The twinning microstructure and growth of amethyst quartz

The characteristic lamellar-twinning of the right-handed (R) and left-handed (L) structures in the major rhombohedral growth sectors of amethyst quartz has been studied by optical techniques, X-ray topography and transmission electron microscopy (TEM). The TEM observations show that the region of each Brewster fringe consists of fine-scale Brazil twin lamellae parallel to one of the r, z{10 \(\overline {\text{1}} \) 1} planes, and structural considerations suggest that it is one of the r-planes. The twin boundary corresponding to a Brewster fringe has the form of a zig-zag structure consisting of Brazil twin boundaries on two r{10 \(\overline {\text{1}} \) 1} planes, with one predominating. The Brewster fringes appear black between crossed polarizers because light travelling along the optic axis [001] passes through almost equal distances of R and L quartz, giving essentially zero optical rotation. From the visibility of the Brazil twin boundaries in electron micrographs and the visibility of the Brewster fringes in X-ray topographs, the fault vector R and the corresponding composition plane of the major Brazil twin associated with each Brewster fringe has been determined. The streaking of the Brewster fringes observed optically and in the X-ray topographs appears to be due to the stair-rod dislocations at the intersections of the Brazil twin boundaries. Experiments in which synthetic quartz was grown hydrothermally on untwinned seeds and on twinned amethyst seeds showed that the initiation of Brazil twins and the development of Brewster fringes was dependent upon the presence of iron in the growth solution.     

Deformation and transformation in experimentally shock-loaded quartz

Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0 \(\bar h\) l, 0k \(\bar k\) l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01 \(\bar 1\) 2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01 \(\bar 1\) 2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.     

An X-ray photoelectron spectroscopic study of shock-loaded quartz

X-ray photoelectron spectra (XPS) for the Si 2p and O 1s signals of quartz recovered after shock-loading at pressures up to 55 GPa revealed the presence of stishovite in the pressure region between 10 and 34 GPa. The stishovite binding energy for both the Si 2p and O 1s is found to be independent of the shock stress level from which it is recovered. Moreover, the binding energy values obtained from 0.5 mm thick samples shocked in the laboratory for times of ca. 1 μs are equal, within experimental uncertainty, to stishovite produced by the Ries impact event. Variations of binding energies observed for the other phases (residual quartz and glass formed simultaneously with or by decomposition of stishovite) are discussed in the framework of previous results obtained by other methods such as X-ray diffraction and infrared spectroscopy. Although unequivocal interpretation of the variation in binding energy with exposure to different shock pressure is not always possible, the XPS method proves to be very well suited for recognition of high pressure phases and for distinction of pressure regions dominated by various shock or post-shock events.     

Thermoluminescence spectra of igneous quartz and hydrothermal vein quartz

Variations in thermoluminescence spectra are reported for four types of geological quartz examined with a new spectrometer featuring dual imaging photon detectors that separately and simultaneously detect (1) uv-blue (200–450 nm) and (2) blue to near infrared (400–800 nm) emission. Samples show striking differences which appear to be characteristic of their geological origin. Volcanic quartz phenocrysts from acid volcanics show red thermoluminescence (TL) emission bands centered at 620–630 nm that are 100 times more intense than similar bands in other quartz, while a violet emission at 420–435 nm was observed exclusively in igneous quartz (volcanic and granitic). A broad emission band centered at 560–580 nm was observed only in quartz formed hydrothermally. Massive quartz from Li-rich pegmatite bodies shows narrow, intense 470 nm emission bands at 230° C apparently related to Al and to Ge defects detected with electron paramagnetic resonance (EPR), and emission bands at 330 and 280 nm, possibly related to recombination at oxygen vacancies. The common 380 nm emission band of quartz was observed in both volcanic and granitic quartz, but was not detected in either the pegmatitic or the hydrothermal vein quartz. Observed spectral variation is identified as a potential source of error in luminescence dating.     

Deformation of experimentally shock-loaded quartz powders: X-ray line broadening studies

Quartz powders (mean grain size: 22 μm) were pressed into sample discs of different green densities (ρ₀=1.65; 1.85 and 2.05 g/cm³) and subjected to shock pressure between 1.5 and 17.0 GPa. Peak shock pressures were determined by the impedance method using the Hugoniot curves of steel and quartz powders. Fourier techniques were used to analyse the line broadening of 5 X-ray reflections in each case. The Fourier coefficients were subjected to a Rothman-Cohen correction before further processing. The coherent domain size decreases abruptly from several thousand Ångström in the unshocked state to about 800 Å at 1.5 GPa, and reaches a constant mean value of about 200 to 300 Å at dynamic pressures of about 4 GPa. At very high dynamic pressures (?15 GPa) there is further fragmentation to very small domain size. There is no systematic correlation between sample density and coherent domain size produced by the shock event. A more or less linear dependency exists between microstrain and pressure for each starting density of the quartz powder. The observation that microstrain decreases with increasing starting density could be explained by the fact that increasingly larger portions of the input energy are consumed to create fresh surfaces by comminution. Consequently, less energy will be available for strain strengthening. The stored energies in the shocked quartz powders are of the same order as the surface energy. Shock-treated quartz should therefore be suitable for accelerating any activated process such as sintering.     

Correlation of Fe4+ optical anisotropy,Brazil twinning and channels in the basal plane of amethyst quartz

The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe⁴⁺ and a peak at 3.02 eV—which is the A₃ band related to the [AlO₄]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σ_max/σ_min, of the Fe⁴⁺ band is that of the A₂ absorption band in smoky quartz as would be expected if Fe³⁺ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A₁ and A₂ absorption bands, the A₃ absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO₄]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe³⁺ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation.     

Thermoluminescence and hydroxyl defects associated with broad-band infra red absorption in synthetic quartz

A study has been made of thermoluminescence from synthetic quartz with varying hydroxyl impurity concentrations up to approximately 300 H/10⁶ Si which are associated with a “broad-band” IR absorption in the range 2600–3700 cm⁻¹. These hydroxyl defects are known to be important in the hydrolytic weakening of quartz. We have found only minor differences in the glow curves of unheated crystals but significant intensity increases when “wet” crystals are heated sufficiently to cause bubble formation. It would seem that the electron traps are unaffected by the bubble formation, but the electron/luminescence centre radiative recombination probability is increased.     

Thermoluminescence dating of coarse-grain quartz from the Malan loess at Zhaitang section, China

Thermoluminescence (TL) ages were obtained for five loess samples taken from the Zhaitang section near Beijing, China, using the coarse-grain quartz technique. The paleodose values have been determined by the method of total sample bleaching and regeneration of the TL growth curve. The method appears to be suitable for the age determination of loess samples up to about 150,000 yr where the annual dose-rate values are of the order 3–4 mGy/yr. This limit is a function of the total accumulated dose. The ages are in good agreement with those obtained by a fine-grain TL technique and are consistent with geological and geomagnetostratigraphic evidence.     

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) Å, 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.     

Studies of an artificially shock-loaded H group chondrite

Five samples of the naturally unshocked Kernouve (H6) meteorite were artificially shock-loaded to pressures of 70, 165, 270, and 390 kbar and the silicates and metal examined optically, by scanning and transmission electron microscopy and by thermoluminescence (TL). Olivine deformation is closely comparable to that in naturally shocked meteorites, producing dislocations with Burgers vector [001]. At pressures of ?165 kbar, these are formed in well-defined slip planes. At 270 kbar, olivine develops optical mosaicism, has high dislocation densities throughout and is also highly fractured. Recovery, due to heating is minimal. In orthopyroxene, the deformation mechanism changes, from the clino-inversion to unit-dislocation slip, between 70 and 165 kbar. In diopside, (001) and (100) twinning was produced. Plagioclase is inferred to have been progressively converted to maskelynite, but some is still present in 270 kbar sample.The microhardness of the kamacite in the samples increases with shock pressure. The α → ? transformation pressure in the kamacite is 30–40 kbar higher than observed for iron meteorites. Annealed kamacite displays incipient polycrystallinity and α-martensite and taenite sometimes contains slip lines. Troilite acquired cracks, undulose extinction, twins, polycrystallinity and finally melted as the shock pressure increased.At pressures over 200 kbar there was a systematic decrease in the natural TL and the TL sensitivity. Detailed considerations of changes in the natural $\text{TL}\text{TL}$ sensitivity ratio for various regions of the TL glow curve suggest that two processes were effective during shock; thermal drainage of electron traps and a reduction in the effective trap density. It is suggested that the latter process associated with the vitrification of feldspar, the TL phosphor.An additional sample was subjected to a shock pulse which was “spiked” instead of square. Very distinctive changes were apparent; thermal effects are conspicuous and with widespread annealing (~600–800°C) of metal and sulfide. Glassy, opaque veins were produced which are analogous to the black veins in shock-lithified gas-rich meteorites. Anomalous low-temperature TL was induced, suggesting that a new or modified phase or mineral has become the dominant TL phosphor.     

Thermoluminescence of quartz from Shihu gold deposit, western Hebei province, China: some implications for gold exploration

Thermoluminescence (TL) of monomineralic separates have been widely used in various geosciences fields in order to trace the thermal history and aid in prospecting for gold deposits. Quartz is a ubiquitous mineral in the Shihu gold deposit, which is situated in the northern part of the Taihang orogenic belt in the North China craton (NCC). The deposit is hosted by ductile-brittle faults within an Archean metamorphic core complex of the Fuping Group. This deposit is characterized by gold-bearing quartz-polymetallic sulfides and quartz veins. New TL results have been obtained for quartz, in which four type-TL glow curves were identified. The gold-bearing quartz present type III glow curves that consist of two peak glow curves at the middle and high peak temperatures with the similar TL intensity. In addition, the cross-sections of peak temperatures and TL intensity highlight the valuable area where the Au-bearing quartz present weak TL intensity and low-middle peak temperatures. Our results significantly enhance the usefulness of quartz in metallogenic studies of the North China craton and as an indicator mineral in mineral exploration of the Taihang Mountain region.     

X-Ray investigations on the deformation of experimentally shock-loaded quartzes

Quartz single crystals submitted to dynamic pressures higher than 200 kbar show intensive anisotropic postshock cell expansions and lattice disordering which gradually increase along with the degree of shock compression. Maximum expansion and lattice distortion occur parallel to [10.0] followed by [21.0] and [20.1], whereas the lowest expansion rates and comparatively little lattice damage can be observed parallel to [10.2] and [00.1]. [10.0], [21.0], and [20.1] represent short Burgers vectors within the quartz lattice. They are probably preferred directions of a structure-controlled deformation.Annealing experiments carried out at 300, 605 and 900 ° C make the expanded cell parameters approach the values of unshocked quartz. Two different types of recrystallization can be observed:

Deformation of shock-loaded andalusite studied with X-ray diffraction techniques

The structural deformation of an andalusite single crystal, shockloaded up to 400 kb with shock wave direction approximately parallel to c, was investigated by means of X-ray powder (Guinier) and single crystal techniques (Weissenberg, precession). Exposure to the dynamic pressure revealed a fracturing of the crystal into lattice blocks, with a mean size >1,000 Å. No change of the lattice constants could be observed after pressure release. From the streaks of X-ray reflection spots measured within the hk0, h0l, 0kl, and hhl planes the shock-induced lattice deformation is interpreted in terms of rotational gliding and/or microfracturing. The distortion mode is highly structure controlled. It follows preferrably two different structural motion systems: (1) Gliding parallel to (001) occurs, which produces lamellae parallel to (001), mainly arranged in two sublattices with common c-axis. The stacking sequence of lamellae along c is irregular. The lamellae-type structure may also result from an orientated transformation into a high pressure phase of lower symmetry and subsequent inversion into the original phase after pressure release. (2) Gliding parallel to (100) occurs. In this case the deformation mode is asymmetrical with respect to the undistorted crystal. The common direction b of the (001) and (100) deformation planes is probably the main direction of the shock-induced lattice deformation.     

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.     

Intergrowth textures of diaplectic glass and crystal in shock-loaded P-anorthite

The intergrowth of diaplectic glass and primitive anorthite has been observed in fragments shocked to 300 kbar. The textures observed by electron microscopy are similar to those produced in shocked quartz crystals. The diffuseness of reflections, related to antiphase domains, show no noticeable change in comparison with unshocked specimens. These results indicate that no gradual change in original domain size occurs and that the transformation of primitive anorthite to diaplectic glass occurs abruptly.     

Thermoluminescence dating of sediments

The potential for using the thermoluminescence behaviour of sediments for dating them was first recognized by Soviet scientists G.V. Morozov and V.N. Shelkoplyas, and for over a decade their TL dates, obtained from a variety of sediments, have appeared in the Soviet literature. Since 1977 TL sediment dates have been published by six additional groups using a variety of methods.In this review we describe the principles of TL dating, the various methods used, and contrast TL dating of sediments with the now well-accepted TL dating of pottery. We conclude that while TL dating has the potential to solve many sedimentary problems, more fundamental research needs to be carried out before such dates should be accepted. A set of criteria for acceptable dates is proposed.     

Thermoluminescence of carbonates from carbonatites

The Gejiu tin field in southern China consists of six major deposits and many minor ones containing more than 120 million tons (pre-mining resource) at 1% Sn and significant amounts of Cu, Ag, Zn, and Pb. It is one of the largest tin fields of primary deposits in the world. Mineralization is the result of the intrusion of granitic plutons into Permian and Triassic sedimentary rocks, which are dominantly limestone, dolomitic limestone, and dolomite. Five (mostly peraluminous) granitic intrusives (64-115 Ma) are present in the area. The largest orebodies are spatially and temporally related to the Laoka (principally), Beipaotai, and Marsong granites. Tin mineralization is mainly within greisens developed at the outermost zone of a skarn zonal sequence and are mineralogically dominated by fluorite, quartz, and micas.

The deposits are the result of volatile-rich ore solutions that evolved late in the plutonic crystallization history. The solutions produced metamorphic skarns as well as ore skarns, both of which later became “greisenized” skarns. Gejiu is the largest example of what has been, up to now, a style of mineralization reported only in minor amounts.     

Thermoluminescence and radioluminescence in aragonite

Summary Thermoluminescence (TL) and radioluminescence (RL) wavelength spectra of aragonite have been investigated. Despite the different origin of the samples used, a number of common luminescence features emerges. The spectra for TL peaks appearing near 50, 70 and 170 K are identical with a prominent and broad band centered near 400 nm. In the case of TL peaks appearing at about 350, 380, 420 and 455 K, the onset of new emission bands is due to the characteristic emission of some cation impurities (i.e., Mn²⁺, Pb²⁺). Below 200 K, all emission bands encountered for a given TL peak are also present in the RL spectrum obtained at the temperature of the glow peak. The electron-hole recombination mechanisms involved in aragonite appear to be related with those found in rhombohedric carbonates.

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Thermolumineszenz und Radiolumineszenz in Aragonit

Zusammenfassung Wellenlängen-Spektra der Thermolumineszenz (TL) and der Radiolumineszenz (RL) von Aragonit wurden untersucht. Trotz des unterschiedlichen Ursprungs der verwen-deten Proben zeigte die Lumineszenz eine Reihe gemeinsamer Züge. Die bei ungefähr 50, 70 und 170 K auftretenden TL-Spektra sind ident und zeigen eine dominierende und breite Bande mit Zentrum bei ca. 400 nm. Die TL-Maxima, die bei ungefähr 350, 380, 420 und 250 K auftreten, gehören zu Spektren, deren wesentliche Züge Kationen-Verunreinigungen wie Mn²⁺ und Pb²⁺ zugeordnet werden können. Die TL- und RL-Spektra sind ident, wenn die Temperatur des Kristalls kleiner als 200 K ist. Die Elektron-Loch Rekombinationsmechanismen in Aragonit scheinen mit jenen in den rhomboedrischen Karbonaten verwandt zu sein.

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Resumen Dos aragonitos de distinta procedencia se han investigado con et fin de analizar espectralmente sus respectivas emisiones de termoluminiscencia (TL) y radioluminiscencia (RL). A pesar de su distinto origen, ambos aragonitos presentan caracteristicas luminiscentes comunes. Los mâximos de TL que aparecen a 50, 70 y 170 K presentan espectros idénticos. Su principal caracteristica común es la presencia de una prominente y ancha banda cuyo máximo se situa cerca de 400 nm. Los máximos de TL que aparecen alrededor de 350, 380, 420 y 450 K presentan espectros cuya principal caracteristica es la de contener bandas asignables a iones-impureza tales como Mn y Pb. Los espectros de TL y RL son coincidentes siempre que la temperatura del cristal sea inferior a 200 K. Los mecanismos de recombination electron-hueco en et aragonito presentan una cierta similitud con los de los carbonatos romboédricos.

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With 6 figures