Aaron Palke

Aaron Palke
Gemological Institute of America

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71
Publications
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402
Citations

Publications

Publications (71)
Article
Full-text available
Iron oxide inclusions and exsolution lamellae in rainbow lattice sunstone (RLS) from Harts Range, Australia, are examined using optical and electron microscopy and single-crystal X-ray diffraction (SC-XRD). Laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) analyses show a bulk composition of An1.4Ab14.8Or83.0Cn0.8 with < 200 p...
Article
The color of most garnets is derived from multiple color‐producing elements (chromophores). In this study we selectively isolate the ultraviolet–visible–near infrared (UV–Vis–NIR) spectra of four major chromophores (Fe2+, Mn2+, Cr3+ and V3+) by collecting and extrapolating the spectra of four representative pyrope‐almandine‐spessartine (pyralspite)...
Article
Full-text available
The optical properties (scattering, absorption and extinction) of spheroidal Cu particles embedded in intermediate plagioclase feldspar are computed for various sizes and shapes using the Mie theory and T-matrix method. The observed color for Cu-bearing plagioclase, as a function of particle size and shape, is also calculated from the computed exti...
Article
Full-text available
This study compares two purple gem spinel samples with regard to trace element chemistry and visible light absorption spectra, and presents the first reported example of a spinel with a saturated purple color caused predominantly by chromium and cobalt. Sample VN, from a placer deposit in the Luc Yen district of Vietnam, exhibited strong saturation...
Article
A new mineral of the beryl group, johnkoivulaite, Cs(Be2B)Mg2Si6O18, was recovered from the gem gravels of the Pein Pyit area of the Mogok region in Myanmar. Thus far, only a single crystal has been identified. It has dimensions about 5.8×5.7×5.5 mm. This specimen has an irregular shape but still has discernible crystal form with geometric growth p...
Article
Full-text available
Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem; it can also provide a method to quickly distinguish different species within a composite gem. This article provides a summary of this relatively new technology and its instrument...
Article
Full-text available
The red variety of corundum owes its color and strong fluorescence to the presence of Cr, as well as traces of Fe. The latter can reduce the fluorescence and thus impact the appearance of the final gem. Gem quality rubies are rarely available for scientific study and even less common in their rough form. Opaque inclusions in rubies are often remove...
Article
Full-text available
Gem corundum deposits are typically divided into blue sapphire and ruby deposits. However, this classification often overlooks the fact that the precious stones produced are the same mineral with only an overall slight difference in their trace element profiles. It can take only a couple thousand ppm chromium to create the rich, red color expected...
Article
Ethiopia, traditionally known for opal, has become an important source for emerald and sapphire. After these significant discoveries, a new type of Cu-bearing sunstone feldspar, first shown in 2015 to Tewodros Sintayehu (Orbit Ethiopia Plc.), was discovered in the Afar region (L. Kiefert et al., “Sunstone labradorite-bytownite from Ethiopia,” Journ...
Article
Yogo Gulch in central Montana is one of the most important gem deposits in the United States. Although very little material has been recovered there in recent years, it has produced several million carats of rough sapphire over the course of its history (Voynick, 2001). These stones, known for their vibrant untreated blue color and high clarity, ha...
Article
In studies of the structures of silicate minerals and glasses, 29Si NMR spectroscopy has been applied almost exclusively to materials containing relatively low concentrations of ions with unpaired electrons spins, such as most transition metals and rare earths, because of sometimes severe broadening and shifting of resonances in such strongly param...
Article
A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat re...
Article
Pleochroism plays an important role in determining the face-up visual color appearance of faceted, optically anisotropic (non-cubic) gemstones. One area that has received little attention is the interplay between pleochroism and the so-called alexandrite effect wherein the perceived color of a mineral changes with different lighting conditions (i.e...
Article
Trace-element chemistry and microscopic observations of included gem corundum (α-Al2O3) suggests a new model of syngenetic growth of oriented rutile inclusions rather than the usual interpretation of their growth through exsolution. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is now a robust method for measuring trace el...
Article
Most applications of high-resolution NMR to questions of short-range order/disorder in inorganic materials have been made in systems where ions with unpaired electron spins are of negligible concentration, with structural information extracted primarily from chemical shifts, quadrupolar coupling parameters, and nuclear dipolar couplings. In some ca...
Article
We report here compositions of glassy melt inclusions hosted in sapphires (gem quality corundum) from three alluvial deposits in Montana, USA including the Rock Creek, Dry Cottonwood Creek, and Missouri River deposits. While it is likely that sapphires in these deposits were transported to the surface by Eocene age volcanic events, their ultimate o...
Article
Gem corundum (sapphire) has been mined from an ultramafic lamprophyre dike at Yogo Gulch in central Montana for over 100 years. The sapphires bear signs of corrosion showing that they were not in equilibrium with the lamprophyre that transported them; however, their genesis is poorly understood. We report here the observation of minute glassy melt...
Article
Full-text available
Pyrope-rich garnet is an important "sink" for trivalent transition metals such as Cr^3+, V^3+, and Fe^3+ in Earth's upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, 27^Al and 29^Si MAS NMR spectra were collected on synthetic crystals of composition Mg3(Al0...
Article
Full-text available
A type of pink pyrope garnet containing vanadium and chromium, believed to have been mined in Tanzania, appeared at the 2015 Tucson shows. The material shows a noticeable color difference from purplish pink under incandescent light (A) to purple under daylightequivalent light (D65). This study reports a quantitative analysis of the difference in co...
Conference Paper
Full-text available
Cation Order-Disorder Behavior in Silicate Garnet Solid Solutions Most rock-forming silicates are substitutional solid solutions and extensive research has been done to determine their cation order-disorder behavior. In the case of aluminosilicate garnet (X3Al2Si3O12 with X = Mg, Fe^2+, Mn^2+ and Ca) both synthetic and natural solid-solution cryst...
Article
Full-text available
An attempt is made for the first time to measure the local (short-range) distribution of the X-cations in a ternary almandinepyrope-grossular garnet solid solution using paramagnetically shifted NMR resonances. Initial 27Al MAS NMR measurements made on an almandine-rich garnet of composition Alm45Prp42Grs13 proved to be uninterpretable because of t...
Poster
Full-text available
The Local Structural State of a Kimberlitic Grospydite Garnet using Paramagnetically Shifted 27^Al and 29^Si MAS NMR Resonances C.A. Geiger1, A.C. Palke2, and J.F. Stebbins2 1Materials Science and Physics, Salzburg University, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria 2Geological and Environmental Sciences, Stanford University, Stanford, C...
Article
In marked contrast to the single, narrow 29Si MAS NMR resonance for pure forsterite (Mg2SiO4), the spectra for synthetic forsterite containing 0.05 to 5% of the Mg2+ replaced with Ni2+, Co2+, or Fe2+ display between 4 and 26 additional, small, paramagnetically shifted peaks that are caused by interactions of the unpaired electron spins on the trans...
Article
Full-text available
A suite of Fe-bearing natural and synthetic grossular-rich [(Ca,Fe)3(Al,Fe)2Si3O12] and pyrope-rich [(Mg,Fe)3Al2Si3O12] garnets were investigated using 27^Al and 29^Si MAS NMR and 57^Fe Mössbauer spectroscopy. This was done to study the state of cation order-disorder in garnet solid solutions by analyzing paramagnetically shifted resonances in high...
Poster
Full-text available
Most rock-forming silicates are substitutional solid solutions. The distribution of their cations affects a number of properties such as thermodynamic behavior. For aluminosilicate garnet solid solutions (X3Al2Si3O12 with X = Mg, Fe^2+, Mn^2+ and Ca) crystals have random long-range X-cation disorder in space group Ia-3d. It is assumed, as with most...
Article
We present (31)P magic angle spinning nuclear magnetic resonance spectra of flux-grown solid solutions of La1-xCexPO4 (x between 0.027 and 0.32) having the monoclinic monazite structure, and of Y1-xMxPO4 (M = V(n+), Ce(3+), Nd(3+), x between 0.001 and 0.014) having the tetragonal zircon structure. Paramagnetically shifted NMR resonances are observe...
Article
We present 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of Al- and Fe-bearing, high-pressure pyroxene and perovskite samples, synthesized in a multi-anvil apparatus at 26 GPa and 1900 °C at targeted compositions of (Mg1−xFex)(Si1−xAlx)O3 (x = 0.01, 0.025, and 0.05). 27Al MAS-NMR spectra of the perovskite samples i...
Article
The boundary between the Earth's upper and lower mantle is generally attributed to the decomposition of (Mg,Fe)2SiO4 ringwoodite into ferropericlase and perovskite-structured (Mg,Fe)SiO3, hereafter perovskite. Concomitant with this phase change is the more gradual disappearance of a separate Al-bearing phase (majoritic garnet) and dissolution of Al...
Article
We present the results of a variable-temperature (VT) 31P magic angle spinning NMR (MAS-NMR) study of a series of solid solutions between different synthetic rare earth (RE = Y, La, Ce, Pr, Nd, Eu, Dy) orthophosphates (REPO4) taking either the monoclinic monazite or tetragonal xenotime (zircon) crystal structure. Solid solutions were formed by mixi...
Article
We present variable-temperature (VT, 31 to 204 °C) 29Si magic angle spinning NMR (MAS-NMR) spectra of a synthetic forsterite containing dilute paramagnetic impurities and 27Al and 29Si spectra of two iron-bearing natural pyrope garnet specimens from the Dora Maira ultrahigh-pressure metamorphism locality. Previous NMR studies of these materials rep...
Article
The analysis of transparent conducting oxide nanostructures suffers from a lack of high throughput yet quantitatively sensitive set of analytical techniques that can properly assess their electrical properties and serve both as characterization and diagnosis tools. This is addressed by applying a comprehensive set of characterization techniques to...
Article
Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) has long been used to obtain structural information of geologically important minerals and glasses. The presence of paramagnetic ions such as Fe2+, Fe3+, Co2+, Ni2+, and rare earth elements may lead to ``paramagnetically-shifted'' NMR peaks. Previous work on 27Al and 29Si NMR on natural pyro...
Article
Full-text available
The use of ZnO nanowires has become a widespread topic of interest in optoelectronics. In order to correctly assess the quality, functionality, and possible applications of such nanostructures it is important to accurately understand their electrical and optical properties. Aluminum- and gallium-doped crystalline ZnO nanowires were synthesized usin...
Article
The 2005-2006 eruption along a well-surveyed part of the East Pacific Rise (EPR) at 9°50"N, and the subsequent observations and collection of the flow (Soule et al., 2007) provide a rare opportunity to study how submarine basalts vesiculate as they ascend beneath the mid-ocean ridge (MOR) and are emplaced at the surface. We have measured dissolved...
Article
Lake Tanganyika is the oldest and largest of the East African rift lakes and vital to the economy of the surrounding villages and countries. Deforestation around the lake causes many negative effects from an increased sediment flux and has changed dramatically as a function of population. Increases in inorganic sediment flux from deforested watersh...
Article
When lavas ascend and erupt along mid-ocean ridges, they are usually oversaturated with dissolved volatiles (especially CO2) because of the pressure dependence of volatile solubility. If the lava is not quenched to glass immediately after eruption, CO2 may exsolve into vesicles and/or be lost into seawater. This process is time dependent and could...

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Projects (3)
Project
The solid Earth consists for the most part of minerals and rocks, but fluids, glasses, melts and other noncrystalline substances are also found and they play an important role in a number of geochemical and geophysical processes. The mineral sciences and the field of geochemistry are greatly concerned with investigating the nature of all geomaterials. Indeed, one wants to describe and understand their fundamental chemical and physical properties and also their behaviour under different physical conditions. In many cases a level of scientific understanding is best achieved when the atomistic-scale properties and interactions can be described or characterised. This is, for example, the case for investigating the adsorption behaviour of molecules or atoms on the surfaces of minerals or in studying the physical nature of viscosity of a silicate melt. Ultimately, it is the atomistic-scale properties that control the bulk macroscopic properties of a material and, thus, they have to be characterised and understood. One is interested in both the static and dynamic behaviour of atoms and molecules and their energetic properties and interactions with one another. This is where spectroscopy enters the picture, because spectroscopic measurements can provide local or atomistic-level information on a variety of different materials, whether they are gas, liquid or solid phase. This information can be compositional, structural, crystal chemical as well as dynamical. The traditionally trained mineralogist, petrologist or geochemist was instructed to use an optical microscope, a powder X-ray diffractometer and a couple of different devices for analytical chemistry determinations, such as the electron microprobe or an X-ray fluorescence unit, to characterise a rock or mineral, for example. Times have changed and this no longer suffices. Today, the research problems and directions are often different from those of the past and new approaches and ‘tools’ for study are required. Here, the development and application of various spectroscopic methods over the last approximately 30 years is most notable. The number of spectroscopic methods that are presently available to mineral scientists and geochemists is staggering. There has been both a birth of new spectroscopic techniques and rapid advances and refinements of the older more traditional methods (e.g., IR, Raman, XAS). Both developments are having a major influence on the type of investigations undertaken on a wide variety of geomaterials and, in addition, under different physical conditions. The range of methodologies and the types of investigations are so great that it is sometimes difficult to decide on what kind of spectroscopic measurement should be made to address a scientific question. In general, many spectroscopic methods deliver chemical or structural information that is narrow or specific in scope in comparison to the important X-ray diffraction experiment used in structural investigations of crystalline materials, for example. However, the information that they can deliver can be very quantitative and unique. For example, 29Si MAS NMR spectroscopy can be used to determine quantitatively the short-range distribution of Si atoms in silicates or the type of SiO4 polymerisation in silicate melts. Such information cannot be easily obtained by nearly any other experimental method. On the other hand, NMR spectroscopy can only be applied to nearly paramagnetic ion free substances and in the case of crystals one needs to know the crystal structure before a spectrum can be fully interpreted. A second example involves IR spectroscopy. If one wants to determine small concentrations of structural H2O or OH- in a nominally anhydrous mineral or in a silicate glass and also their energetic interactions with the environment, then IR spectroscopy is essentially unique in its ability to provide such information. Indeed, in the case of noncrystalline materials like gases or fluids or even nanoparticles, spectroscopy offers the best way to characterise their physical and chemical properties. A difficulty for the beginner is to decide what method should be used or how one should start a spectroscopic investigation. Experience shows that, in general and in many cases, the different spectroscopic methods should be used in a complementary fashion (Calas & Hawthorne, 1988). These authors state “one can view the different spectroscopies … , as a series of tools that one uses to solve or examine a problem of interest; a single tool is generally not sufficient for ones needs – you cannot drive a nail and drill a hole with just a hammer”. It can be stated further that one cannot build a house with just a saw. Hence, if the goal is to understand the physical and chemical nature of some material or a system of phases (e.g., a rock or a mineral-fluid interface) in a complete sense, one must apply a number of different techniques. Ultimately, one wants to understand the properties of a material or a physicochemical system from the atomistic level through the nano and microscopic scale up to the macroscopic state. In addition, the atomistic-level dynamic properties sometimes require a description over different time scales. Needless to say, there remains much work to be done in the Earth Sciences. Observed in a historical context, the general scientific problem has not changed greatly with time. The ancient Greeks struggled to understand the nature of matter, as did Kepler almost 2000 years later (Schneer, 1995). Today, mineral scientists and geochemists are still struggling to understand why various geomaterials behave the way they do under certain geologic conditions or why they display the properties that characterise them.
Project
A main research thrust of the mineral sciences lies in investigating the thermodynamic, crystal-chemical, and physical properties of minerals. In terms of the thermodynamic properties of end-member phases, a number of different compilations listing the standard functions Cp, V°, S°, and deltaHf° can be found (e.g., Holland and Powell, 2011). Crystal-chemical and other properties (e.g., compressibility, thermal expansion, magnetic and electronic, etc.) are given in a number of books and monographs (e.g., AGU ¬- Mineral Physics and Crystallography: A Handbook of Physical Constants and various MSA Reviews of Mineralogy and Geochemistry). It is a fact, though, that most rock-forming silicates are not compositionally simple end members, but are substitutional solid solutions, and it is imperative to investigate their thermodynamic, crystal-chemical and physical properties. In terms of the composition of many minerals, the element Fe is important. Because of its different electronic states (+2 and +3, as well as mixed valence charge-transfer states), Fe gives rise to complex and interesting behavior in minerals. The importance of the exchange of Fe2+ and Mg cations, as in many rock-forming silicate groups, has been recognized for many years (e.g., Ramberg, 1952) and much work has been done. However, a complete and precise understanding of the thermodynamic and physical properties (e.g., elastic constants and compressibility) of Fe2+-Mg silicate solid solutions is still not at hand. Microscopic- or atomistic-scale aspects determine the macroscopic behavior of minerals (Navrotsky and Kieffer, 1985; Geiger, 2001). It follows that electronic and magnetic states arising from Fe can significantly affect thermodynamic (e.g., heat capacity) and other properties (e.g. compressibility). However, in the mineral sciences, not much experimental research has been done and several areas are not well understood. Indeed, it can be argued, in an even broader scientific sense that an understanding of the relationships between microscopic crystal-structure properties and macroscopic thermodynamic behavior of minerals is still in its beginning phase. These issues are important and not just strictly for academic reasons, as applied aspects are also involved. Namely, if a more fundamental understanding of various Earth processes is to be achieved, whether they involve metamorphism, mantle melting, or deep earthquakes, to name but three, more quantitative research in terms of the thermodynamic, crystal-chemical and physical properties of rock-forming minerals is necessary.
Project
. To determine the primary origin and age of some alluvial gem quality Sapphire associated with alkali volcanism in Upper Benue Trough NE Nigeria To determine the condition of formation of the sapphire-zircon-spinel megacrysts associations. To develop a model for the origin of sapphire-zircon-spinel megacrysts associations in a rift setting which will then be evaluated within various genetic models for basalt-related alluvial sapphire-zircon-spinel megacrysts fields in a global context.