Michael A. Wise

Laser-induced breakdown spectroscopy-based geochemical fingerprinting for the rapid analysis and discrimination of minerals: the example of garnet

Laser-induced breakdown spectroscopy (LIBS) is an analytical technique real-time geochemical analysis that is being developed for portable use outside of the laboratory. In this study, statistical signal processing and classification techniques were applied to single-shot, broadband LIBS spectra, comprising measured plasma light intensities between 200 and 960 nm, for a suite of 157 garnets of different composition from 92 locations worldwide. Partial least squares discriminant analysis was applied to sets of 25 LIBS spectra for each garnet sample and used to classify the garnet samples based on composition and geographic origin. Careful consideration was given to the cross-validation procedure to ensure that the classification algorithm is robust to unseen data. The results indicate that broadband LIBS analysis can be used to discriminate garnets of different composition and has the potential to discern geographic origin.

Compositional and structural systematics of the columbite group

The kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data. We have experimentally studied the kinetics of the interconversion reaction between H20 molecules and OH groups in natural rhyolitic glasses (0.5-2.3% total water) and a synthetic albitic glass (l % total water) at 400-600 0c. The reaction rate increases with temperature and total water content. Equilibrium is not always approached monotonically; the speciation may first depart from equilibrium and then come back to equilibrium. Experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850°C. The experimental data are modeled successfully by considering both the reaction and the diffusion of OH that brings OH groups together to react. This study shows that species concentrations in glasses quenched from ::;600 °C reflect those at experimental temperatures unless the water content is higher than that used in the present study. Species concentrations in glasses with total water contents ~0.8 wt% and which were rapidly quenched in water from 850°C do not represent their equilibrium concentrations in the melt at 850 °C, but record a lower apparent equilibrium temperature that depends on water content and quench rate. Natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation, though total water content may be conserved. The experimental data are used to infer cooling rates for natural obsidian glasses. Pyroclastic glass fragments from the bb site of Mono Craters have cooling rates similar to air-cooled experimental charges (3 °C/s). Different types of glasses from the Mono Craters have different cooling rates, which cover four orders of magnitude. Some natural obsidians appear to have had complex cooling histories. The wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the Mono Craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption, which led to variable degrees of transient heating followed by rapid cooling and deposition. These experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how, at least in principle, quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models.

Can the provenance of the conflict minerals columbite and tantalite be ascertained by laser-induced breakdown spectroscopy?

Conflict minerals is a term applied to ores mined in conditions of armed conflict and human rights abuse. Niobium and tantalum are two rare metals whose primary natural occurrence is in the complex oxide minerals columbite and tantalite, the ore of which is commonly referred to as coltan. The illicit export of coltan ore from the Democratic Republic of the Congo is thought to be responsible for financing the ongoing civil conflicts in this region. Determining the chemical composition of an ore is one of the means of ascertaining its provenance. Laser-induced breakdown spectroscopy (LIBS) offers a means of rapidly distinguishing different geographic sources for a mineral because the LIBS plasma emission spectrum provides the complete chemical composition (i.e., “chemical fingerprint”) of any material in real time. To test this idea for columbite–tantalite, three sample sets were analyzed. Partial least squares discriminant analysis (PLSDA) allows correct sample-level geographic discrimination at a success rate exceeding 90%.

Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence

Abstract The Crown emerald veins are somewhat enigmatic, displaying characteristics that are common to emerald deposits of tectonic–hydrothermal origin and of igneous origin. The veins cut the Fire Lake mafic meta-volcanic rocks, occurring within 600 m of an outcrop of Cretaceous S-type granite. Field work and vein petrography are consistent with a polythermal origin for the veins. The primary vein mineralogy is quartz and tourmaline with variable sized alteration haloes consisting of tourmaline, quartz, muscovite, chlorite and emerald. The veins weather a buff brown colour due to jarosite, scheelite and minor lepidocrocite, which were precipitated during the waning stages of vein formation. Microthermometic studies of primary fluid inclusions within emerald growth zones are consistent with emerald precipitation from H 2 O–CO 2 –CH 4 (±N 2 ±H 2 S) bearing saline brines. The estimated fluid composition is approximately 0.9391 mol% H 2 O, 0.0473 mol% CO 2 , 0.0077 mol% CH 4 and 0.0059 mol% NaCl (∼2 wt.% NaCl eq.). Fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365–498 °C, with corresponding pressures along fluid inclusion isochore paths ranging from 700 to 2250 bars. These data correlate with a very slow uplift rate for the region of 0.02–0.07 mm/year. Emerald deposits are generally formed when geological conditions bring together Cr (±V) and Be. Cr and V are presumed to have been derived locally from the mafic and ultramafic rocks during hydrothermal alteration. The Be is most likely derived from the nearby Cretaceous granite intrusion.

Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites

SummaryGranitic pegmatites characterized by advanced accumulation and fractionation of incompatible rare lithophile elements (Li, Rb, Cs, Be, Ta ≶ Nb, B, P and F), often contain mineral assemblages which host lithium-rich micas. Lepidolite and lithian muscovite occur in high-pressure spodumene, low-pressure petalite, phosphorus-enriched amblygonite and fluorine-rich lepidolite subtypes of orogenic affiliated complex type granitic pegmatites and rarely in anorogenic affiliated amazonite-bearingTrace element data determined by X-ray fluorescence for lepidolite of various pegmatite subtypes, morphology (“book”, “scaly”, “fine-grained”), position within the pegmatite (primary zones, replacement units, pockets), mineral assemblages and tectonic affinity (orogenic vs anorogenic) show extreme fractionation of Rb and Cs; modest levels of T1, Ga, Nb, Ta, Sn and Zn; and typically low abundances of Ba, Sr, Ni, Pb, Y, V, W and Zr. Extreme fractionation is indicated by low values of K/Rb, K/Cs and Nb/Ta which are lowest in lepidolite from petalite subtype pegmatites.No systematic differences in trace element content is evident among the different lepidolite morphologies or paragenetic position. Lepidolite from spodumene subtype pegmatites are generally slightly less fractionated than those from petalite or lepidolite subtype pegmatites.ZusammenfassungGranitische Pegmatite, die durch fortgeschrittene Anreicherung und Fraktionierung von inkompatiblen, seltenen, lithophilen Elementen (Li, Rb, Cs, Be, Ta Nb, B, P und F) charakterisiert sind, enthalten häufig Mineralparagenesen mit Lithium-reichen Glimmern. Lepidolith und Li-Muskowit treten in Hochdruck-Spodumen, in Niedrigdruck-Petalit, in mit Phosphor angereichertem Amblygonit und in Fluor-reichen Lepidolith-Unterarten aus komplexen orogenen granitischen Pegmatiten und selten auch aus anorogenen, Amazonit-führenden Pegmatiten, auf.Spurenelement-Daten aus der Röntgenfluoreszenzanalyse von Lepidolith aus verschiedenen Pegmatit-Untertypen, die Morphologie (tafelig, schuppig, feinkörnig), die Position innerhalb des Pegmatits (primäre Zonen, verdrängte Einheiten, Taschen), Mineralbestände und tektonische Affinität (orogen gegen anorogen) zeigen eine extreme Fraktionierung von Rb und Cs, bescheidene Gehalte an TI, Ga, Nb, Ta, Sn und Zn; und typischerweise geringe Häufigkeiten von Ba, Sr, Ni, Pb, Y, V, W und Zr. Die extreme Fraktionierung wird durch niedrige Werte von K/Rb, K/Cs und Nb/Ta angezeigt, die in Lepidolith von Pegmatiten des Petalit-Subtyps am niedrigsten sind.Aus den verschiedenen Morphologien oder paragenetischen Positionen von Lepidolith sind keine systematischen Unterschiede im Spurenelementgehalt ersichtlich. Lepidolith aus Pegmatiten des Spodumen-Subtyps sind generell etwas weniger fraktioniert als jene von Pegmatiten des Petalit- oder Lepidolith-Subtyps.

Topaz: A Mineralogical Review

Abstract Coveted by mineralogists and gemologists alike, Novembers birthstone has been named the featured mineral at the 1995 Tucson Gem & Mineral Show.

Chemical composition of coexisting columbite-group minerals and cassiterite from the Black Mountain pegmatite, Maine

Detailed examination of hand samples and backscatter electron images of Nb-,Ta-,Sn-oxide minerals from the Black Mountain pegmatite has revealed the presence of discrete crystals, as well as cassiterite-hosted micrograins of columbite-group minerals (CGM). Micrograins and discrete grains of CGM show a wide range of Mn/(Mn + Fe) values, but only the micrograins have limited Ta/(Ta + Nb) values. Trace amounts of Ti, Sc, Fe3+ and Sn are common in both generations of CGM, but their total rarely exceeds 1.0 oxide wt%. The chemistry of the host cassiterite is relatively homogeneous and pure; the total concentration of FeO, MnO, Nb2O5, Ta2O5 and TiO2 rarely exceeds 2.0 wt%. While discrete CGM grains show weak progressive zonation and minor replacement, micrograins display complex oscillatory zonation, resorption and/or replacement textures. Textural evidence suggests that the micrograins crystallized after the formation of discrete grains of columbite and cassiterite. On the whole, both discrete crystals and micrograins of CGM show increasing Mn/(Mn + Fe) with fractionation as commonly observed in F-enriched granitic pegmatites. Micrograins of CGM typically show higher Ta/(Ta + Nb) values compared to their discrete crystal counterparts. Textural characteristics of CGM micrograins are inconsistent with an origin by exsolution, and instead, indicate primary precipitation.

CRYSTALLIZATION OF ‘POCKET’ BERTHIERINE FROM THE PULSIFER GRANITIC PEGMATITE, POLAND, MAINE, USA

A new occurrence of berthierine has been found in the Pulsifer granitic pegmatite, near Auburn, Maine. Berthierine was found in miarolitic cavities (‘pockets’) as aggregates of radial platy crystals with albite, as fracture-fillings in microcline and as fine-grained anhedral flakes with muscovite. Berthierine samples from all associations are depleted in Mg, but show variable enrichment in Mn. The textural relationships of the berthierine assemblage indicate that its formation is probably related to the dissolution of microcline in the presence of a low-temperature, neutral to alkaline, Fe-rich hydrothermal solution. This mode of occurrence is in direct contrast to the typical formation of berthierine in granitic pegmatites that is generally related to the alteration of cordierite-group minerals.