Crystal trees

Crystal trees

In the photo - not the imprints of ancient plants, but crystals pyrolusite in Zolnhofen limestone (the scale at the bottom of the photo is given in millimeters). Such crystal forms are called because of their resemblance to real trees. dendrites , from Greek ??????? - "wood". Sometimes they are really mistaken for plant prints.

Pyrolusite MnO 2 most often found in the form of cryptocrystalline aggregates (that is, those in which the crystals are indistinguishable by the eye), resembling kidneys, soot, or earth - these forms of discharge are called kidney-shaped, sooty, or earthy. This metallic black metallic mineral forms sedimentary 3-3-33282. deposits of manganese and is of industrial importance. In deposits, it is found together with other manganese and ferrous oxides and hydroxides - 3–3–333. Hausmanite , manganite , brownite , psilomelan , limonite .

Crystal trees

Various forms of pyrolusite: a Fine needle crystals, b - cryptocrystalline kidney-shaped form. Photo from the site

Pyrolusite is rare in the form of dendrites: specific conditions are needed for their formation. Dendritic forms of crystals are a variety of skeletal crystals (see picture of the day 3-3-375. Skeletal spinel 3-3-33282.), which split into separate branches during growth. Cleavage occurs, as a rule, in cramped conditions and /or with an uneven process 3-3-377. crystallization of - for example, when the temperature of the solution drops sharply or when the feed solution is not homogeneous. Crystal growth begins with the embryo - crystallization center . If the solution parameters (temperature, pressure, concentration of compounds) do not change with time or change weakly, a crystal of the correct form will form. However, if one or several parameters change rapidly, nucleation will also occur faster, and they will stick together. The intake of the substance (3-3-3381. Diffusion 3-3-33282.) To the growing embryos becomes even more uneven, and, as a result, characteristic branching forms are formed. The growth direction of the "branches" occurs in the direction of the most intense 3-3-3383. mass transfer - that is, in the direction of receipt of the nutrient solution. The process of dendrite formation itself is commonly called dendritic growth.

Crystal trees

Pyrolyusite dendrites (black) in the Zolnhofen limestone. Photo from the site

Dendrites are two-dimensional and three-dimensional. Two-dimensional dendrites grow on the surface of thin rock cracks. Three-dimensional dendrites grow in open rock voids. So, the growth of pyrolusite crystals from the main photo occurred in a limestone crack, through which a solution enriched with manganese passed.

Crystal trees

Two-dimensional (3-3-3173. A 3-3-3174., 3-3-3173. 3-3-3174., 3-3-3173. 3-3-3174.) And three-dimensional (3-3-3173. a - dendrites goethite in the layer agate ; b - dendrites of iron hydroxides in quartz, developing inside a crack; c - dendrites of manganese oxides in 3-3-3151. chalcedony ; d - native gold; e - native copper dendrite; f - Native copper dendrite from a crack in the median 3-3-3159. quartzite , covered with a thin crust malachite and atacamite . Photo a - d , f - from the site , e - from the site

Dendrites are formed not only in nature, but also in 3-3-3183. foundry . The size of dendritic crystals is an important factor determining the properties of cast alloys. For example, if the melt is cooled slowly, then new dendrites will form slowly, and as a result they will be larger. Conversely, if cooling is fast, the number of dendrites will be greater and their total size smaller. The finer the dendrites, the better the mechanical and technological properties of the finished product, but it is difficult to completely get rid of dendrites.

The first to draw attention to dendrites in steels was the Russian scientist 3-3-3187. Dmitry Konstantinovich Chernov . He comprehensively investigated the iron – carbon system and laid the foundations of 3–3–3189. metallography . In honor of him, one of the largest metal dendrites discovered during the smelting of metal was named - Chernov crystal : its height is 39 cm, and its mass is 345 kg.

Crystal trees

Crystal Chernova. Photo © Vladimir Korobitsyn from

Due to the fact that dendrites directly affect the properties of the obtained alloys, the study of dendritic growth is important for industry. In the 1990s, NASA spent an experiment to study the growth of dendrites in zero gravity. Under terrestrial conditions, due to the presence of gravity, areas with an uneven concentration of elements and, as a consequence, with an uneven diffusion of matter to growing faces, appear in solution or melt. That is, dendrites grow in a heterogeneous environment. To avoid the influence of gravity, NASA employees designed a device with remote control that allows you to control the progress of the experiment from Earth (while the device itself was in orbit). In the first two experiments, 3–3–3215 acted as model substances. succinic dinitrile in the third - pivalic acid (see 3-3-33217. Pivalic acid 3-3-33282.). Succinic acid dinitrile was used as an analogue of ferrous metals (iron and alloys based on it, manganese, chromium, vanadium), pivalic acid - an analogue of non-ferrous metals (copper, lead, tin, aluminum, titanium and others). Thanks to the experiment, information was obtained on the shapes of the vertices of dendritic crystals and the effect of cooling conditions on their growth, which is important for the theory of crystal growth.

However, researchers encountered dendritic growth much earlier. Compounds capable of crystallizing in the form of "trees" were known to medieval alchemists. The most famous are the Diana Tree (see Diana's Tree ) And the Saturn Tree. In the original recipes, water was mixed with mercury, silver (Diana's Tree) or lead (Saturn's Tree; Saturn is the alchemical name of lead). Crystallization of 3–3–3223 occurred in the resulting solution. Amalgam 3-3-33282. in the shape of a tree. Such "trees" were so impressive to medieval alchemists that they believed in the existence of "living" crystals.

Crystal growth occurs on a seed of amalgam of silver and mercury. As a solution - nitric acid with mercury and silver

dissolved in it.

Experiments with trees have not lost their relevance so far. Conducted, for example, 3-3-33237. experiments 3-3-33282. with copper and an aqueous solution of silver nitrate: the solutions were mixed and fixed the process of growth of dendrites in them. This setting of the experiment is largely reminiscent of the experiments that were set by medieval alchemists.

Crystal trees

Diana's tree. Crystallization of silver amalgam in water. Photo from the site

But, probably, the most common form of dendrites is ice crystals on window panes. The appearance of ice dendrites directly depends on the cooling conditions. In the interval temperatures from 0 to minus 6 ° C on the surface of the window pane a uniform layer of opaque, loose ice is deposited, and dendrites are not formed. But if the cooling of the glass surface begins at a positive temperature and high relative humidity, then a film of water is deposited on its surface, which crystallizes as dendrites even at lower temperatures.

Crystal trees

Ice dendrites on a window pane. Photo © Gary Lane from

Defects of the glass surface — microscopic scratches, chips, cracks — act as crystallization centers. Moreover, the shape of the dendrites in the upper and lower parts of the glass is different: below, where more water accumulates under the influence of gravity, dendrites are usually more. In the upper part of the glass, where the film of water is smaller, the dendrites are correspondingly thinner.

Photo from , the scale is given in millimeters.

Alexander Marfin

15 июнь 2020 /
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