Compositional closure, spurious negative correlations in data sets of a fixed sum (e.g., fractions and percent), is often encountered in geostatistical analyses, particularly in mineralogy, petrology, and geochemistry. Techniques to minimize the effects of closure (e.g., log-ratio transformations) can provide consistent geostatistical results. However, such approaches do not remove these effects because closure does not result from mathematical operations but is an inherent property of the physical systems under study. The natural world causes physical closure; mathematics simply describes that closure and cannot alter it by manipulations. Here, we examine the distinct types of geologic systems and samples to determine in which situations closure (physical and mathematical) does or does not ensue and the reasons therefor. We parse compositional systems based on (1) types of components under study, immutable (e.g., elements) or reactive (minerals), and (2) whether the system is open or closed to component transfer. Further, open systems can be (1) displacive in which addition of a component physically crowds out others, or (2) accommodative in which addition or subtraction of components does not affect the others. Only displacive systems are subject to compositional closure. Accommodative systems, even with components expressed as percent or fractions, are not closed physically or, therefore, mathematically.

Critical and rare earth elements are in high demand for their increasing incorporation in modern technological devices for applications in the military, industrial, commercial, and consumer sectors. Round Top Mountain, a rhyolite laccolith in Sierra Blanca, west Texas, U.S.A. is a unique mineral deposit that offers opportunity for development of rare earth elements, especially the heavy rare earths, as well as associated critical elements. The main objective here is to evaluate the distances between accessory minerals of potential economic value (yttrofluorite, cryolite, uraninite, thorite, cassiterite, and columbite), and to major (potassium feldspar, albite, and quartz) and minor minerals (annite mi-ca, magnetite, and zircon). In this study we explore the proximity and clustering of these minor and accessory minerals, at the mi-cron-to-millimeter scale, from mineral maps constructed in a previous application of ArcGISTM tools to electron probe microanal-ysis (EPMA) element maps. Our goal is to determine whether specific minerals cluster spatially and, if so, at what distances. We noted that the high-value target yttrofluorite grains often neighbor potassium feldspar and quartz grains, but less commonly magnetite and mica grains. With regard to cluster analysis, most minor and accessory minerals were found to group together at small scales (low micrometer) and were dis-persed or random at larger (up to 1 mm) distances.

Rare earth elements (REEs), especially heavy rare earth elements (HREEs), are in demand for their current and emerging applications in advanced technologies. Here we perform computer-driven micro-mapping at the millimeter scale of the minerals that comprise Round Top Mountain, in west Texas, USA. This large rhyolite deposit is enriched in HREEs and such other critical elements as Li, Be, and U. Electron probe microanalysis of 2 × 2 mm areas of thin sections of the rhyolite produced individual maps of 16 elements. These were superimposed to generate a 16-element composition at each pixel. Principal components analysis of elements at each pixel identified the specific mineral at that site. The pixels were then relabeled as the appropriate minerals, thereby producing a single mineral map. The overall mineral composition of the 7 studied samples compared favorably with prior analyses of the Round Top deposit available in the literature. Likewise the range of porosity in the maps was consistent with that of previous direct measurements by water saturation. This new statistical and GIS-based technique provides a robust and unbiased approach to electron microprobe mapping. The study further showed that the high-value yttrofluorite grains exhibited little tendency to cluster with other late-stage trace minerals and that the samples extended the previously documented overall homogeneity of the deposit at field scale to this microscopic scale.

Elemental characterization of fine particulate matter was undertaken at schools and residences in three low income neighborhoods in Quito, Ecuador. The three zones were located in the northern (Cotocollao), south central (El Camal), and south east (Los Chillos) neighborhoods and were classified as zones 1–3, respectively. Forty elements were quantified via ICP-MS analysis. Amongst the geogenic elements, the concentration of Si was the most abundant followed by S, Al, and Ca. Elements with predominantly anthropogenic sources such as Zn, V, and Ni were higher in zone 3 school followed by zone 2 and zone 1 schools. Enrichment factors were calculated to study the role of crustal sources in the elemental concentrations. Geogenic elements, except K, all had values <10 and anthropogenic elements such as Ni, V, Zn, Pb, As, Cr had >10. Principal Component Analysis suggested that Ni and V concentrations were strongly attributable to pet coke and heavy oil combustion. Strong associations between As and Pb could be attributed to traffic and other industrial emissions. Resuspended dust, soil erosion, vehicular emissions (tailpipe, brake and tire wear, and engine abrasion), pet coke, heavy oil combustion, and heavy industrial operations were major contributors to air pollution.

The combination of global warming and urbanization is expected to exacerbate the well-established urban heat island effect. Unfortunately, little is known about nighttime urban heat retention and its possible health effects on residents. Here we use infrared satellite imagery to evaluate the distribution of nighttime heat and its decadal changes in a large desert metroplex.

Separation of target elements or minerals from their host rock or ore is essential to successful mining operation. The inevitable loss of a portion of the desired material that accompanies each step in the extraction process must be documented to develop the operational protocol. Superposition of the characteristic X-ray fluorescence spectra of head (crushed rock ore particles, pre-processing) and tail (post-processing particles) samples provides a direct visual comparison of relative peak sizes, and thereby the relative concentrations, of elements of interest. If the head and tail peaks are identical, none of the element was recovered in the extraction process. At the other extreme if the tail peak “flat lines”, i.e., there is no peak, there was 100% recovery of that element. Standardless visual comparison is valid if the same mass of identical starting material is incorporated into the head and tail sample analysis pucks, and XRF analytical conditions are identical. The considerable time and expense of acquiring and calibrating the standards associated with XRF analysis of 75 or more elements are avoided, a significant advantage during initial broad screening of an experimental extraction procedure. Full quantitation by XRF or an alternate technique can proceed at a later project stage, if desired. The approach retains and presents all features of the original data, thus eliminating questions about data quality, standards and their calibration, and data manipulation in processing from raw counts to concentrations in printout tables. This form of display is ideal for both the mining professional and such less technical groups as corporate staff, investors, regulators, and the public. Examples presented herein are for heap leaching; the protocol can be applied as well to any of the other traditional ore processing and beneficiation procedures, e.g., gravity concentration, magnetic and electrical separation, froth flotation, and ore sorting.

Water-saturation porosity and dye-penetration permeability measurements of Round Top Mountain rhyolite confirm that a ½-inch (13-mm) crush size would permit efficient acid heap leaching of yttrium and heavy rare earth elements (YHREEs) hosted in yttrofluorite, a YHREE-substituted variety of fluorite. Laboratory acid leaching has extracted up to 90% of the YHREEs. The bulk insoluble gangue mineralogy of the rhyolite, 90% to 95% quartz and feldspars, assures low acid consumption. Different crush sizes were weighed, soaked in water, and reweighed over time to determine water-penetration estimated porosity. Typical porosities were 1% to 2% for gray and 3% to 8% for pink varieties of Round Top rhyolite. The same samples were re-tested after soaking in dilute sulfuric to simulate heap leaching effects. Post-leach porosity favorably increased 15% in pink and 50% in gray varieties, due to internal mineral dissolution. Next, drops of water-based writing ink were placed on rhyolite slabs up to ~10 mm thick, and monitored over time for visual dye breakthrough to the lower side. Ink penetration through 0.5 to 2.5-mm-thick slabs was rapid, with breakthrough in minutes to a few hours. Pink rhyolite breakthrough was faster than gray. Thicker slabs, 4 to 10 mm, took hours to three days for breakthrough. Porosity and permeability of the Round Top rhyolite and acid solubility of the yttrofluorite host should permit liberation of YHREEs from the bulk rock by inexpensive heap leaching at a coarse and inexpensive nominal ½-inch (13-mm) crush size. The rate-limiting step in heap leach extraction would be diffusion of acid into, and back-diffusion of dissolution products out of, the crushed particles. The exceptional porosity and permeability that we document at Round Top suggest that there may be other crystalline rock deposits that economically can be exploited by a coarse-crush bulk heap leach approach.

Round Top Mountain in Hudspeth County, west Texas, USA is a surface-exposed rhyolite intrusion enriched in Y and heavy rare earth elements (HREEs), as well as Nb, Ta, Be, Li, F, Sn, Rb, Th, and U. The massive tonnage, estimated at well over 1 billion tons, of the deposit makes it a target for recovery of valuable yttrium and HREEs (YHREEs), and possibly other scarce elements. Because of the extremely fine grain size of the mineralized rhyolite matrix, it has not been clear which minerals host the YHREEs and in what proportions. REE-bearing minerals reported in the deposit included bastnäsite-Ce, Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Ca-Th-Pb fluoride, and possibly ancylite-La and cerianite-Ce. Extended X-ray absorption fine structure (EXAFS) indicated that virtually all of the yttrium, a proxy for the HREEs, resided in a coordination in the fluorite-type crystal structure, rather than those in the structures of bastnäsite-Ce and xenotime-Y. The YREE grade of the Round Top deposit was just over 0.05%, with 72% of this consisting of YHREEs. This grade was in the range of the South China ionic clay deposits that supply essentially all of the world's YHREEs. Because the host Y-bearing fluorite is soluble in dilute sulfuric acid at room temperature, a heap leaching of the deposit appeared feasible, aided by the fact that 90%–95% of the rock consists of unreactive and insoluble feldspars and quartz. The absence of overburden, remarkable consistency of mineralization grade throughout the massive rhyolite, proximity (a few km) to a US interstate highway, major rail systems and gas and electricity, temperate climate, and stable political location in the world's largest economy all enhanced the potential economic appeal of Round Top.

An examination of a specimen of aguilarite from the type locality provides new data on the chemistry and structure of this mineral. The chemical formula of the crystal used for the structural study is (Ag3.98Cu0.02)(Se0.98S0.84Te0 18), on the basis of 6 atoms. The mineral was found to be monoclinic, crystallizing in space group P2(1)/n, with a = 4.2478(2), b = 6.9432(3), c = 8.0042(5) angstrom, beta = 100.103(2)degrees, V = 232.41(2) angstrom(3) and Z = 4. The crystal structure [refined to R-1 = 0.0139 for 958 reflections with I> 2 sigma(I)] is topologically identical to that of acanthite, Ag2S. It can be described as a body-centred array of tetrahedrally coordinated X atoms (X = S, Se and Te) with Ag2X3 triangles in planes nearly parallel to (010); the sheets are linked by the Agl silver site, which has twofold coordination.Aguilarite is definitively proved to be isostructural with acanthite; it does not have a naumannite-like structure, as previously supposed. Our data support the hypothesis that there are two solid solution series in the system: a monoclinic `acanthite-like' series (from Ag2S-Ag2S0.4Se0.6), and an orthorhombic 'naumannite-like' series (from Ag2S0.3Se0.7-Ag2Se). This is supported by data gathered on synthetic counterparts. Aguilarite remains as a valid as a mineral species, but it should be described as the Se-analogue of acanthite.In this study we also (1) review the history of the aguilarite; (2) compare properties of synthetic and natural aguilarite; and (3) demonstrate how earlier researchers erred in describing aguilarite as orthorhombic.The Te-bearing composition of the studied aguilarite crystal suggests the possibility of a solid solution with cervelleite (Ag4TeS).

En el primero de los estudios individuales que comprende este capitulo, las concentraciones del aire son presentadas para cuatro metales toxicos—cobre (Cu), plomo (Pb), arsenico (As), y cromo (Cr)—basados en la materia particulada (PM) atrapada en filtros de ocho estaciones de muestreo en la cuenca atmosferica Paso del Norte en 1994, 1995, y 1996. Se eligio un dia representativo de bajos vientos en cada estacion (primavera, verano, otono, e invierno) para el analisis por espectrometria de masas (ICP-MS) acoplado con plasma. El conjunto de datos resultante revela ambas tendencias geograficas y estacionales de variacion en metales toxicos aerotransportados. Se encontro que las concentraciones de cobre, plomo, cromo, y arsenico aumentan durante el otono e invierno, con relacion a la primavera y el verano. Se cree que inversiones atmosfericas en niveles del suelo durante las estaciones mas frias en esta region desertica atrapan particulas antropogenicas al prevenir la mezcla vertical. Niveles elevados de metal caracterizaron a las estaciones de prueba en el nucleo urbano, con valores inferiores en muestras distantes. Esto sugiere la importancia de las fuentes locales de los metales. El capitulo tambien detalla un estudio en el cual se encontro que los niveles de plomo, cobre, y arsenico en PM en la cuenca atmosferica de El Paso-Ciudad Juarez exhiben disminuciones significativas en el pasado cuarto de siglo. Estas tendencias generales son evidentes aun en un conjunto de datos limitado de pruebas de 1977, 1980, 1987, 1994, y 2001. Las disminuciones de plomo son dramaticas y una contribucion significativa para la salud publica de la comunidad binacional El Paso- Ciudad Juarez. El capitulo detalla sobre la coleccion de muestras del suelo de areas que rodean diversas instalaciones en El Paso que son fuentes puntuales potenciales (Historicas y actuales) de contaminacion por metales. Las areas de interes incluyen a la American Smelting and Refining Company (Asarco) en el lado del oeste de El Paso, el Parque Memorial en el centro de El Paso (donde anteriormente estaba ubicada la fundicion Federal), y la refineria de cobre Phelps Dodge en el lado Este de El Paso. Muestras de suelo fueron tambien obtenidas de areas en las afueras al Norte y Este de El Paso en un esfuerzo por observar los efectos de la distancia con fuentes urbanas. Las pruebas de suelo fueron preparadas y analizadas para arsenico, bario, calcio, cadmio, cobre, cromo, plomo, niquel, selenio, y zinc. Los valores para el arsenico, plomo, cobre, y cromo son presentados en mapas del area de El Paso y se obtuvieron las correlaciones entre las concentraciones de los diversos metales. Las concentraciones de arsenico, plomo, cobre, y cromo fueron las mas altas en el area alrededor de la fundicion Asarco. Concentraciones superiores fueron observadas en las muestras superficiales (2.5 centimetros [cm]) que en las de profundidades mayores. El area del Parque Memorial exhibio niveles ligeramente elevados de estos metales en relacion con valores anteriores urbanos de El Paso. Las pruebas de la region de la refineria Phelps Dodge fueron generalmente indistinguibles de los valores anteriores de referencia. Finalmente, la distribucion geografica de plomo en suelos en El Paso se presenta en mapas basados en mas de 300 muestras del suelo obtenidas en la region. El uso de los mapas permite resaltar la distribucion de plomo en el nivel vecindad, y resta importancia a cualquier nivel anomalo elevado asociado a una estructura o casa individual. Los niveles de la plomo son mas altos en el distrito comercial del centro de la ciudad; en el area adyacente hacia el Este, que comprende una vieja central de negocios, de transporte, y un complejo de industria ligera; y hacia el oeste en el area de la fundicion Asarco. La continuidad de esta zona y la edad de sus estructuras dificultan el diferenciar fuentes de plomo. Los valores de plomo disminuyen sistematicamente fuera de esta zona urbana nucleo, con los niveles mas bajos generalmente encontrados en las comunidades y desarrollos perifericos poco poblados. Esta distribucion geografica de plomo en el suelo es consistente con medidas de plomo reportadas en PM tomada de cuatro estaciones de monitoreo de aire durante los 1990s. Los datos del terreno pueden asi complementar estudios de aire al proveer una red geografica esencialmente infinita de sitios de muestreo que, con exactitud variable, registran e integran condiciones del aire a traves de anos y decadas.