate asymmetric bending vibration [31e33]. In this context, all vibrations are active in Raman spectroscopy, whereas only n 3 and n 4 are active in infrared. On the basis of structure refinement of afwillite it was established a presence of (SiO 3 OH) 3unit in its structure instead of ideal (SiO 4 ) 4tetrahedron. In the structure of some minerals belonging to the uranophane and sklodowskite groups, this unit is also observed [31]. As the authors reported, the presence of (SiO 3 OH) 3may be explained by a decreasing symmetry of ideal tetrahedral coordination from T d to C 3v , which implicate the local structural re-arrangement, and as a consequence a splitting of individual bands in the spectrum. Moreover, local structural deformation (mutual rotation of tetrahedral units) probably corresponds     (Fig. 4a). In turn, the nature of the bands in the range 895-970 cm -1 can be related to n 3 asymmetric stretching vibrations. The nature of bands at 786 cm -1 and 1290 cm -1 is interesting, in our interpretation they can be assigned to the Si-OH deformation and stretching modes, but the band positions are strictly determined by the character of hydrogen bonds.
In the Raman spectrum of the heated afwillite (Fig. 4b), visible changes are observed in the stretching vibration region in the range 800-960 cm -1 . The bands connected with the Si-O symmetric and asymmetric stretching vibration of (SiO 4 ) 4group are still present in the spectrum at 818 and 960 cm -1 , respectively. The positions of these bands are shifted to the lower frequencies in comparison to their positions in the unheated spectrum (Fig. 4a). In turn, the intensity of the bands related to the stretching vibration of (SiO 3 OH) 3unit has been reduced and its position was also shifted to the lower wavenumbers, in range 835-885 cm -1 . Similarly, the bands at 930 cm -1 and 786 cm -1 observed in the unheated spectrum disappeared after heating. It can suggest a weakening of the hydrogen bonds due to the thermal effects.
The low-frequency range in the Raman spectrum of the unheated sample (Fig. 4a) is determined by the presence of many overlapping bands with relatively low intensity. There, the bands in the ranges 390-420 cm -1 and 480-530 cm -1 , are attributed to the n 2 and n 4 bending modes of Si-O-Si in (SiO 4 ) 4groups (Fig. 4a). The formers may overlap also with the vibration of the Ca-O in octahedral coordination. Other bands observed at 463 and 625 cm -1 can be associated with the vibrations in (SiO 3 OH) 3unit. It is worth to notice that the assignation of the latter one may be questionable and is also related to the bending Si-OH out-of-plane mode [31]. Raman bands below 320 cm -1 are usually ascribed to the lattice vibrations. The spectral region related to the bending vibration also have been regrouped in the Raman spectrum of the heated sample (Fig. 4b), and here only symmetric and asymmetric bending vibrations of (SiO 4 ) 4group are noted in the 375e417 and~470-530 cm -1 ranges. Identification of bands in-between the 1500 and 3200 cm -1 is problematic in both spectra, but one can assume that their origin can be a result of a fluorescence effect, conditional by a presence of structural defects and the atypical protonated silica tetrahedral units. The bands related to the O-H stretching vibrations in H 2 O were observed in the range 3300-3450 cm -1 for unheated afwillite (Fig. 4a). For heated afwillite the band shift is also noted, the broad band related to the OH stretching vibration is centred at~3420 cm -1 (Fig. 4b). The presence of the weak and broad band may confirm the hypothesis on reorganization of (SiO 3 OH) 3unit and weakening of hydrogen bonds stabilizing the afwillite structure.
The changes observed in the Raman spectra of natural and heated afwillite sample suggest that the presence or absence of some bands may be associated mostly with the vibration of the (SiO 3 OH) 3unit. As a result of the heating process, the presence of the proton of such group which in the unheated spectrum probably forms a hydrogen bond system yield to distract due to the temperature and in consequences provide to the disappearance of some bands. The specific band arrangement in the unheated spectrum, especially due to the marker bands at 463, 625, 786, in between 870-930 and 1290 cm -1 provide a conclusion about the existence of protonated silica tetrahedral units defined as (SiO 3 OH) 3because of the vanishing due to the heating, of the band pattern associated with the vibration within this group.
Afwillite belongs to the orthosilicates (nesosilicates), which contain isolated SiO 4 tetrahedra with T d symmetry in the structure [34]. Handke and Urban [35] performed Raman investigation for some orthosilicates, in which Raman active symmetric and asymmetric stretching vibrations are in the ranges 820-865 cm -1 for n 1 and 880-978 cm -1 for n 3 , respectively. The symmetric stretching vibrations of the Si-O bond in tetrahedra were also observed by many authors, who assigned its to the following positions in the Raman spectra: 850 cm -1 [36], 861 cm -1 [37], and the 800-950 cm -1 range [33]. Griffith [38], using IR and Raman spectroscopy, analysed a dozen orthosilicates belonging to the different mineral groups and showed that the position of both symmetric vibrations is changeful. For example, n 1 for forsterite is at 823 cm -1 , whereas for phenakite at 877 cm -1 . Our results on the stretching vibrations of (SiO 4 ) 4group in afwillite are in good agreement with previously reported data.
Symmetric and asymmetric bending vibrations in orthosilicates according to Handke and Urban [35] are in the~350-440 cm -1 and 490-610 cm -1 ranges, respectively. Griffith [38], as in the case of stretching vibrations, stated that the position of bending modes is distinct and is related to the mineral group. For example, n 4 in forsterite is at 602 cm -1 , whereas in phenakite -at 444 cm -1 . The literature data confirmed the correctness of afwillite spectroscopic results of bending vibration assignation.
A tetrahedron consisting of silica surrounding by three oxygens and one hydroxyl group e (SiO 3 OH) 3-, is recognized not only in afwillite structure. It was reported in the structures of some uranyl silicates like uranophane, sklodowskite, boltwoodite [31], bultfonteinite [39], olmiite [40] and poldervaartite [41]. During detailed spectroscopic investigations of uranyl minerals Frost et al. [31] assigned Raman bands in the 900e1150 and 390-570 cm -1 ranges to the (SiO 4 ) 4stretching and bending vibrations, respectively. There are no Raman spectroscopic data on bultfonteinite. In the Raman spectra of olmiite and poldervaartite Si-O symmetric and asymmetric stretching vibrations of (SiO 3 OH) 3unit were attributed to bands at~852 cm -1 and~900-955 cm -1 , respectively [42,43]. In our work bands generated by (SiO 3 OH) 3unit are shifted to the higher wavenumbers. It can be connected by the presence of two distinct tetrahedra in afwillite structure. In this case, bond length, oxygen position and type of connection causing between tetrahedra and polyhedra case resulting in high distortion, which is confirmed by structural analyses. In studied minerals Frost et al. [31] assigned bands as vibration associated with Si-OH deformation modes in the spectral region~790-810 cm -1 . In unheated afwillite (Fig. 4a) band related to these vibrations at 786 cm -1 is very close to this range, which is in keeping with data reported before.
At high wavenumber region situation is more complicated because in the afwillite spectrum we observe only the broad bands placed below~3400 cm -1 . In olmiite and poldervaartite several bands assigned to the OH stretching vibrations of the (SiO 3 OH) were reported [42,43]. We suggest that in afwillite this type of vibration is absent because OHion is usually indicated as a sharp band between 3450 and 3700 cm -1 . For example, in zeolites bands associated with SiOH stretching vibrations are placed at wavenumbers >3600 cm -1 [31]. In Raman spectra of the uranophane group minerals bands in the range 3400-3600 cm -1 are assigned to OH stretching vibrations of water molecules. Bands assignation in this spectral region in the Raman spectrum of afwillite is in accordance with the published data.
In the infrared spectrum of unheated afwillite shown in Fig. 5b, strong bands are observed at 860 cm -1 and in the range 909-960 cm -1 . These bands are attributed to the Si-O symmetric and asymmetric stretching vibrations of (SiO 3 OH) 3unit. Other infrared bands noted at 775 cm -1 and 810 cm -1 , according to the Raman band assignation, can be related to hydroxyl deformation modes. Symmetric and asymmetric bending vibrations connected with SiO 3 OH tetrahedra are presented in the infrared spectrum as bands at~470 cm -1 and~630 cm -1 , respectively. The two bands between 1280-1325 cm -1 are related to the Si-O vibrations, while the band at 1097 cm -1 is associated with the Si-O-Si asymmetric stretching vibrational mode. The infrared spectrum over the 2300 cm -1 frequencies contains two very broad absorption bands centred at approximately 2352 and 2719 cm -1 and two less broad bands at 3130 and 3329 cm -1 . All these bands are attributed to the y OH stretching vibrations of OH and H 2 O groups. The frequencies of the y OH vibrations depend on the strength of the hydrogen bond. Strong hydrogen bonds, which corresponding to the short O-O distances, are usually related to broad infrared bands in the spectral region 2000-3000 cm -1 , whereas weak H-bonds are associated with sharper bands between 3000-3500 cm -1 [44]. There are two strong hydrogen bonds between the hydroxyl group and oxygen (O2eH2/O1, O4eH4/O3) in afwillite structure, then it is highly probable, that the bands at 2352 and 2719 cm -1 are connected with these strong bonds. In turn, the bands at 3130 and 3329 cm -1 can be related to the weak hydrogen bonds forming in the following configurations O10eH10a$$$O4 and O9eH9a$$$O10. Summarise, the bands noted <3000 cm -1 in the infrared spectrum of afwillite are attributed to the y OH vibrations of hydroxyl group linked to the Si atom in tetrahedra. In turn, the bands above this frequency are related to the y OH vibrations of H 2 O. In this case, a broad band at 1685 cm -1 is connected with the water d H-O-H bending modes.
Changes in the spectrum of the heated sample are observed in the intensities of bands (Fig. 5). Comparing spectra b and c in Fig. 5 we noted that the spectra below 1325 cm -1 look similar and only band at 1097 cm -1 has been strongly reduced. The greater changes have been observed above 1500 cm -1 . As a result of the heating process hydrogen bonds system has been disturbed and almost all infrared bands disappeared above this frequency. Only one band at 3330 cm -1 related to OH vibrations in water can be discerned.
There are a few examples of infrared spectrum of colourless afwillite from different localities around the world in literature: Yoko-Dovyren massif, Buryatia Republic, Transbaikal Territory, Siberia, Russia [4], Crestmore quarry, north of Riverside, Riverside Co., California, USA [45], and Lakargi Mt., Upper-Chegem caldera, Northern Caucasus, Republic of Kabardino-Balkaria, Russia [45]. Despite similarities in the bands' frequencies, only infrared spectrum from the first locality was described so far. For comparison, an adequate spectrum was added to Fig. 5 (a). The assignation of the Si-O stretching vibrations related to the isolated SiO 4 tetrahedra by Rastsvetaeva et al. [4] is comparable with our description. Other bands were assigned in a different way and were correlated with the presence of the hydronium cation H 3 O þ solvated by one water molecule and formed during this process the Zundel complex H 5 The infrared spectrum of afwillite sample from Dutoitspan Mine, Kimberley, South Africa [44] has been examined in the region 2000-3500 cm -1 to identify of OH stretching vibrations connected with a hydroxyl group and water molecules. Their results are comparable with our prediction and confirmed that the stability of afwillite structure depends on hydrogen bonding system. Moreover, published data suggest that in minerals where Si with three apical O atoms is present, the Si-O-H group with covalent O-H bond can be formed and in the infrared spectrum several bands in region 1300-3200 cm -1 appear [31]. The identical feature is noted in the afwillite IR spectrum. Furthermore, the similar frequencies of bands associated with (SiO 3 OH) 3unit in afwillite and other minerals like olmiite [42] and poldervaartite [43] and correlation with Raman band assignation, presented herein for the first time, can provide the correct interpretation of spectroscopic data.
Afwillite sample was also investigated using the hydrogendeuterium exchange of the hydrogen protons by IR spectroscopy technique. FTIR measurements included the natural and deuterated afwillite samples. The absorption area, occurring in the spectral range between 1300 to 400 cm -1 in IR spectra of both samples, was omitted for the interpretation due to their similarities, as shown in Fig. 6b. Thus, we focused on the detailed analysis of the bending (Fig. 6a) and the stretching (Fig. 6b) . 6a). Two bands at 1435 cm -1 and 1642 cm -1 in IR spectrum of natural afwillite (black line) were in a good agreement with the bending modes in IR spectrum of unheated afwillite from Ma'ale Adummim (Fig. 6b), which was represented by the wide band centred at 1685 cm -1 . The identification of these two characteristic bands may be associated with measurements conditions or sample quality. The band at 1325 cm -1 , as was described earlier, is connected with the Si-O vibrations.
In the IR spectrum of the deuterated sample (red line), the band at 1640 cm -1 was almost completely reduced, whereas the peak associated with the O-D bending vibration appeared at lower frequencies at 1437 cm -1 with a simultaneous increase of band intensity. In the case of the IR spectrum of natural afwillite (black line in Fig. 6b), a broad band centred at 3366 cm -1 was assigned to the O-H stretching vibrations of H 2 O groups. The assignment of the vibration frequencies at 2650 cm -1 was questionable. It could be correlated with the band at~2720 cm -1 in Fig. 5b

The electron absorption and luminescence spectra
The absorption spectra of the natural and heated afwillite sample measured at room temperature are shown in Fig. 7. In the Fig. 6. FTIR spectra of bending (a) and stretching (b) vibration regions in natural and deuterated afwillite. ultraviolet range, two absorption bands at 259 nm (4.78 eV) and 369 nm (3.35 eV) were measured. The half-width of the first band is 3750 cm -1 , while the second is a little bit less means 3281 cm -1 . In far UV the absorption edge at 221 nm (5.60 eV) was recorded. For the heated sample, we observe the change of maximum band position. It can be noticed, however, that from the longer waves of the 369 nm band pronounced shoulder at 430 nm (2.86 eV) appeared. In visible region weak and broad absorption band within 580-780 nm (2.13 -1.58 eV) was measured. The maximum of this band, as well as maximum of transmission, shifted to the longer wavelength for a heated sample. As a result, the original blue-green colour of the sample changed to green. Of course, in pastel shades, i.e. with little colour saturation.
The absorption bands peaked at 259 nm and 369 nm are associated with the hole oxygen defect, most probably SiO 3 - [46,47]. This hole defects can be associated with a certain number of Cavacancies, or Ca 2þ shifted to other lattice sites. These vacancies or defects can form clusters, as in some fluorite CaF 2 species. There is a known case of M-centres, i.e. two conjugated F-centres which changed their size under the effect of heating and irradiation and caused the change the fluorite colour. It was proposed [48] than in Ca clusters with 1 nm in size, the absorption band falls within the range of 2.3 eV, i.e. 536 nm. It was recognized as the band from the centre of M. For this reason, some changes in the size of the cluster could take some Si-O or Si-OH bonds may have changed, most likely these are shortened. Consequently, a certain modification of the energy levels of the absorption centres occurs, i.e. an additional band at 430 nm could appear. Absorption in the range 580-780 nm does not originate from electron levels, because no luminescence was measured at the energy of excitation corresponding to these wavelengths. It is caused by light scattering on the clusters of these defects. As a result of the heating, the size of these clusters may change. Therefore, it is possible to recognize this absorption in the 600e800 nm range place and some, ineffective changes in absorption could follow them.
Under continuous excitation at 259 nm and 369 nm, the emission band at 468 nm (2.65 eV) was measured. The luminescence intensity for l exc ¼ 369 nm and was three times weaker than for l exc ¼ 259 nm (Fig. 8a). For the heated sample, the maximum of emission was shifted to longer wavelengths in relation to the position of the unheated sample (Fig. 8b). The difference in energy is  rather significant i.e., 0.327 eV. Now the intensity of luminescence for l exc ¼ 369 nm is 7 times weaker than for excitation at 259 nm ( Fig. 8c). During ultra-short laser excitation, the luminescence spectrum has been measured for the natural sample only for l exc ¼ 369 nm, but no luminescence was obtained for l exc ¼ 259 nm. Similar, for the heated sample the luminescence spectra with the femtosecond excitation have been measured only for excitation at 369 nm and 430 nm (Fig. 8d). The intensity of luminescence for excitation at 430 nm was 8 times lower than for excitation at 369 nm. It is related to both types of defect. The luminescence decay time was measured only under ultra-short laser excitation for l exc ¼ 369 nm and it was equal to 2.8 ns.

Conclusions/summary
Based on the obtained results the following conclusions can be listed: 1. The blue colour of afwillite crystals from Ma'ale Adummim locality came from the hole oxygen defect, most probably SiO 3 -, which can be associated also with a certain number of Cavacancies, Ca or their shifting to other lattice sites. As a result of the heating experiment, the blue colour of afwillite crystals transforms into pastel green at 250 C.
2. The SC-XRD investigations confirm a presence of the (SiO 3 OH) 3units linked among themselves by the hydrogen bond system, which is responsible for structural stability. The presence of (SiO 3 OH) 3unit can cause a decreasing symmetry of ideal tetrahedral coordination from T d to C 3v , which implicate the local structural re-arrangement. 3. For the first time, the assignment and interpretation of Raman bands were performed. The Raman spectrum for natural afwillite revealed a great number of bands in the range 780 and 970 cm -1 , which can confirm previously assumption about structural deformation enforced by protonation of the ideal silica tetrahedron. Another hypothesis explaining observed effects is the presence of two different kinds of structural units: (SiO 3 OH) 3and its deprotonated counterpart (SiO 4 ) 4-. 4. The changes observed in the heated Raman spectrum are mainly connected with bands attributed to the (SiO 3 OH) 3unit. The bands were reduced and shifted to the lower wavenumbers. Some of them disappeared as a result of distracted hydrogen bonds related to the protonated silica tetrahedron. 5. The interpretation of bands in the infrared and Raman spectra is comparable with respect to bands associated with the Si-O stretching and bending vibration of (SiO 3 OH) tetrahedra. We concluded, that the absorption bands over 2300 cm -1 corresponding to the stretching vibrations (y OH) of the OH or H 2 O groups, depends on the strength of the hydrogen bond in the crystal structure.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.