Activation possibilities and geochemical correlations of photoluminescing carbonates,particularly calcites

We studied carbonates of mainly postmagmatic origin. As a result of extensive measurements of the absorption spectra, light of λ=253 nm of a Xenon lamp was used for excitation. The emitted luminescence was measured by a photometer. Luminescence of considerable intensity is exhibited by calcites, aragonites, smithsonites, strontianites as well as by witherites and cerussites. Calcites have the most strongly differentiated spectrum. Each of their five different luminescence types is caused by a special activation. Particularly characteristic are calcites, whose luminescence is activated (a) by lattice defects and/or Zn-contents (broad emission band with maximum at about 420 nm), (b) by Mn/Pb-substitution (maxima at 325 and 420 nm), (c) by REE (maxima at 340–360, 480, 545, 575, 623 and 670 nm). The other, also luminescing carbonates, show a close relationship to the calcite of the ‘fundamental type’ (a) but with varying position of their emission maxima. Also most of the phosphorescing samples of calcite are of this type (a). The majority of these calcites originate from telethermal, surface-near formation regions and from resolutions, respectively. Fe, Ni, and in the case of smithsonite also Cu, are ‘killing elements’, which are less efficient by REE-activation than by Mn/Pb-activation. In particular phosphorescence and activation by lattice defects will be disturbed. The luminescence analysis represents a possibility for mineral identification. Furthermore it gives criteria for chemical homogenity of minerals, and locates concentrations of certain elements in inhomogeneous samples. For the most important activators it is a tool for qualitative element detection, especially in the case of REE.     

NMR-spectroscopy of synthetic potassium-bearing cordierites

Potassic cordierites with the chemical composition K _x Mg₂Al_4+x Si₅–xO₁₈ (x = 0.00, 0.10, 0.20, and 0.25) were synthesized by annealing glasses at 1290° C for different lengths of time. The procedure resulted in cordierites with different states of Al,Si-order for the tetrahedral sites in the structure. The dependence between the potassium-content and the state of order on one side and between annealing time and the state of order on the other side was then studied using ²⁹Si MAS nuclear magnetic resonance (NMR) spectroscopy. The spectra show that the state of order is a continuous function of annealing time for all compositions considered, but the rate of ordering decreases with increasing K-content. Since the substitution K+Al Si leads to higher Al/Si-ratios; the lower rate of ordering is discussed as a consequence of changed statistics for Al, Si site exchanges. The Al atoms replacing silicon in the structure to balance the charge of potassium cations are not located close to the potassium ion but at a maximum distance from it. This is shown to be a consequence of an improvement in coordination of all oxygen atoms in the cordierite framework.     

Towards an understanding of the Of?p star HD 191612: optical spectroscopy

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe–O8fp). The Balmer and He  i lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. He  ii absorptions and metal lines (including many selective emission lines but excluding He  ii λ4686 Å emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with   P _orb= 1542 d, e = 0.45  . We conduct a model-atmosphere analysis of the spectrum, and find that the system is consistent with a ∼O8 giant with a ∼B1 main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying 'clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.     

Drought adaptation policy development and assessment in East Africa using hydrologic and system dynamics modeling

Drought is a natural disaster that affects millions of people across the globe. Lack of rainfall reduces crop yields and livestock productivity and, in turn, food availability and income. In developing countries, these effects are even more detrimental. As droughts become more frequent, adaptation is a fundamental concern for countries and their policymakers. To support a development of drought adaptation policies, a combined hydrologic and system dynamics model was developed for a region in East Africa, focused on the Horn of Africa (i.e., a region bordering Kenya, Somalia, and Ethiopia), an area that has endured multiple droughts in the last few decades. The model simulates the interdependencies between water availability, land degradation, food availability, and socioeconomic welfare. The impacts of new adaptation policies on the region were evaluated over a 10-year simulation period using historical weather data. It was found that a combination of increased hydraulic infrastructure and innovative agricultural practice policy can reduce domestic water deficits by 54–100 % while increasing the income per capita up to 285 % over the 10 years. Innovatively combining hydrologic and systems dynamic modeling produces a realistic simulation of water scarcity and the effects on natural systems. Implementation of policies within the model aids the selection process by evaluating multiple options, quantifying the effectiveness the policies have on individual stakeholder livelihood (i.e., pastoralist, agro-pastoralists, and farmers), and analyzing the overall outcome to ensure equitable costs and benefits among the stakeholders.     

NMR,XRD and IR study on microcrystalline opals

Microcrystalline opal-CT and opal-C were investigated by ²⁹Si MAS NMR and ²⁹Si {¹H} cross polarisation MAS NMR spectroscopy, X-ray small angle scattering, X-ray powder diffraction and infrared absorption spectroscopy. The results are compared with those for non-crystalline precious opal (opal-AG), non-crystalline hyalite (opal-AN), moderately disordered cristobalite and with well ordered low-cristobalite and low-tridymite. Opal-C is confirmed to be strongly stacking disordered low-cristobalite with about 20 to 30% probability for tridymitic stacking. More extensively stacking disordered opal-CT does not contain detectable domains of low-cristobalite or low-tridymite. The stacking sequence is close to 50% cristobalite and 50% tridymitic. The local order decreases with increasing stacking disorder, so that the structural state of microcrystalline opals lies between cristobalite, tridymite and non-crystalline opals.