Computer simulation of crystal structures

It is impossible to calculate from first principles the details of crystal structures. However, it is possible to rely on information which is certain, namely on interatomic distances, and calculate the details of a crystal structure based on these. Individual distances between atoms can be predicted accurately if one considers the coordination numbers of anions and cations, the extended electrostatic valence rule, and the effects of shared edges between different coordination polyhedra. The predicted interatomic distances are used as observations in a distance least squares refinement in which the positional parameters and the cell edges are adjusted until the calculated interatomic distances correspond as closely as possible to the predicted distances. The topology of the simulated structure has to be known or assumed. Appropriate weighting can facilitate proper modelling. Applications include: a) solution of pseudosymmetric structures; b) preliminary refinement of trial structures; c) geometric refinement without X-ray intensities; d) comparison of hypothetical structures with observed polymorphs of the same compositions; e) simulation of temperature dependence of structures; f) simulation of pressure dependence; g) calculation of structures isomorphous to known structures; h) calculation of thermal ellipsoids; i) calculation of local environments deviating from the overall symmetry of a structure; j) testing of hypotheses about the behavior of structures at varying conditions.     

Estimation of mean water residence times and runoff generation by0 measurements in a Pre-Alpine catchment (Rietholzbach, Eastern Switzerland)

Stable isotopes (¹⁸0) are used to investigate hydrological processes in the small Pre-Alpine research catchment Rietholzbach in NE Switzerland. The catchment covers an area of 3.18 km² and ranges from 680 to 960 m a.s.l. The¹⁸0 concentration in rainfall and in runoff at various sites, including a lysimeter installed within the catchment, over a 2-a period are investigated. First, conceptual flow models are applied to determine the mean water residence times of different systems of the basin and their storage properties. As a second approach, the separation of runoff components using¹⁸O and Mg ions as tracer during some storm flow events was carried out. The following flow mechanisms can be identified from the modelling results: (i) lateral hillslope fluxes, which are faster in the part of the Tertiary Molasse sediments and slower in the part of the Quaternary deposits, and (ii) relatively slow vertical infiltration processes through the soil layer within the lysimeter. Mean residence times of 12.5 months result for the total catchment baseflow component and 24.5 months for the baseflow from the upper part of the catchment, respectively. For the lysimeter seepage water, a mean residence time of about 200 days was obtained. During the intensive storm events, a fast reaction of the stored subsurface water due to heavy precipitation and an expulsion of this storage water amounting to a considerable portion of the total basin runoff can be detected.     

Hollandite–strontiomelane solid solutions coexisting with kanonaite and braunite in late quartz veins of the Stavelot Massif, Ardennes, Belgium

In a newly found type of quartz vein cross-cutting the famous "viridine"-bearing phyllites at Le Coreux, hollandite, ideally BaMn₈O₁₆, was discovered for the first time at this locality and in Belgium. Because the crystals contain up to 60 mol% of the Sr end member, this is also the second occurrence of strontiomelane. The coexisting "viridine" (= kanonaite) contains the highest amount (88 mol%) of the ideal end member MnAlSiO₅ ever found worldwide. The hollandite-type minerals are intimately intergrown with braunite containing appreciable Ca and Mg. Ba-bearing muscovite, Fe-poor excess-Al clinochlore (not quite trioctahedral), and albite are the remaining accessory minerals in the dominant quartz matrix. Microprobe analyses of all phases show rather extreme element fractionations: nearly all K is located in muscovite and none in the hollandite phase despite the existence of the end member KMn₈O₁₆ (cryptomelane). Similarly, nearly all Na is in albite and not in hollandite (no NaMn₈O₁₆=manjiroite component). Nearly all Mn resides in the two oxide phases and in kanonaite. Mg is strongly fractionated into chlorite. The small amounts of Fe and Ti present are predominantly partitioned into the hollandite phase, which also accommodates most of the Ba and Sr. Indeed, the hollandite phase is stabilized by the latter two elements relative to other Mn oxides. Kanonaite is stabilized by Al. Although no requisite sites are available in its crystal structure, braunite always contains small amounts of Ba and Sr. However, the Sr/(Sr+Ba) ratios in braunite are spurious and unrelated to those of the directly adjoining hollandite phases The conditions of formation of these veins may be well below 300 °C at low pressures (1-2 kbar), in agreement with the experimental results that the maximum Mn contents in kanonaite increase with falling temperatures.