ABSTRACT Beryl (Be3Al2Si6O18) is a well-known mineral, most famously in its vivid green form of emerald, but also as a range of other colors. Prominent varieties of beryl aside from emerald include aquamarine, red beryl, heliodor, goshenite, and morganite. There has not been a significant amount of research dedicated to comparing the crystal-chemical differences among the varieties of beryl except in determining chromophoric cations. While the H2O content within structural channels of emerald has been explored, and the H2O content of individual beryl specimens has been studied, there has not yet been a study comparing the H2O content systematically across beryl varieties. In this study we consider single-crystal X-ray diffraction data and electron probe microanalyses of 80 beryl specimens of six primary varieties, to compare and contrast their crystal chemistry. Beryl cation substitutions are dominantly coupled substitutions that require Na to enter a structural channel site. The results indicate that with increasing Na content beryl varieties diverge into two groups, characterized by substitutions at octahedral or tetrahedral sites, and that the dominant overall cation substitutions in each beryl variety tend to be different in more than just their chromophores. We find that the relation between Na and H2O content in beryl is consistent for beryl with significant Na content, but not among beryl with low Na content. Natural red beryl is found to be anhydrous, and heliodor has Na content too low to reliably determine H2O content from measured Na. We determined equations and recommendations to relate the Na and H2O content in emerald, aquamarine, goshenite, and morganite from a crystallographic perspective that is applicable to beryl chemistry measured by other means. This research will help guide future beryl studies in classifying beryl variety by chemistry and structure and allow the calculation of H2O content in a range of beryl varieties from easily measured Na content instead of requiring the use of expensive or destructive methods.
Beryl (Be3Al2Si6O18) is a well-known mineral, most famously in its vivid green form of emerald, but also as a range of colors. Prominent varieties of beryl aside from emerald include aquamarine, red beryl, heliodor, goshenite, and morganite. There has not been a significant amount of research dedicated to comparing the crystal-chemical differences among the varieties of beryl except in determining chromophoric cations. While the water content of emerald has been explored, and the water content of individual beryl specimens has been studied, there has not yet been a study to compare the water content systematically across beryl varieties. In this study we consider single-crystal X-ray diffraction data and electron probe microanalyses of dozens of beryl specimens, of six primary varieties, to compare and contrast their crystal chemistry. Beryl cation substitutions are dominantly coupled substitutions that require Na to enter a structural channel site. The results indicate with increasing Na content beryl varieties diverge into two groups, characterized by substitutions at octahedral or tetrahedral sites, and that the dominant overall cation substitutions in each beryl variety tend to be different in more than just their chromophores. We find that the relation between Na and H2O content in beryl is consistent for beryl with significant Na content, but not among beryl with low Na content. Natural red beryl is found to be anhydrous, and heliodor has too low of Na content to reliably determine H2O content from measured Na. We determined equations and recommendations to relate the Na and H2O content in emerald, aquamarine, goshenite, and morganite from a crystallographic perspective that is applicable to beryl chemistry measured by other means. This research will help guide future beryl studies in classifying beryl variety by chemistry and structure, and allow the calculation of H2O content in a range of beryl varieties from easily measured Na content instead of requiring use of expensive or destructive methods.
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ABSTRACT Gadolinite, REE2FeBe2Si2O10, is a monoclinic orthosilicate member of the gadolinite supergroup of minerals and occurs in beryllium and rare earth element (REE) bearing granites, pegmatites, and some metamorphic rocks. Gadolinite from the White Cloud pegmatite, South Platte Pegmatite district, Colorado, USA, has been investigated and shows unusually variable REE compositions and distinct Be-Si disorder. Crystal structure and chemistry of two petrographically distinct gadolinite samples from this locality have been studied by electron microprobe chemical analysis, laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), single-crystal X-ray diffraction (XRD), and micro-Raman spectroscopy. Within these samples, the gadolinite was found to range from gadolinite-(Y) to gadolinite-(Ce). Regions of nearly full occupancy of Fe at the M site, and partial substitution of Si for Be at the Q tetrahedral site, as well as substitution of Be for Si at the T site were observed, with up to 15% vacancy at the Fe site and up to 15% disorder between Be and Si at distinct tetrahedral sites elsewhere. The layered nature of the crystal structure allows for large variation of the radius of the cation at the A site which contains the REE. This study shows that Be may substitute for Si and that Be may be more abundant in geochemical systems than previously assumed.
ABSTRACT Emerald is the most well-recognized beryl (Be3Al2Si6O18) variety, and although it has been extensively studied, a satisfactory method for quantifying the water content within the structural channels of the crystal lattice has yet to be proposed. Water is frequently present in the structural channels of beryl and can occur in two orientations (Type I and Type II). While spectroscopic methods are ideal for determining the orientation of the water molecules, measuring the overall water content often requires expensive or destructive analytical techniques. Sodium is necessary to charge-balance divalent cation substitutions at the Al site of beryl; it is also correlated with H2O in the structural channels, which typically occurs as Type II water. In this study, we present equations that can be used to easily calculate the H2O content of an emerald beryl with significant Na+ content based on either Na+apfu or Na2O weight percent. Unlike previous work, these equations are derived from single-crystal X-ray diffraction data which can be used to accurately measure both the Na+ and H2O contents. We checked the validity of the data using electron probe microanalyses for elements heavier than O. We compared the results with hypothetical scenarios in which different cation substitutions are prevalent, as weight percentages are variable based on the elemental contents. Our results indicate that Na+ or Na2O weight percent can be used to calculate H2O content in emerald beryl with reasonable accuracy, which will allow future researchers to use a simple calculation instead of expensive or destructive techniques when determining H2O content in emeralds.
Abstract Crystal-structure prediction is a challenging topic. Few models have been developed that use the chemical composition of a known compound to determine a complete crystal structure. A complete structural model should include all major bond lengths and angles, atomic coordinates, polyhedral volumes and distortions, and unit-cell parameters. The mineral beryl is used here to develop such a model. Beryl (Be3Al2Si6O18) is an ideal mineral to show that predicting the crystal structure using chemistry is possible: the framework structure is known, this structure has only two cation sites that experience substitutions, and these substitutions only minimally occur simultaneously. Vacant channel sites are involved in coupled substitutions, allowing alkali cations (typically Na+) to enter the structure, and the channel regularly contains molecular H2O correlated to Na content (Henry et al. 2022). The research employed single-crystal X-ray diffraction and electron probe microanalyses of 80 samples to create a model which was subsequently tested using 33 samples. Results show that the complete crystal structure of beryl can be accurately calculated using the Al-site average ionic radius (Al-SAIR) for octahedrally trending beryl, or the Be-site average ionic radius (Be-SAIR) for tetrahedrally trending beryl. Beryl for which Al-SAIR > (0.45 × Be-SAIR) + 0.414 is considered octahedrally trending and that for which Al-SAIR ≤ (0.45 × Be-SAIR) + 0.414 is considered tetrahedrally trending. Red beryl (differentiated by high Fe and Mn) exhibits a different trend, forming a subset of the octahedrally trending beryl. There is an upper limit to the predictable range of beryl structures of 0.604 Å Al-SAIR or 0.326 Å Be-SAIR. This model makes it possible to explore limitations on the crystal structure of beryl and the potential for unusual cation substitutions, or conversely, to compute the structure of a hypothetical pure endmember beryl. It is robust for true beryl (beryl for which Be and Al are the dominant non-Si cations) up to a high limit of cation substitutions, but not for other beryl-group minerals, including stoppaniite, bazzite, avdeevite, and johnkoivulaite. Future studies on beryl will be able to estimate basic crystal-structure features arising from standard chemical analyses as used in this research. It enables the creation of an extensive beryl database, aids comparisons of natural beryl to synthetics, and helps provide further guidance on provenance studies. It also invites future crystal-structure prediction research. This approach is applicable to broader fields, as crystal structures are linked to the physical characteristics of minerals and rocks in which they form.
Crystal structure prediction is a challenging topic.A model has not previously been developed that uses the chemical composition of a known mineral to determine the complete crystal structure, including all major bond lengths and angles, atomic coordinates, polyhedral volumes and distortions, and unit cell parameters.The mineral beryl is used here to create such a model.Beryl (Be 3 Al 2 Si 6 O 18 ) is an ideal mineral to prove that modeling the crystal structure using chemistry is possible: the base structure is known, it has two cation sites that experience substitutions, and these do not occur simultaneously.This research uses single-crystal X-ray diffraction and electron-probe micro-analyses on 80 samples to create the model.The model's validity is tested with 33 samples to verify that the measured structures fall within the predicted boundaries.Results indicate that a complete crystal structure of beryl can be calculated accurately using chemical composition by utilizing the average ionic radii of the measured cations within the Al-and Be-sites.Beryl can be separated into two categories: octahedrally and tetrahedrally trending.The full structure is predictable using the Al-site average ionic radii for octahedrally trending beryl, or the Be-site average ionic radius for tetrahedrally trending beryl.Red beryl (differentiated by high Fe and Mn) has a slightly different trend, forming a subset of octahedrally trending beryl.This model makes it possible to explore the limitations on the beryl structure and the possibility of unusual cation substitutions.It is robust for all true beryl up to a high limit of substitutions, but not for other beryl group minerals.This research is beneficial for beryl studies, which will be able to determine crystal structures during standard chemical analyses.It enables the creation of an extensive beryl database, aids comparisons of natural beryl to synthetics, and helps provide further guidance on provenance studies.It also invites future crystal structure prediction research.This approach is applicable to broader fields, as crystal structures are linked to the physical characteristics of minerals and rocks in which they form.
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