As is known from the elementary course in crystallography, to determine the symbol [r₀, r₁, r₂, r₃] of the zone axis of a hexagonally isotropic complex from the symbols of two non-parallel faces (p₀, p₁, p₂, p₃) and (q₀, q₁, q₂, q₃) belonging to this zone, one can use the following method (see the article).
Ammonium bromostannate (NH₄)₂SnBr₆ was obtained by Prof. N. S. Kurnakov by mixing SnBr₄ + 2(NH₄)Br in an aqueous solution in the presence of hydrobromic acid. Crystals of cubic system, lemon-yellow in color. To determine the structure, about 50 crystals were examined.
Artificial ullmannite NiSbS. A compound corresponding in chemical composition to the mineral ullmannite was obtained by Prof. N. S. Kurnakov and stud. Ya. Posternak (at the St. Petersburg Polytechnic Institute) and was transferred to the Mineralogical Institute of the Mining Institute for crystallographic study. The compound CoSbS. This compound, obtained (also like NiSbS) by student Ya. Posternak, was given to me by Prof. N. S. Kurnakov. The cavity walls in the alloy were dottded with finely tabular crystals, arranged without any regularity, having a strong metallic luster and a steel-gray color.
For the correct orientation, 100 crystals were re-examined. The results of the re-examination and calculations of the probability of correct orientation are given in the tables (see the article). Using asparagine as an example, the regularity of the decrease in the number of developed forms as the crystal sphere grows is demonstrated with exceptional clarity and distinctness. Furthermore, during subsequent crystallization, the appearance of any new forms not observed in the preceding crystallization was never observed. From all the experiments described above, it clearly follows that there exists possibility of obtaining a much greater number of forms during the crystallization of a sphere, compared with what we observe during the free growth of crystals of the same compound.
Crystallization of this compound was carried out at a temperature of about +20°C from solutions in methyl alcohol, in which it dissolves quite easily even in the cold. Solubility increases upon heating. A total of 57 crystals were examined. The results of the examination are presented in the table (see the article).
When crystallization is accelerated by cooling the initially heated solution, the crystals of these compounds begin to take on an increasingly needle-like appearance. The crystals of the iodide compound are so small (no more than 1/2 mm in length) that their goniometric study appears extremely difficult, and I preferred not to perform it, especially since this compound decomposes very easily in solution. The crystals of the chloride compound have already been described earlier. However, after E. S. Fedorov's final clarification of the principles of correct orientation, the face symbols of these crystals must be changed according to the transition determinant (see the article).
CuSO₄+5H₂O crystals have been studied by several authors. The most comprehensive investigation of crystals of this compound was carried out relatively recently by Th. V. Barker in the laboratory of R. Groth. Copper sulfate crystals, as is known, belong to the cynacoidal class of the triclinic system. CuSO₄+5H₂O crystallizes (at t° = + 20°C) from an aqueous solution into well-formed thick tabularcrystals. The article includes a diagram for the correct orientation of crystals.
In the article by Prof. E. S. Fedorov, "Representation of Crystal Structure by Vector Circles," a graphical method is presented for finding the reticular densities of crystal faces (more precisely: the squares of the reticular densities of the corresponding nets). Anyone who has dealt with determining the reticular densities of faces by this method knows that it involves making certain constructions—not particularly complicated, but nonetheless quite time-consuming. This article aims to show how the task can be simplified, reducing the drafting work to a minimum.
Cobalt-nitro-aquo-dimethylglucimine was first prepared by L.A. Chugaev, who transferred it to the Mineralogical Institute (of the Mining Institute) for crystallographic research. For the research results and crystal measurement tables, please see the article.
Cobalt-diamine-dimethylglucimine chloride was obtained prepared by L.A. Chugaev and presented by him to the Mineralogical Institute for crystallographic research (see the article). Goniometric measurements (using a universal goniometer by E. S. Fedorov) and optical observations in polarized light revealed that the crystals belonged to the tetragonal system, although their specific class could not be determined.
Especially for triclinic crystals, when determining the lattice plane densities of the vertical zone from tables, it is necessary to apply a shift to the defining faces, and such a shift should be made with greater precision, compared with that which we can achieve graphically. In this case, it will be necessary to calculate the spherical coordinate ρ, which defines the faces after the shift. Solving the problem of such calculation does not present any difficulty, and is already partly contained in one of the formulas given in the article by Sokolov and myself “Determination of the lattice plane densities of crystal faces without the help of constructions"
During the slow crystallization of cobalt - amino - chloro - dimethyl - glucosimine at + 18-20 C, certain crystals of this compound develop special, enigmatic faces. These faces differ primarily by their size, since they are usually larger than the others and formed so perfectly that when measuring, one can rely on measurement accuracy within a few seconds. However, by their orientation they do not seem to belong to the crystal's normal morphological set, since their indices are extraordinarily complex, almost irrational.
Let's imagine a crystal being measured on a universal, theodolite goniometer. Let's assume we have to measure a face that has various defects in formation and produces not one but several signals (due to the development of vicinal forms). Under such conditions, the measurement accuracy is significantly reduced and rather large errors can be made in the readings of the angles φ (along the vertical limb of the goniometer) and ρ (along the horizontal limb). Even if a face is formed reasonably well , one can often still vouch for an accuracy of only a few minutes, and higher accuracy is achieved only in exceptional cases.
Cobalt‑amino‑chloro‑dimethylglyoxime was first obtained by L. A. Chugaev, who offered it to one of the authors for crystallographic study. Crystallization was carried out at about +20 °C from solutions in water containing 5% acetic acid. In this solvent, the compound under investigation is rather sparingly soluble in the cold, its solubility increasing upon heating. An excess of the powdered substance was heated in the solvent on a water bath to approximately the boiling point; the hot solution was then filtered and cooled at room temperature (about +20°C). For the results of the investigations, see the article.
A hemisphere with a radius of 5 mm and a diametral plane (110) (the cleavage plane) was cut from a cleavage fragment of calcite. This hemisphere, attached with wax to a glass hemisphere of the same diameter, was suspended on a thread in a supersaturated NaNO3 solution. Prior to this, to clean the surface of the calcite hemisphere, the sphere assembled as described above was immersed for several seconds in a dilute hydrochloric acid solution.
Experiments on the crystallization of a hemisphere with a diameter of 5 mm, prepared from a K₂Cr₂O₇ crystal with a central plane (100), were generally carried out in exactly the same way as with the crystallization of hemispheres cut from sodium chloride, aluminum alum, and chrome alum.
Crystals of potassium dihydroxide were measured by Schabus (Wien. Ak. Veg. 1850) and he also stated a very perfect cleavage along {001} and less perfect along {100} and {010}. Potassium dichromate releases from aqueous solutions (t = + 20° C.) as well-formed crystals with growth planes {001} or {101}, which, as is known, belong to the pinacoidal class of the triclinic system of cubic type.
In brief reports of the 1st issue of vol. I of the "Notes of the Mining Institute" (p. 83), I reported on some experiments on the growth of spheres (hemispheres) prepared from crystals of chrome and potassium alum. Similar experiments were carried out with crystals of rock salt (NaCl) from Stassfurt. A welded piece of such salt was ground into a hemisphere with a diameter of 10 m.m. with a central plane (100) and glued with wax onto a glass hemisphere of the same diameter.
When studying certain physical properties of crystals, it is often advantageous to operate with crystals ground into spherical forms, since in this case we directly obtain the specific magnitude of its change for each vector. However, as far as I know, no experiments have yet been undertaken on the crystallization of spheres artificially prepared from a crystal of a given solid substance. Meanwhile, this form of crystal is important in terms of reducing the influence of crystal planes on the crystallization currents that arise during the deposition of a substance from a solution.
