Natural Amethyst Research Articles

Broadband EPR Spectroscopy of the Ground Electron State of the Fe4+ Impurity Ion in Amethyst

Fine structure of ground electron state of Fe4+ impurity ion in a natural amethyst crystal was studied by broadband electron paramagnetic resonance spectroscopy in the frequency range of 34–500 GHz. It is established that energy levels scheme consists of ground quasi-doublet Sz = ± 2, quasi-doublet S z= ± 1 and singlet Sz=0 with zero-field energies ± 4.9 GHz, 435.2 ± 45.4 GHz and 584 GHz, respectively. Parameters of effective spin Hamiltonian describing dependences of electron spin levels on magnetic field are determined.

Characterisation of the Fe(III):H+ Defect Centre in Natural Amethyst

A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre. In this centre, Fe(III) replaces Si(IV) in the tetrahedral site, whereas H+ is coupled to Fe(III) to maintain the charge balance. The spectroscopic investigations, mainly the interpretation of the Electron Spin Echo Envelope Modulation, allowed a detailed localisation of the proton to be obtained. H+ occurs in the channels crossing the crystal parallel to the crystallographic c axis, in a largely eccentric position. The Fe(III)-H+ distance, evaluated in 2.70 Å, is found associated with a non-negligible isotropic hyperfine coupling, which can be linked to the relative stability of the S2 centre in natural amethyst.

Determination of dosimetric properties of MgO doped natural amethyst samples

In this paper, the thermoluminescence (TL) dosimetric characteristics of MgO doped natural amethyst samples (Mg-NA) are presented. The morphologies and chemical structures of the powder form samples were identified using XRD, FTIR, SEM, SEM mapping and EDX. Comparison of the TL intensities showed that 10wt% Mg-NA was nearly 150 times more sensitive than undoped amethyst and the main dosimetric properties proved that 10wt% Mg-NA may be a promising phosphor for clinical and radiotherapy purposes.

Thermoluminescence properties of non-stoichiometric Li2Si2O5 synthesized from natural amethyst quartz

In the present study, on the contrary of lots of researches, stoichiometric (s-LD) and non-stoichiometric (n-LD) lithium disilicate polycrystalline were tried to synthesize using easily available and cheap natural amethyst quartz samples collected from Balıkesir Dursunbey Turkey as a starting material by the high temperature solid state method. Structure and morphology of s-LD and n-LD were done using XRD, FTIR and SEM, showing the main structure as lithium disilicate for n-LD. The findings showed that n-LD had better crystal structure and gave better TL response, so a detailed investigation on its main dosimetric properties was studied and presented in this paper. The thermoluminescence (TL) glow curve of n-LD consisted of 3 distinguishable peaks and the main peak was stable since TL sensitivity increased only 12% after 2kGy of high dose exposure. The reusability results showed that TL intensity decreased maximum 21% and 16% for 5Gy and 50Gy, respectively after 11 repeated cycles and n-LD had a superlinearity index (g(D)=1.019). The main peak was just 5% faded in 4 weeks. Consequently, n-LD as a phosphor might be used in many fields concerning with the dose range from 0.1Gy up to 100Gy.

Study of thermoluminescence response of purple to violet amethyst quartz from Balikesir, Turkey

In thermoluminescence (TL) dosimetry, the phosphor amethyst quartz as a thermoluminescent, appears to be one of the materials arousing the highest interest. In this study the dosimetric characteristics of natural amethyst quartz crystals collected from Balikesir–Dursunbey (Turkey) were investigated for the purpose of determination of the general properties that phosphors should have in order to be useful for thermoluminescence dosimetry. The natural thermoluminescence was drained by annealing the powder samples at 450°C for 1.5h. The effects of high temperature annealing, dose response curves, glow curves after a postirradiation annealing, reusability of the samples and storage of trapped electrons in dark at room temperature were clarified through irradiating the samples with the desired exposures by 90Sr/90Y beta particles.Isothermal annealing before and after irradiation was found to have a definite effect upon the TL glow curve of amethyst crystal powder. The same sample varied in sensitivity depending upon its previous thermal and radiation history. The peak heights of the glow peaks were examined with respect to dose response at dose levels between 1Gy and 5kGy. The intermediate temperature (IT) and high temperature (HT) peaks of 230°C and 300°C, respectively, exhibit dose–response curves as superlinear when dose is on the logarithmic scale except the dose response of 300°C peak for the dose values of 1<D<20Gy in which linear dose response was acquired. At the end of the storage time between exposure and readout which was about one month at room temperature, the emitted light reduction was 14% comparing to the initial state. Repeating the measurements of the same sample, exposed with 0.1, 0.5, 0.8 and 1kGy beta exposures, resulted in between 4% and 11% increase in the TL sensitivity of the material.

Spectroscopic study of natural quartz samples

In this work we performed a spectroscopic characterization of natural amethyst, citrine, and prasiolite samples from different localities. These materials were examined by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy (UV–vis), electron paramagnetic resonance (EPR), and inductively coupled plasma atomic emission spectrometry (ICP-AES). Samples were used in this study in as-received, gamma-irradiated, UV-irradiated, and heat-treated conditions. We observed the changes in the FTIR, UV–vis, and EPR spectra of these samples as a function of the condition they were analyzed. We noticed that gamma radiation had a great effect on the color of amethyst and citrine samples used in this work. It was observed that light colored samples showed a deepening of their colors upon gamma-irradiation and a bleaching upon heat treatment at 450°C. However, we observed that gamma radiation had a slight effect on the color of citrine. UV-irradiations revealed that the coloration of both amethyst and prasiolite can be bleached by UV radiation. On the other hand, the color of citrine was not affected by UV radiation.

Photo-induced dipole relaxation current in natural Amethyst

Thermally stimulated depolarization current (TSDC) measurements were carried out for SiO2 in the amethyst form, aiming to investigate the relationship of observed current with relaxation phenomena related to quartz impurities. In addition to TSDC conventional dark procedure, photo-induced TSDC was also carried out, where the exciting light came from an Ar+ laser, tuned either at 488 nm or at 541 nm. X-ray diffraction and optical absorption measurements were used as complement for the interpretation of TSDC data. Optical absorption data, mainly in the range 400-700 nm, allow identifying the characteristic bands of amethyst as well as to relate them with TSDC and photo-induced TSDC data, leading to a relationship between absorption bands and light irradiation with selected wavelengths. These results allow determining how the formation of a TSDC band in the range 220-260 K, is affected by the light absorption, modifying the formation and the dipole orientation distribution in the samples. Results also help the verification of defects formed by Fe3+ or Fe4+ ions in the amethyst structure, as well as suggest that these defects, besides the participation in the amethyst structure as color centers, also play a role in the formation of TSDC bands, contributing for the observed effect of monochromatic light irradiation on these bands.

Thermodynamic and magnetic properties of surface Fe3+ species on quartz: effects of gamma-ray irradiation and implications for aerosol–radiation interactions

Samples of a natural amethyst, pulverized in air, and irradiated for gamma-ray doses from 0.14 to 70 kGy, have been investigated by powder electron paramagnetic resonance (EPR) spectroscopy from 90 to 294 K. The powder EPR spectra show that the surface Fe3+ species on the gamma-ray-irradiated quartz differ from its counterpart without irradiation in both the effective g value and the observed line shape, suggesting marked radiation effects. This suggestion is supported by quantitatively determined thermodynamic properties, magnetic susceptibility, relaxation times, and geometrical radius. In particular, the surface Fe3+ species on gamma-ray-irradiated quartz has larger Gibbs and activation energies than its non-irradiated counterpart, suggesting radiation-induced chemical reactions. The shorter phase-memory time (T m) but longer spin–lattice relaxation time (T 1) of the surface Fe3+ species on the gamma-ray-irradiated quartz than that without irradiation indicate stronger dipolar interactions in the former. Moreover, the calculated geometrical radius of the surface Fe3+ species on the gamma-ray-irradiated quartz is three orders of magnitude larger than that of its counterpart on the as-is sample. These results provide new insights into radiation-induced aerosol nucleation, with relevance to atmospheric cloud formation and global climate changes.

Infrared Spectroscopy of Natural vs. Synthetic Amethyst: An Update

When microscopic identification is not feasible, FTIR spectra at high resolution (0.5 cm-1) can distinguish natural and synthetic amethyst. The 3595 cm-1 band is characteristic of natural amethyst and has a full width at half maximum (FWHM) of 3.3 ± 0.6 cm-1. In synthetic amethyst, this band is either absent or (very rarely) about twice as broad. Exceptions to this criterion include natural amethyst with pronounced near-colorless zones, which do not always display this band, and natural specimens that either have an intense, unusually broad 3595 cm-1 band or show total absorption in the X-OH region (3800-3000 cm-1) of the spectrum.

Electron paramagnetic resonance spectroscopy of Fe3+ ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe3+ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E1′. However, the two broad signals do not correspond to any known Fe3+ centers in the quartz lattice, but are most likely attributable to Fe3+ clusters on surfaces. The absolute numbers of spins of the Fe3+ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy Ea, entropy of activation ΔS and enthalpy of activation ΔH for the Fe3+ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe3+ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.

A Fe K-edge XAS study of amethyst

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of α-quartz, SiO2) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe2+ is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell: = 1.78(2) A, the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe3+ variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe4+ is stabilised by the favourable local structural arrangement.

AN UPDATE IN THE SEPARATION OF NATURAL FROM SYNTHETIC AMETHYSTS

Since the first synthetic amethyst was commercially manufactured about 30 years ago, the separation from natural material has been difficult for gemologists. Even today, the separation of natural from hydrothermally grown in K2C03 synthetic amethyst is still an issue. With only the help of classical gemological criteria (such as inclusions, twinning and color zoning) it is difficult to separate synthetic stones from their natural counterparts. The separation problem is even more complex in the case of the highest quality (and value) of amethysts presenting neither inclusion nor twinning nor color zoning. IR absorption spectra of amethyst in the region of the ΧΟΗ group stretching (particularly from 3000 to 3900 cm'1) reveal several bands that have been used for the separation of natural from synthetic amethyst. Using a resolution at 0.5 cm the 3595 cm1 band is present in all natural amethyst and in some rare synthetic ones. When present in synthetic amethysts, its full width at half maximum (FWHM) is about 7 cm , whereas it is about 3.3 cm' in all naturals. This new criterion worked for all of our samples.

Appearance of basal faces in natural amethyst crystals from Four Peaks, Arizona

The surface microtopography of rough basal faces and internal inhomogeneities of amethyst crystals from Four Peaks, Arizona were investigated by means of optical, scanning electron microscopes and cathodoluminescence tomography. Rough interface overgrowth was shown to have taken place along the c-axis on mechanically broken surfaces and the present rough basal surfaces were demonstrated to have formed because of later heavy dissolution.

Distinguishing natural from synthetic amethyst: the presence and shape of the 3595 cm−1 peak

The infrared absorption spectrum of amethyst in the region of stretching vibrations of X–OH groups reveals several bands that have been used for the separation of natural from synthetic amethyst. The intensity and shape of these bands have been measured as a function of crystallographic orientation. Using a resolution of 0.5 cm−1 the 3595 cm−1 band is present in all infrared spectra of natural amethyst and in some rare synthetic ones. If present in synthetic amethyst, its full width at half maximum (FWHM) is about 7 cm−1 whereas it is about 3 cm−1 in all natural samples. This new criterion, unlike the previous ones, seems appropriate to separate natural from synthetic amethyst in all cases.

The 3543 cm&lt;sup&gt;-1&lt;/sup&gt; Infrared Absorption Band in Natural and Synthetic Amethyst and Its Value In Identification

The 3543 cm&lt;sup&gt;-1&lt;/sup&gt; Infrared Absorption Band in Natural and Synthetic Amethyst and Its Value In Identification

Natural amethyst from the Caxarai Mine, Brazil, with a spectrum containing an absorption peak at 3543 cm−1

Natural amethyst from the Caxarai Mine, Brazil, with a spectrum containing an absorption peak at 3543 cm⁻¹

Electron paramagnetic resonance and thermally stimulated depolarisation currents in natural brazilian amethyst

EPR measurements have been performed in powder and crystal samples of natural Brazilian amethyst for two magnetic field directions. The dominant features of the EPR spectrum are provided by iron centres. Thermal treatments performed in powder samples show that the reduction of the spectrum is larger for increasing temperatures and for longer time intervals. The thermally treated samples were X and γ-irradiated and UV-illuminated. The spectra of the samples bleached by heating could not be recovered. TSDC measurements in amethyst crystals show four bands of dipole origin. The dipoles were attributed to Fe3+ in a Si4+ site adjacent to an alkali-ion. The difference in the temperature positions of the TSDC peaks is originated from the unequal distribution of the Fe3+ among the three equivalent sites.

X-ray topography study of Brazil twin boundaries in natural amethyst

X-ray topography study of Brazil twin boundaries in natural amethyst

Structure of Brazil twin boundaries in amethyst showing brewster fringes

Optical investigations of Brewster fringes in natural amethyst crystals have indicated that the fringes are Brazil twin boundaries consisting of McLaren and Pitkethly type zig-zag structures and closed parallelograms. The two types of Brazil twin boundaries appear alternately in the successive fringes and in different orientations in one fringe. The Brewster fringes develop by a process of incorporation of many Brazil twin lamellae, when growth conditions are stabilized after this process. If perturbation occurs in growth conditions, distinct growth banding with and without Brazil twin lamellae will develop, and not Brewster fringes.

Brazil twinning in natural and synthetic amethyst crystals

The structures of Brazil twin boundaries, the micro-textures due to Brazil twinning, and the sites and the conditions for the generation of Brazil twins have been investigated on natural amethyst crystals with and without Brewster fringes, synthetic amethyst crystals hydrothermally grown under various conditions, and natural milky quartz crystals of epithermal origin. Their generation is triggered by the precipitation of solid inclusions on the growing smooth surfaces due to a perturbation of growth parameters. Brewster fringes appear through a process of incorporation of a large number of Brazil twin lamellae, whereas fibrous or other micro-textures appear when there is no such process. The former is assumed to have been formed under more stable conditions than the latter.