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Es gibt nir­gend­wo im Uni­ver­sum ei­nen per­fek­ten Kris­tall, denn jed­er Kris­tall hat eine Ober­fläche und für die Atome auf der Ober­fläche ist die Umge­bung an­ders als für Atome im Vol­u­men. Die Ober­fläche ist somit ein De­fekt. Reale Kris­talle sind damit al­so Kris­talle, die De­fekte en­thal­ten.

Eine ein­fa ... moreEs gibt nirgendwo im Universum einen perfekten Kristall, denn jeder Kristall hat eine Oberfläche und für die Atome auf der Oberfläche ist die Umgebung anders als für Atome im Volumen. Die Oberfläche ist somit ein Defekt. Reale Kristalle sind damit also Kristalle, die Defekte enthalten.

Eine einfache Definition für Defekte in Kristallen ist die Betrachtung der Umgebung der Atome im Kristall. Falls die unmittelbare Umgebung - streng genommen im zeitlichen Mittel, da die Atome im Kristall wegen der Temperatur um ihre Position wackeln - um ein beliebig herausgegriffenes Atom anders ist als die Umgebung eines Referenzatom in einem perfekten Teil des Kristalls, ist ein Defekt Ursache für diese Änderung. Für ein Atom auf der Oberfläche eines Kristalls ist diese Bedingung zweifellos erfüllt, da die eine Hälfte des Raumes keine Atome des Kristalls hat. Es gibt also prinzipiell keine perfekten Kristalle.
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In­di­ca­tor stoneA Scandinavian indicator stone is a glacial erratic composed of a characteristic rock type derived from a small known source area in Scandinavia. This term not only applies to igneous and metamorphic rocks but also to some sedimentary rocks. The Jotnian Sandstone and most of the Palaeozoic limestones, and the Old Red Sandstone are not included in the definition, although the presence of these and other rocks provide some evidence about the source area(s) of erratics and should certainly not be neglected in a stone count. In all cases we recommend including the whole assemblage of erratics/stones in such account. This makes it possible to use several methods of Interpretation. However, it should be pointed out that the practise used in the past, whereby each researcher had her/his own method of interpreting stone counts, has proved to be unsatisfactory. We demonstrate on the bases of over 2000 counts of indicator stones that we have carried out on assemblages mostly from Lower Saxony and Schleswig-Holstein, but also from other N.German states and neighbouring countries, that the TGZ method (LÜTTIG 1958) yields the most reliable results. In addition to this method, the sources of individual indicator stones may be plotted on a so-called circle map and can be integrated with possible source data and the relative frequencies of other erratics in the assemblage. Some rock types are more suitable as indicator stones then others. It is unwise to use clearly unsuitable rock types; this would considerably reduce the reliability of the method and lead to erroneous results.
A Scan­di­na­vian in­di­ca­tor stone is a gla­cial er­rat­ic com­posed of a char­ac­teris­tic rock type de­rived from a small known source area in Scan­di­navia. This term not on­ly ap­plies to ig­neous and me­ta­mor­ph­ic rocks but al­so to some sed­i­men­tary rocks. The Jot­nian Sand­s­tone and most of the Palaeo­zoic lime­s­tone ... moreA Scandinavian indicator stone is a glacial erratic composed of a characteristic rock type derived from a small known source area in Scandinavia. This term not only applies to igneous and metamorphic rocks but also to some sedimentary rocks. The Jotnian Sandstone and most of the Palaeozoic limestones, and the Old Red Sandstone are not included in the definition, although the presence of these and other rocks provide some evidence about the source area(s) of erratics and should certainly not be neglected in a stone count. In all cases we recommend including the whole assemblage of erratics/stones in such account. This makes it possible to use several methods of Interpretation. However, it should be pointed out that the practise used in the past, whereby each researcher had her/his own method of interpreting stone counts, has proved to be unsatisfactory. We demonstrate on the bases of over 2000 counts of indicator stones that we have carried out on assemblages mostly from Lower Saxony and Schleswig-Holstein, but also from other N.German states and neighbouring countries, that the TGZ method (LÜTTIG 1958) yields the most reliable results. In addition to this method, the sources of individual indicator stones may be plotted on a so-called circle map and can be integrated with possible source data and the relative frequencies of other erratics in the assemblage. Some rock types are more suitable as indicator stones then others. It is unwise to use clearly unsuitable rock types; this would considerably reduce the reliability of the method and lead to erroneous results.
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Hel­go­land, was im nied­erdeutschen "Heiliges Land" be­deutet, ist Deutsch­lands einzige Hoch­seeinsel, wen­n­gleich sie die Kri­te­rien ein­er solchen nicht er­füllt. Die ca. 2 km2 große Insel liegt mit­samt ihr­er kleineren Nach­barinsel, der Düne, ca. 60 km vor der Elb­mün­dung in der Deutschen Bucht (Ger­manisch ... moreHelgoland, was im niederdeutschen "Heiliges Land" bedeutet, ist Deutschlands einzige Hochseeinsel, wenngleich sie die Kriterien einer solchen nicht erfüllt. Die ca. 2 km2 große Insel liegt mitsamt ihrer kleineren Nachbarinsel, der Düne, ca. 60 km vor der Elbmündung in der Deutschen Bucht (Germanisches Becken), wie dieser Teil der östlichen Nordsee genannt wird. Die Insel ist neben den beiden dänischen Inseln Møn und Seeland, und der Kreideinsel Rügen in der Ostsee ebenfalls aus festem Gestein aufgebaut, was sie deutlich von den aus Lockersedimenten aufgebauten Nordfriesischen Inseln unterscheidet. Mit 245 Millionen Jahre altem Gestein ist sie zugleich auch die älteste Insel. Die auffällige Rotfärbung des Inselgesteins wurde durch intensive lateritische Verwitterung und die Oxidation von Eisen hervorgerufen. Ihr Alter, das Erscheinungsbild und die isolierte Lage macht sie also zu etwas besonderem, weshalb ihr jedes Jahr ca. eine halbe Million Menschen einen Besuch abstatten. Sie ist aber ... Ein Beitrag von René Gervers und Sebastian Möller
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Ara­g­onite is a cal­ci­um car­bo­nate, chem­i­cal­ly iden­ti­cal with calcite. The min­er­al calcite, how­ev­er, dif­fers from Ara­g­onite due to its in­ter­nal crys­tal struc­ture. While the crys­tal sys­tem of calcite is trig­o­n­al, the sys­tem of ara­g­onite is rhom­bic. Dense mass­es of small ara­g­onite crys­tals are dif­fi­cul ... moreAragonite is a calcium carbonate, chemically identical with calcite. The mineral calcite, however, differs from Aragonite due to its internal crystal structure. While the crystal system of calcite is trigonal, the system of aragonite is rhombic. Dense masses of small aragonite crystals are difficult to distinguish from calcite, but they are larger, they show a distinct habit.

A lot of chapters in this portrait will give you much more details about this interesting mineral. Written and investigated by Peter Seroka. (Article in german)
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