12 results
Search Results
2. Luminescence spectroscopy and microscopy applied to study gem materials: a case study of C centre containing diamonds.
- Author
-
Hainschwang, Thomas, Karampelas, Stefanos, Fritsch, Emmanuel, and Notari, Franck
- Subjects
LUMINESCENCE spectroscopy ,MICROSCOPY ,MINERAL analysis ,GEMS & precious stones ,CASE studies ,DIAMONDS ,ULTRAVIOLET radiation - Abstract
The methods of luminescence spectroscopy and microscopy are widely used for the analysis of gem materials. This paper gives an overview of the most important applications of the analysis of laser and UV excited luminescence by spectroscopy and visually by microscopy with emphasis on diamond, and specifically natural type Ib diamond, little studied so far. Luminescence based techniques are paramount to the gemmological analysis of diamond, in order to determine whether it is natural, treated or synthetic. The great sensitivity of luminescence helps detect some emitting centres that are undetectable by any other analytical method. Hence, especially for diamond, luminescence is an enabling technology, as illustrated by its pioneering use of imagery for the separation of natural and synthetic diamond, and of spectroscopy for the detection of High Pressure-High Temperature treatment. For all other gemstones the applications are at the moment less numerous, but nevertheless they remain highly important. They provide quickly information on the identification of a gem material, and its treatment. Besides the study of broad band emissions caused by various colour centres, the typical PL-causing trace elements (amongst others) are chromium, manganese, uranium and rare earth elements. In pearls the study of broad band luminescence can be useful, and particularly the study of pink to red porphyrin luminescence in pearls from certain species such as Pinctada and Pteria and others can help identify the pearl-producing mollusc, or if a pearl has been dyed or not. Type Ib diamonds are representative of the importance and complexity of the analysis of luminescence by microscopy and spectroscopy. They show a wide range of sometimes very complex emissions that result in luminescence colours from green to yellow to orange or red. These emissions show generally very inhomogeneous distribution. They are caused by a range of defects, however only a few of them are well characterized. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
3. Formation of corundum in metapelites around ultramafic bodies. An example from the Saualpe region, Eastern Alps.
- Author
-
Riesco, M., Stüwe, K., and Reche, J.
- Subjects
CORUNDUM ,GEMS & precious stones ,ULTRABASIC rocks ,METAMORPHISM (Geology) - Abstract
This paper describes corundum formation in a metasomatic reaction zonation around an ultramafic body within a metapelitic sequence. The investigated body is about 100 m in diameter and is located in the Saualpe of the Austroalpine nappe complex in the Eastern Alps. The body is surrounded by a 10 m wide reaction zone (here called zone 3) containing the paragenesis garnet-staurolite-biotite-margarite-chlorite-corundum. Beyond a further metasomatic transition zone (here called zone 2), there are undisturbed metapelitic host rocks (zone 1) that have the metamorphic peak paragenesis garnet-biotite-plagioclase-staurolite-muscovite-quartz. It is shown that reaction zonation formed around 7.2?kbar and 615?°C during regional metamorphism, just above the serpentine breakdown reactions in the system MgO-SiO
2 -H2 O. Detailed analysis of the whole rock compositions shows that the reaction zonation formed by infiltration metasomatism that caused significant mass loss in the two alteration zones. These zones are particularly depleted in SiO2 , Na2 O and possibly K2 O. AnXNa2 O-XSiO2 thermodynamic pseudosection is presented that includes the parageneses of both the unaltered metapelitic host rock and the corundum-bearing parageneses. This suggests that the metasomatic process can be explained by the transfer of SiO2 and Na2 O alone. We interpret that the process is driven by water liberated from the previously serpentinised ultramafic body during prograde dehydration during regional Eo-Alpine metamorphism. This fluid flowed outwards from the ultramafic body depleting the surrounding pelites in silica and causing margarite formation from plagioclase and muscovite. This interpretation of the driving mechanism is consistent with our knowledge of the low water activities of the Saualpe during the Eo-Alpine orogenic cycle. [ABSTRACT FROM AUTHOR]- Published
- 2005
- Full Text
- View/download PDF
4. Petrogenesis of gem sapphire in a pegmatite-aplite vein from the Alvand batholith, Western Iran.
- Author
-
Sheikhi Gheshlaghi, Rasoul, Ghorbani, Mansour, Sepahi, Ali Asghar, Deevsalar, Reza, and Shinjo, Ryuichi
- Subjects
SAPPHIRES ,BATHOLITHS ,PETROGENESIS ,SUBDUCTION zones ,VEINS ,SEAWATER ,GEMS & precious stones ,METASOMATISM - Abstract
In the Hamadan region, Sanandaj-Sirjan Zone (SaSiZ), W Iran, a pegmatite-aplite vein occurs that contains up to 15 vol% sapphire. The Hamadan sapphire is black to blue, and translucent to opaque, which reveals its semi-gem and gem quality. On the basis of petrographic studies, the Hamadan sapphire-bearing pegmatite-aplite vein represents alkali syenitic to syenitic composition and its adjacent granitoids have a range of composition from syenitic to quartz monzonite and granodiorite. New petrographic, whole-rock geochemical data and δ
18 O values [9.30–12.45‰VSMOW (Vienna Standard Mean Ocean Water)] suggest crustal origin for the peraluminous, S-type and alkaline sapphire-bearing pegmatite-aplite vein. The presence of syngenetic quartz inclusions in the sapphire, and some similar physical-gemological signatures (e.g., parting patterns and angles) of the Hamadan sapphire and kyanite support the generation of the sapphire at the expense of kyanite. In addition, a secondary metasomatic stage may have played a minor role in formation of the sapphire grains. The results indicate an important role for the mafic magmas in triggering crustal anatexis and gem crystallization in continental subduction zone settings. [ABSTRACT FROM AUTHOR]- Published
- 2020
- Full Text
- View/download PDF
5. Genetic significance of the 867 cm− 1 out-of-plane Raman mode in graphite associated with V-bearing green grossular.
- Author
-
Thomas, Rainer, Rericha, Adolf, Pohl, Walter L., and Davidson, Paul
- Subjects
GEMS & precious stones ,GRAPHITE ,SILICATE minerals ,MINERALIZATION ,MINERALOGY - Abstract
SE Kenya is the world’s largest producer of green vanadium grossular gemstones (tsavorite). Samples from one of the mines near Mwatate, and of occurrences in Tanzania yielded remarkable new insights into the genesis of tsavorite. Graphite is intimately associated with V-grossular and is one of the keys to understanding its origin. In the course of this study we found five different types of graphite. Surprisingly, in one graphite type the “Raman-forbidden” and IR-active 867 cm
− 1 band was observed. In this communication, we attempt to find an explanation for this unusual phenomenon. Additionally, our observations also address some of the issues pertaining to the origin of the green grossular-dominated rocks (grossularites), as well as the gem quality tsavorite crystals, since we propose that the anomalous spectroscopic behavior of the graphite is related to the unusual conditions during crystallization of both the grossular and graphite from a near-supercritical volatile- and sulfur-rich silicate melt. The massive green vanadium grossular contains abundant unequivocal crystallized melt inclusions, while the transparent gem quality grossular (tsavorite) displays only fluid inclusions. On the basis of inclusion studies we suggest that anatectic melts originated in the peculiar evaporitic host lithology of the tsavorite deposits. Near peak metamorphic temperatures (~ 700 °C) these liquids occurred as a supercritical volatile-rich “fluid/melt phase” characterized by complete miscibility between H2 O and silicate liquid. Relatively dry liquid batches precipitated non-transparent green grossular, whereas wet batches segregated fluids that formed transparent tsavorite. [ABSTRACT FROM AUTHOR]- Published
- 2018
- Full Text
- View/download PDF
6. On the presence of a hydrous component in a gemstone variety of intermediate olivine-type triphylite-lithiophilite, Li(Fe,Mn)PO.
- Author
-
Libowitzky, Eugen, Beran, Anton, Wieczorek, Arkadiusz, and Wirth, Richard
- Subjects
GEMS & precious stones ,OLIVINE ,TRIPHYLITE ,LITHIUM compounds ,SPECTRUM analysis ,ELECTRON microscopy ,ABSORBANCE scale (Spectroscopy) - Abstract
A single-crystal gemstone of intermediate olivine-type triphylite-lithiophilite with the composition Li(FeMn)PO was investigated by vibrational spectroscopy and electron microscopy techniques. Though gem-quality, the crystal reveals lamellar, platy inclusions parallel to (100) with a length of up to > 10 μm and a thickness of up to 1-2 μm. These were identified as sarcopside, (Fe,Mn)(PO), yielding a higher Fe:Mn ratio than the host triphylite. Polarised infrared spectra of oriented triphylite sections show a weak, pleochroic band in the O-H stretching region that is centred at ~ 3444 cm. Its integrated total absorbance corresponds to a hydrogen content of only 3.7 wt.ppm HO. Using peak-fitting techniques the band is decomposed into three component bands at ~ 3419, 3444, and 3474 cm which are least intense in the [010] spectra and which show stronger absorptions in the [001] and [100] directions. Thus, O-H vectors almost parallel to the (010) plane, pointing closer to the a or to the c axes are assumed. Three ways of hydrogen incorporation are in agreement with this observation: (a) hydrous species at the oriented interfaces between triphylite host and sarcopside lamellae, (b) hydrogen from submicroscopic inclusions of hydrous phosphate minerals such as phosphoferrite-reddingite, (Fe,Mn)(PO) · 3HO, (c) hydrous point defects in the structure of triphylite and/or sarcopside, which are possible under the assumption of vacancies at the Li ( M1) and/or P sites. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
7. Chabazite in spodumene-bearing Alpine-type fissure veins from Hiddenite, North Carolina, USA.
- Author
-
Wise, Michael A.
- Subjects
QUARTZ ,GEMS & precious stones ,CRYSTALS ,CRYSTALLIZATION ,SPODUMENE - Abstract
Alpine-type fissure vein mineralization in the Hiddenite area of western North Carolina, USA consists mostly of quartz, but locally contains Cr-bearing beryl (emerald) or Cr-bearing spodumene (hiddenite). These gem minerals occur in mineral-lined cavities and may be accompanied by euhedral crystals of quartz, calcite, muscovite, rutile, albite, pyrite, siderite and dolomite. Chabazite-Ca occurs as a late stage phase in spodumene-bearing veins, but is absent in emerald-bearing veins. Chabazite-Ca occurs as simple penetrating twins of pseudocubic rhombohedra and as the lens-shaped variety, phacolite. Chabazite-Ca from Hiddenite contains minor amounts of Na, Mg, Fe and K. Phacolitic chabazite-Ca shows Fe-enriched but Mg-depleted cores relative to the rims. Chemical zoning is absent in rhombohedral chabazite. The Hiddenite chabazite apparently precipitated under low temperature (< 250°C) and low pressure (< 2 kbar) conditions during the waning stages of crystallization of an alkaline hydrothermal fluid. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
8. Garnet Lu–Hf dating of retrograde fluid activity during ultrahigh-pressure metamorphic eclogites exhumation.
- Author
-
Hao Cheng, Nakamura, Eizo, and Zuyi Zhou
- Subjects
GARNET ,GEMS & precious stones ,SILICATE minerals ,LUTETIUM ,HAFNIUM ,TITANIUM group - Abstract
Previous studies on the atoll-shaped garnets in ultrahigh-pressure (UHP) metamorphic eclogites from the Dabie orogen, east-central China, suggest a fluid-enhanced overgrowth origin at the onset of exhumation. The atoll-garnets bearing eclogite place better constraints on the timing of the retrograde fluid activity and are a straightforward target to gain insight into the isotopic equilibrium and/or disequilibrium during exhumation. Comprehensive textural, chemical and Lu–Hf geochronological analyses on the atoll garnet-bearing eclogite show that the retrograde fluid activity event likely occurred at ca. 221 Ma. The Lu–Hf age of 221.0 ± 2.3 Ma marks the last garnet overgrowth episode during exhumation rather than prograde metamorphism. This somewhat restricted study suggests that dating the prograde-zoning-preserved garnets may bias results towards a particular metamorphic event rather than the prograde timing, as previously thought. The general assumption that larger garnet crystals in metamorphic rocks are older should be made with caution, and it is likely invalid in atoll garnet-bearing metamorphic eclogites because the preliminary garnet cores have been largely consumed. These observations highlight that linking textural and chemical analyses is crucial for interpreting geochronological data. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
9. A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece.
- Author
-
Katerinopoulou, A., Katerinopoulos, A., Voudouris, P., Bieniok, A., Musso, M., and Amthauer, G.
- Subjects
GARNET ,CRYSTALS ,X-ray diffraction ,CHEMISTRY ,GEMS & precious stones ,MARONEIA (Extinct city) - Abstract
Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO
2 , 8 wt.% Cr2 O3 and 4 wt.% ZrO2 , representing a solid solution of andradite (Ca3 Fe3+ 2 Si3 O12 ≈46 mol%), uvarovite (Ca3 Cr2 Si3 O12 ≈23 mol%), grossular (Ca3 Al2 Si3 O12 ≈10 mol%), schorlomite (Ca3 Ti2 [Si,(Fe3+ ,Al3+ )2 ]O12 ≈15 mol%), and kimzeyite (Ca3 Zr2 [Si,Al2 ]3 O12 ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al3+ , Fe3+ and Ti4+ . Cr3+ and Zr4+ are found at the octahedral site along with Fe3+ , Al3+ and Ti4+ . The measured H2 O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca2.99 Mg0.03 )Σ=3.02 (Fe3+ 0.67 Cr0.54 Al0.33 Ti0.29 Zr0.15 )Σ=1.98 (Si2.42 Ti0.24 Fe0.18 Al0.14 )Σ=2.98 O12 OH0.11 . Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
10. Age and geochemical constraints for partial melting of granulites in Estonia.
- Author
-
Soesoo, A., Košler, J., and Kuldkepp, R.
- Subjects
METAMORPHIC rocks ,GRANULITE ,ZIRCON ,METAMORPHISM (Geology) ,GEMS & precious stones - Abstract
The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
11. Late Neoproterozoic strongly peraluminous leucogranites, South Eastern Desert, Egypt--petrogenesis and geodynamic significance.
- Author
-
Moghazi, A. M., Hassanen, M. A., Mohamed, F. H., and Ali, S.
- Subjects
GARNET ,GEOCHEMISTRY ,SILICATE minerals ,GEMS & precious stones ,ROCKS ,GEOLOGIC faults ,TITANIUM dioxide ,IRON - Abstract
Late Neoproterozoic garnet-bearing leucogranites are developed locally along thrust faults in the South Eastern Desert, Egypt. This work presents field observations, whole rock major and trace element abundances, Rb--Sr isotope data and mineral chemistry for three occurrences in the Sikait-Nugrus area. Field observations show that the leucogranites cut the faults and their contact with the country rocks is sharp with no indication of contact metamorphism. They were intruded into a low-grade metamor-phosed ophiolitic melange and a high-grade metamorphosed metasedimentary succession of biotite schist composition. Numerous biotite schist enclaves, having irregular and diffuse contacts, are recorded within the leucogranites. Whole rock Rb--Sr ages of the leucogranites from two different localities are 610 ± 20 and 594 ± 12 Ma respectively; they are interpreted as emplacement ages. The leucogranites contain more than 70% SiO
2 , and they are strongly peraluminous (A CNK >1.1) with low TiO2 ,Fe2 O3 *, MgO, CaO, Ba, Sr, LREE, Eu/Eu* and Sr/Ba and high Rb, Rb/Zr, Rb/Sr and Rb/Ba. These geochemical parameters and the low initial87 Sr/86 Sr ratios (∼ 0.703) indicate crustal derivation by dehydration partial melting from a juvenile protolith similar to the exposed biotite-rich metasediments. Models for the tectonic setting of these leucogranites suggest their emplacement during an extensional tectonic stage that follows continental collision. It is proposed that crustal heating, caused by decompression along shear zones, is responsible for the production of these granitic melts. The results support previous hypotheses and further document a regional late Neoproterozoic extensional tectonic event, which is probably related to the initial break-up of Gondwana. [ABSTRACT FROM AUTHOR]- Published
- 2004
- Full Text
- View/download PDF
12. Synthesis of pyrope–grossular garnets: an experimental study at P = 2.5 GPa.
- Author
-
Orlando, A. and Borrini, D.
- Subjects
GARNET ,GEMS & precious stones ,MAGNESIUM ,CALCIUM ,MELILITE ,DIOPSIDE - Abstract
Summary¶In order to evaluate the stability of Mg–Ca garnets at P = 2.5 GPa/T = 800–1500 °C, some experiments were performed using both natural and synthetic starting materials. Particular attention was paid to the investigation of the Ca rich side of the pyrope–grossular join and the maximum pyrope content in grossular was found to be about 10 mol.%. The addition of Cr to the system only slightly increases the observed pyrope solubility in grossular whereas Na stabilises melilite and merwinite. Clinopyroxene with a composition lying on the diopside – Ca Tschermak join appears to be the only stable crystalline phase in the middle part of the pyrope–grossular join. [ABSTRACT FROM AUTHOR]
- Published
- 2003
- Full Text
- View/download PDF
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.