An X-ray excited optical luminescence study of a zoned quartz crystal from an emerald-bearing quartz vein,Hiddenite, North Carolina,USA

The optical luminescence excited with synchrotron radiation along a preferential orientation of a quartz crystal has been investigated. It is found that the crystal is composed of two distinct regions, only one of which luminesces upon X-ray excitation. This luminescence is generally uniform and exhibits emission bands in the blue (470 nm with a shoulder at 522 nm) and in the UV (340 nm) regions of the spectrum. The branching ratio for the intensity of these bands is sensitive to the excitation energy across the Si K-edge. XANES spectra collected by partial luminescence yield (PLY) suggest that both emission bands originate from the de-excitation of Si atoms in the quartz. The possible defect sites within the crystal structure that could account for the observed luminescence are investigated and discussed. Additional experiments are proposed to verify this assignment of the optical emission bands.     

New ASTER derived thermal indices to delineate mineralogy of different granitoids of an Archaean Craton and analysis of their potentials with reference to Ninomiya's indices for delineating quartz and mafic minerals of granitoids—An analysis in Dharwar Craton,India

We processed five thermal infrared (TIR) bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) “at-sensor radiance” (Level 1B) data to derive few new indices to delineate variation in quartz, feldspar and mafic minerals in three different variants of granitoid; commonly occurring quartz bearing intrusive rock group. In this regard, three indices named as quartz-bearing rock index (QRI), feldspar-bearing rock index (FRI) and mafic-bearing rock index(MRI) were proposed.QRI index was derived using band 10, band 12 and band 13((band 10/band 12) 1 (band 13/band 12)) of ASTER radiance image. MRI index was derived using band 12, band 13 and band 14 ((band 12/band 13) 1 (band 13/band 14)) of ASTER radiance bands while band 10, band 11 were combined to derive FRI ((band 10/band 11) index. Three indices were combined in false colour composite image (FCC) and three-dimensional scatter plot to delineate granite, alkali granite and mafic rich granodioritic gneiss from each other as these granitoids had variable abundances of quartz, feldspar and mafic minerals. QRI and MRI were compared with the corresponding quartz and mafic indices proposed by Ninomiya (2005). It was observed from the respective ratio images and their regression plots that MRI and Ninomiya's mafic index (NMI) were complementary to each other. On the other hand, QRI image was better in enhancing quartz enrichment in alkali granites than Ninomiya's quartz index (NQI).However, QRI index was comparable with the quartz index proposed by Rockwall and Hofstra (2008) in terms of delineating quartz enrichment in alkali granite. Mutual exclusive nature of mafic minerals and quartz in granitoids was also evident from the negative correlation between MRI and QRI indices of the granitoids. On the other hand, FRI and QRI were negatively correlated with low regression value. This was resulted due to the combined effect of inverse relation of abundance of two dominant feldspars with quartz in different granitoids. In granitoids, abundance of plagioclase is known to increase with decreasing quartz content in granodiorite and tonalite although alkali feldspar bearing granites are characterised with high silica content. Results of discrimination of granitoids using proposed indices were validated based on deriving emissivity spectra of rocks and comparing them with ASTER TIR band resampled laboratory spectra of respective granitoids in addition to use geological map of the study area. Emissivity spectra of granitoids were derived from emissivity image (derived using emissivity normalisation method) after geospatially tagging it with QRI–FRI–MRI image composite; which was used to delineate exposures of granitoids. Further, we also found that the QRI, MRI and FRI indices had poor temperature dependence; when these indices were compared with relative surface temperature image derived from radiance bands using emissivity normalisation algorithm. Therefore, proposed indices can be implemented for delineating mineralogical variations of granitoids irrespective of surface temperature condition. Hence, proposed indices may be used successfully to delineate different granitic intrusions and relating their mineralogical variations with metallogeny.     

Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz – a review

Summary Investigations of natural and synthetic quartz specimens by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) and trace-element analysis showed that various luminescence colours and emission bands can be ascribed to different intrinsic and extrinsic defects. The perceived visible luminescence colours in quartz depend on the relative intensities of the dominant emission bands between 380 and 700 nm. Some of the CL emissions of quartz from the UV to the yellow spectral region (175 nm, 290 nm, 340 nm, 420 nm, 450 nm, 580 nm) can be related to intrinsic lattice defects. Extrinsic defects such as the alkali (or hydrogen)-compensated [AlO₄/M⁺] centre have been suggested as being responsible for the transient emission band at 380–390 nm and the short-lived blue-green CL centered around 500 nm. CL emissions between 620 and 650 nm in the red spectral region are attributed to the nonbridging oxygen hole centre (NBOHC) with several precursors. The weak but highly variable CL colours and emission spectra of quartz can be related to genetic conditions of quartz formation. Hence, both luminescence microscopy and spectroscopy can be used widely in various applications in geosciences and techniques. One of the most important fields of application of quartz CL is the ability to reveal internal structures, growth zoning and lattice defects in quartz crystals not discernible by means of other analytical techniques. Other fields of investigations are the modal analysis of rocks, the provenance evaluation of clastic sediments, diagenetic studies, the reconstruction of alteration processes and fluid flow, the detection of radiation damage or investigations of ultra-pure quartz and silica glass in technical applications. Zusammenfassung Ursachen, spektrale Charakteristika und praktische Anwendungen der Kathodolumineszenz (KL) von Quarz – eine Revision Untersuchungen von natürlichen und synthetischen Quarzproben mittels Kathodolumineszenz (KL) Mikroskopie und -spektroskopie, Elektron Paramagnetischer Resonanz (EPR) und Spurenelementanalysen zeigen verschiedene Lumineszenzfarben und Emissionsbanden, die unterschiedlichen intrinsischen und extrinsischen Defekten zugeordnet werden k?nnen. Die sichtbaren Lumineszenzfarben von Quarz werden durch unterschiedliche Intensit?tsverh?ltnisse der dominierenden Emissionsbanden zwischen 380 und 700 nm verursacht. Einige der KL Emissionen vom UV bis zum gelben Spektralbereich (175 nm, 290 nm, 340 nm, 420 nm, 450 nm, 580 nm) stehen im Zusammenhang mit intrinsischen Defekten. Die kurzlebigen Lumineszenzemissionen bei 380–390 nm sowie 500 nm werden mit kompensierten [AlO₄/M⁺]-Zentren in Verbindung gebracht. Die KL-Emissionen im roten Spektralbereich bei 620 bis 650 nm haben ihre Ursache im “nonbridging oxygen hole centre” (NBOHC) mit verschiedenen Vorl?uferzentren. Die unterschiedlichen KL-Farben und Emissionsspektren von Quarz k?nnen oft bestimmten genetischen Bildungsbedingungen zugeordnet werden und erm?glichen deshalb vielf?ltige Anwendungen in den Geowissenschaften und in der Technik. Eine der gravierendsten Einsatzm?glichkeiten ist die Sichtbarmachung von Internstrukturen, Wachstumszonierungen und Defekten im Quarz, die mit anderen Analysenmethoden nicht oder nur schwer nachweisbar sind. Weitere wesentliche Untersuchungsschwerpunkte sind die Modalanalyse von Gesteinen, die Eduktanalyse klastischer Sedimente, Diageneseuntersuchungen, die Rekonstruktion von Alterationsprozessen und Fluidmigrationen, der Nachweis von Strahlungssch?den oder die Untersuchung von ultrareinem Quarz und Silikaglas für technische Anwendungen. Received March 29, 2000 Accepted October 27, 2000     

Advances in lithium analysis in solids by means of laser-induced breakdown spectroscopy: an exploratory study

Lithium is an important geochemical tracer for fluids or solids. However, because the electron microprobe cannot detect Li, variations of Li abundance at the micrometric scale are most often estimated from bulk analyses. In this study, the Li intense emission line at 670.706 nm in optical emission spectroscopy was used to perfect the analysis of Li at the micrometric scale by means of laser-induced breakdown spectroscopy (LIBS). To estimate lithium content for different geological materials, LIBS calibration of the emission line at 670.706 nm was achieved by use of synthetic glasses and natural minerals. The detection limit for this method is ∼5 ppm Li. Three applications to geological materials show the potential of LIBS for lithium determination, namely for Li-bearing minerals, melt inclusions, quartz, and associated fluid inclusions.For spodumene and petalite from granite pegmatite dikes (Portugal), the Li₂O concentrations are 7.6 ± 1.6 wt% and 6.3 ± 1.3 wt%, respectively, by use of LIBS. These values agree with ion microprobe analyses, bulk analyses, or both. For eucryptite crystals, the Li concentrations are scattered because grain size is smaller than the LIBS spatial resolution (6 to 8 μm). Lithium concentrations of melt inclusions from the Streltsovka U deposit (Siberia) are in the range of 2 to 6.2 wt% (Li₂O) for Li-rich daughter minerals. Lithium estimations on silicate glasses display values between 90 and 400 ppm.Lithium was also analyzed as a trace element in quartz. Transverse profiles were performed in hydrothermal barren quartz veins from the Spanish Central System (Sierra de Guadarrama). The highest Li concentrations (250 to 370 ppm) were found in specific growth bands in conjunction with the observed variation in optical cathodoluminescence intensity. Considering the fluid inclusion analysis, the source of fluid responsible to the Li enrichment in quartz is probably high-salinity fluids derived from sedimentary basins.     

Origin of the color in cobalt-doped quartz

Synthetic Co-doped quartz was grown hydrothermally in steel autoclaves at the Technological Center of Minas Gerais (CETEC), Brazil. The quartz samples, originally yellow in the as-grown state acquired blue coloration after prolonged heat treatment times at 500°C near the alpha–beta transition temperature. UV–VIS–NIR absorption spectroscopy shows the characteristic spectra of Co³⁺ before heat treatment. After heat treatment, the optical absorption spectrum is dominated by two split-triplet bands the first in the near infrared region centered at about 6,700 cm⁻¹ (1,490 nm) and the second in the visible spectral range at about 16,900 cm⁻¹ (590 nm). Both split-triplet bands are typical for Co²⁺ ions in tetrahedral coordination environments. From the absence of electron paramagnetic resonance (EPR) spectra, we conclude that the Co²⁺ found in the optical absorption spectra of the blue quartz is not due to an isolated structural site in the quartz lattice. Instead, the blue color is associated with electronic transitions of Co²⁺ in small inclusions in which the Co site has tetrahedral symmetry. The non-observation of polarization-depend optical absorption spectra is also in agreement with this model. The results for Co²⁺ in quartz are different from Co-bearing spinel and staurolite and other silicates like orthopyroxene, olivine, and beryls. The formation process of the color center is discussed.     

Geological mapping of the Francistown area in northeastern Botswana by surface temperature and spectral emissivity information derived from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared data

We identified geological units in the Francistown area in northeastern Botswana by using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) thermal infrared (TIR) data, which contains both surface temperature and spectral emissivity information. A scatter plot of ASTER L1B daytime TIR digital values and L2 daytime temperature indicates that in the ASTER L1B daytime TIR data, bands 10, 11, and 12 contain spectral emissivity and temperature information, whereas surface temperature dominates the spectral emissivity information in bands 13 and 14. Visual interpretation of the ASTER TIR false color composite (FCC) images generated by assigning red, green, and blue to band 14:band 12:band 10 using L1B daytime data allowed us to identify mafic-to-ultramafic units and quartz-rich felsic units. Mafic-to-ultramafic units such as gabbro, dolerite, and dunite appear white in ASTER L1B daytime TIR FCC images due to their high spectral emissivities in the 8–9 μm region (bands 10 and 12) and high surface temperatures. Mafic-to-ultramafic units have higher surface temperatures than other geological units because they absorb more solar radiation due to their lower albedos and they have a lower thermal inertia. Quartz-rich felsic units such as granite and dry river sand appear reddish in the ASTER L1B daytime TIR FCC image because the spectral emissivity of quartz is lower in the 8–9 μm region (bands 10 and 12) than in the 10–12 μm region (band 14). Mafic-to-ultramafic and granitic units are important targets for mineral exploration because they are potential geological units to host or accompany mineralization. The proposed ASTER L1B daytime TIR FCC images can be prepared very simply and they provide valuable information for geological mapping and mineral exploration.     

胶东三山岛北部海域金矿床石英热释光和晶胞参数特征及其找矿意义

胶东是中国最重要的黄金资源基地之一,近年来深部找矿取得重大突破,特别是在三山岛北部海域发现超大型金矿床。深入研究其成矿规律,对于指导今后的找矿勘探意义重大。笔者应用成因矿物学和找矿矿物学理论,选取该矿床的贯通性矿物石英,对其热释光和晶胞参数特征的时空分布规律及与金矿化的关系进行了系统研究。结果表明:(1)石英热释光曲线以中温单峰、肩峰和不对称双峰为特征。主成矿阶段发光曲线出现中温峰和高温峰的双峰,发光强度大;(2)石英热释光峰位温度、发光强度在垂向上的波动变化与矿石金品位套合较好;(3)石英(Δa₀/a₀)/(Δc₀/c₀)值的变化范围在1.92~3.30,指示微量元素进入石英的方式以填隙为主;(4)随着成矿阶段由早至晚,石英晶胞参数a₀和V₀逐渐升高,c₀/a₀逐渐降低,表明石英的晶胞参数可作为识别其结晶阶段的标识之一;(5)钻孔深部石英热释光峰位温度较高,发光强度波动性较大,峰型呈单峰和肩峰,石英晶胞参数a₀、V₀仍较大,指示该矿深部仍有较好的找矿前景。     

Direct and indirect luminescence dating of tephra: A review

In Quaternary studies, tephras are widely used as marker horizons to correlate geological deposits. Therefore, accurate and precise dating is crucial. Among radiometric dating techniques, luminescence dating has the potential to date tephra directly using glass shards, volcanic minerals that formed during the eruption or mineral fragments that originate from the shattered country rock. Moreover, sediments that frame the tephra can be dated to attain an indirect age bracket. A review of numerous luminescence dating studies highlights the method's potential and challenges. While reliable direct dating of volcanic quartz and feldspar as a component in tephra is still methodically difficult mainly due to thermal and athermal signal instability, red thermoluminescence of volcanic quartz and the far-red emission of volcanic feldspar have been used successfully. Furthermore, the dating of xenolithic quartz within tephra shows great potential. Numerous studies date tephra successfully indirectly. Dating surrounding sediments is generally straightforward as long as samples are not taken too close to the tephra horizons. Here, issues arise from the occurrence of glass shards within the sediments or unreliable determination of dose rates. This includes relocation of radioelements, mixing of tephra into the sediment and disregarding different dose rates of adjacent material.     

Deformation and fluid flow in the Huab Basin and Etendeka Plateau,NW Namibia

The Lower Cretaceous Twyfelfontein sandstone formation in the Huab Basin in NW Namibia shows the effects of volcanic activity on a potential reservoir rock. The formation was covered by the Paraná-Etendeka Large Igneous Province shortly before or during the onset of South-Atlantic rifting. Deformation bands found in the sandstone trend mostly parallel to the continental passive margin and must have formed during the extrusion of the overlying volcanic rocks, indicating that their formation is related to South-Atlantic rifting. 2D-image porosity analysis of deformation bands reveals significant porosity reduction from host rock to band of up to 70%. Cementation of the sandstone, linked to advective hydrothermal flow during volcanic activity, contributes an equal amount to porosity reduction from host rock to band when compared to initial grain crushing. Veins within the basaltic cover provide evidence for hot fluid percolation, indicated by spallation of wall rock and colloform quartz growth, and for a later low-temperature fluid circulation at low pressures indicated by stilbite growth sealing cavities. Sandstone samples and veins in the overlying volcanic rocks show that diagenesis of the Twyfelfontein sandstone is linked to Atlantic rifting and was affected by both hydrothermal and low-thermal fluid circulation.     

Dynamic analysis and two types of kink bands in quartz veins deformed under subgreenschist conditions

In order to clarify deformation mechanisms and behaviours of quartz in a low-temperature regime in the earth's crust, microstructural analyses, particularly on kink bands have been carried out for quartz veins moderately deformed under subgreenschist conditions. Both the dominance of subbasal deformation lamellae and geometry of kink bands suggest that basal (0001) slip was the sole active slip system in the deformed quartz. On a morphological basis, kink bands in the quartz were classified into two types: type I is characterized by conjugate and narrow bands with angular hinge zones, and type II by a wide monoclinal band. Dynamic analyses using deformation lamellae and kink bands have revealed that type I kink bands were formed in grains with basal plane (sub-)parallel to the compression axis, whereas type II kink bands were formed in grains with basal planes inclined to it. Using a numerical model of kinking of elastic multilayers modified after Honea and Johnson (Tectonophysics 30, 197–239, 1976), changes of the level of yielding stress for kinking and the width of kink bands as a function of the angle θ between the slip plane and the compression axis have been examined. The theory predicts that type I kink bands were formed at a higher stress level than type II kink bands, and hence occurrence of type I kink bands suggests that a significant strain hardening occurred in the deformed quartz veins. The theory also well explains the fact that the width of type I kink bands (θ=0 to 10°) is narrower by an order of magnitude than type II kink bands (θ=10 to 80°).     

中国东北晚全新世火山爆发最新的一次活跃期

中国东北晚全新世火山活动十分强烈,1200 AD±的一次活跃期及其所产生的地质灾害已被很好地研究.根据历史记载、热释光测年和¹⁴C测年等资料,在1720 AD±存在最新的一次活跃期.确证这一活跃期的存在并开展火山地质灾害调查与研究,对探索火山活动节律和火山地质灾害预警等均有重要的理论和实际意义.     

Cathodoluminescence (CL) and electron paramagnetic resonance (EPR) studies of clay minerals

Summary ?Sheet silicates of the serpentine–kaolin-group (serpentine, kaolinite, dickite, nacrite, halloysite), the talc–pyrophyllite-group (talc, pyrophyllite), the smectite-group (montmorillonite), and illite (as a mineral of the mica-group) were investigated to obtain information concerning their cathodoluminescence behaviour. The study included analyses by cathodoluminescence (CL microscopy and spectroscopy), electron paramagnetic resonance (EPR), X-Ray diffraction (XRD), scanning electron microscopy (SEM) and trace element analysis. In general, all dioctahedral clay minerals exhibit a visible CL. Kaolinite, dickite, nacrite and pyrophyllite have a characteristic deep blue CL, whereas halloysite emission is in the greenish-blue region. On the contrary, the trioctahedral minerals (serpentine, talc) and illite do not show visible CL. The characteristic blue CL is caused by an intense emission band around 400 nm (double peak with two maxima at 375 and 410 nm). EPR measurements indicate that this blue emission can be related to radiation induced defect centres (RID), which occur as electron holes trapped on apical oxygens (Si–O centre) or located at the Al–O–Al group (Al substituting Si in the tetrahedron). Additional CL emission bands were detected at 580 nm in halloysite and kaolinite, and between 700 and 800 nm in kaolinite, dickite, nacrite and pyrophyllite. Time-resolved spectral CL measurements show typical luminescence kinetics for the different clay minerals, which enable differentiation between the various dioctahedral minerals (e.g. kaolinite and dickite), even in thin section. Received December 3, 2001; revised version accepted February 27, 2002     

Composition and genesis of the Konevinsky gold deposit,Eastern Sayan,Russia

The Konevinsky gold deposit in southeast Eastern Sayan is distinguished from most known deposits in this region (Zun-Kholba, etc.) by the geological setting and composition of mineralization. To elucidate the cause of the peculiar mineralization, we have studied the composition, formation conditions, and origin of this deposit, which is related to the Ordovician granitoid pluton 445–441 Ma in age cut by intermediate and basic dikes spatially associated with metavolcanic rocks of the Devonian–Carboniferous Ilei Sequence. Four mineral assemblages are recognized: (1) quartz–pyrite–molybdenite, (2) quartz–gold–pyrite, (3) gold–polysulfide, and (4) telluride. Certain indications show that the ore was formed as a result of the superposition of two distinct mineral assemblages differing in age. The first stage dated at ~440 Ma is related to intrusions generating Cu–Mo–Au porphyry mineralization and gold–polysulfide veins. The second stage is controlled by dikes pertaining to the Devonian–Carboniferous volcanic–plutonic association. The second stage is characterized by gain of Hg and Te and formation of gold–mercury–telluride paragenesis.     

Fragmentation of granitic quartz in water

Studies of grain fragmentation in natural streams have the limitation that the full size range of the debris produced is virtually unobtainable. Experiments described here for grain fragmentation in a rotating drum permitted the study of all of the debris, and a fragmentation load technique was used to relate experimentally and naturally fragmented material. The present investigation has been focused on granitic quartz. Relatively gentle collective movement in water can cause significant fragmentation of coarse, nascent, granitic quartz grains. The debris produced by rotating in a drum a range of single sieve fractions, taken from gravel in the headwaters of a stream draining granite, had continuous size distributions down to (and probably beyond) 0·06 μm. Quartz was the dominant fragmentation product in all fractions down to 2 μm and present in finer fractions. When pebbles moved with sand in these experiments, breakage of the latter was greatly increased. In comparison with that of breakage, the effect of attrition on granitic quartz was negligible. At least a proportion of granitic quartz grains are subject to a fatigue effect as a result of impacts in water. Evidently they are thus progressively weakened prior to being broken. Size analysis of debris showed a significant break at 20 μ, suggesting some special production of quartz particles just below this size. Granitic quartz is criss-crossed with partially healed cracks acquired before the zone of weathering is reached. The wholesale breakage that affects it, particularly in pebbly streams, is largely due to the reopening of these cracks. Progressive fragmentation of this material must eventually reach a stage wherein grains comprising single original crack-bounded volume elements are produced. Such grains, lacking significant internal weaknesses, must strongly resist further breakage. Possibly the preferential production of quartz grains just below 20 μm in size may represent an accumulation of these single, crack-bounded volume elements.     

内蒙古蛤蟆石铅银钼多金属矿矿床地质特征

内蒙古阿鲁科尔沁旗蛤蟆石铅银钼多金属矿地处内蒙古大兴安岭成矿带中南段,西拉木伦河断裂以北,工作和研究程度低,通过普-详查工作首次对该矿床有了较为正确、系统的认识。对矿区地质特征、矿体地质特征、矿石地质特征及成矿要素进行了初步分析总结,认为EW向断裂构造为成岩成矿断裂,矿体与EW向展布的沿断裂充填的晚侏罗世次流纹岩时空、成因关系密切,矿体赋存于次火山岩侵入接触构造-硅化蚀变带(次生石英岩化带)中,矿体呈脉状、透镜状EW向分布。通过分析认为矿床成因为次火山热液矿床,与次火山有关的硅化带、次生石英岩化带是该区寻找相似成因矿床的找矿标志。     

Acoustic emissions from stress-induced dauphiné twinning in quartz

Dauphiné (electrical) twins have been introduced into quartz crystals by means of suitably oriented mechanical stresses. Stress-induced twinning was observed optically by means of the photoelastic effect. Accompanying acoustic emission, first detected by ear, was quantitatively measured with a piezoelectric transducer and a suitable filtering and amplification system. Synchronized sound cinematography has been used to correlate the acoustic emission with visual observation of twinning. The resultant motion picture has been used to determine the stress levels required for twinning in Brazilian quartz and to set limits on the twin wall velocity. The possible relevance to rock noise and earthquake precursory phenomena is discussed.     

Geological structure and genesis of quartz veins in the Ufalei complex exemplified by the Vein 2136

The data on the geological structure and petrography of host rocks of the Vein 2136 quartz deposit located in the eastern part of the Ufalei gneiss–amphibolite complex (the Southern Urals) are discussed. It is established that the deposit resulted from the metasomatic replacement of amphibolite by quartz. The following metasomatic succession was formed during metasomatosis: quartz → quartz–feldspar pegmatoid → biotite schist → amphibolite. The investigation of homogenized vapor–liquid inclusions in quartz and feldspars revealed that the deposit was formed at temperatures of 350–400°C.     

Polarized IR spectra of synthetic smoky quartz

Polarized IR spectra of planeparallel (0001) plates of synthetic smoky quartz, with E rotating around [0001], show that the absorption figures of OH^– related absorption bands at 3380 (room temperature), 3365 and 3305 cm^–1 (liquid nitrogen temperature, -196° C) are strongly anisotropic and violate the trigonal symmetry of low quartz. This effect is correlated with a non-uniform substitution of Si by Al on the three symmetrically equivalent Si sites, as revealed by EPR measurements. Random distribution of Al over the three Si sites, obtained by dry annealing of the samples in air, yields isotropic absorption figures in the (0001) plates. It is thus experimentally evident that the absorption bands at 3380, 3365 and 3305cm^–1 are caused by the OH^– stretching vibrations coupled with Al substituting for Si. For each experimentally determined integral absorption coefficient of the three absorption bands a theoretical absorption coefficient was calculated, based on the symmetry of low quartz and the given Al distribution. This was done for various orientations of the OH^– dipoles with respect to the a axes of low quartz. By comparing the experimentally determined and calculated absorption coefficients, the orientation of the corresponding OH^– dipoles with respect to the a axes could be determined.     

Distribution and petrogenetic behaviour of trace elements in granitic pegmatite quartz from South Norway

The present study documents that the trace-element distribution in granitic quartz is highly sensitive to CAFC processes in granitic melts. Igneous quartz efficiently records both the origin and the evolution of the granitic pegmatites. Aluminium, P, Li, Ti, Ge and Na in that order of abundance, comprises >95% of the trace elements. Most samples feature >1 ppm of any of these elements. The remnant 5% includes K, Fe, Be, B, Ba and Sr whereas the other elements are present at concentrations lower than the detection limit. Potassium, Fe, Be and Ti are relatively compatible hence obtain the highest concentrations in early formed quartz. Phosphorous, Ge, Li and Al are relatively incompatible and generally obtain the highest concentrations in quartz that formed at lower temperatures from more evolved granitic melts. The Ge/Ti, the Ge/Be, the P/Ge and the P/Be ratios of quartz are strongly sensitive to the origin and evolution of the granitic melts and similarly the Rb/Sr and the Rb/K ratios of K-feldspars may be utilised in petrogenetic interpretations. However, the quartz trace element ratios are better at distinguishing similarities and differences in the origin and evolution of granitic melts. After evaluating the different trace element ratios, the Ge/Ti ratio appears to be most robust during subsolidus processes in the igneous systems, hence probably should be the preferred ratio for analysing and understanding petrogenetic processes in granitic igneous rocks.Editorial responsibility: J. Hoefs     

Phreatomagmatic boulder conglomerates at the tip of the ca 2772 Ma Black Range dolerite dyke,Pilbara Craton,Western Australia

Unusual volcanic conglomerates with a mixture of well-rounded granitic boulders (to 1.2 m diameter) derived from adjacent basement rocks, and smaller (1 – 10 cm) subspherical basaltic droplets with chilled margins occupy a linear zone along strike of the northern end of the Late Archaean Black Range dolerite dyke in the Pilbara Craton, Western Australia. The matrix of the volcanic conglomerates becomes more angular with decreasing grainsize and grades to rock flour, a trend opposite to that in sedimentary conglomerates. In other places, the matrix consists of chlorite that cuts through, and resorbs, granitic clasts, indicating an origin as volcanic melt. The volcanic conglomerates have peperitic contacts with immediately adjacent flows of the Mt Roe Basalt of the Fortescue Group. A welded volcanic tuff at the peperitic contact is dated at 2767 ± 3 Ma, within error of the 2772 ± 2 Ma Black Range dolerite dyke and the Mt Roe Basalt (2775 ± 10 Ma), confirming the contemporaneity of formation of these geological elements. Subsequent normal faulting has juxtaposed the higher level conglomerates down into their present exposure level along strike of the Black Range dolerite dyke. The linear zone of volcanic conglomerates is interpreted to represent a phreatomagmatic pebble dyke that formed immediately above, and as a result of intrusion of, the Black Range dolerite dyke. Interaction of magma with groundwater caused phreatomagmatic brecciation of the country rock, in situ milling of granitic boulders, incorporation of basaltic melt droplets, and the formation of a mixed matrix of devitrified volcanic glass and granitic material. This process was accompanied by along-strike epithermal Cu – Hg – Au mineralisation.