Isometric Crystals

Fig. 6.1.6. Block structure of shiny smooth faces (001), (Oil), (110), (111) 0f sulphur isometric crystal shown by hardness testing.

CUPRITE. The mineral cuprite, cuprous oxide. Cu 0. occurs as isometric crystals, usually octahedrons, hut may he cubes, dodecahedrons or modified combinations, it also is found as a massive, earthy material. Its fracture is conchoidal to uneven brittle hardness,, 1.5-4 specific gravity, 6.14 luster, submetallic to earthy color, red nearly transparent to nearly opaque. Its streak is shining brownish-red. Cuprite is a secondary mineral resulting doubtless from the oxidation of copper sulfides. It is often found associated with native copper, malachite and azurile. 

Ruthenium Sesquisulphide, RuaSs, occurs in the platinum washings of Borneo and Oregon as the rare mineral lauriie, usually in the form of minute octahedra hardness 7-5, density 6-99. It has a bright metallic lustre and contains a little osmium. Deville and Debray 5 prepared a similar sulphide artificially in the form of isometric crystals, octahedra, and cubes by heating ruthenium with pyrites and borax. 

Figure 9. Preferred orientation- crystal morphology can affect ZSM-5 XRD powder patterns. Here, crystals with very dissimilar length and width (140 x 20 jim) produce XRD intensities that differ greatly from those of nearly isometric crystals (15 x 15 jim). (Reproduced with permission from ref. 20. Copyright 1983 Butterworth.)...

Metatitanates of divalent metals Ca, Sr, Ra, Cd and Pb were obtained in aqueous solutions of NaOH and KF [25], The synthesis is carried out using Me(OH)2 + TiOj + H2O mixture at 380-450°C and pressure 30-50 MPa. The isometric crystals are formed. Since the formation of titanates occurs in the presence of an alkaline metal, the synthesis is carried out in low concentrated NaOH solutions ( 10 % mass.) The synthesis of these compounds proceeds better if metatitanium acid, H2TiOj, is used instead of titanium dioxide [25]. 

At small values of In / , larger and more isometric crystals tend to form, especially for crystallization in the (5-form. At very low supersaturation, a special kind of (large) spherulites often form they consist of narrow platelets growing out from one point, and they bifurcate when growing longer. A kind of spherical, prickly sponge is formed, partly consisting of liquid fat. 

Often the super saturation is smaller, leading to larger crystals and a weaker network. Isothermal recrystallization, e.g., into another polymorph, generally occurs at very low supersaturation, and it may cause the formation of larger and more isometric crystals (e.g., spherulites) that make very weak networks. 

Yes. Galena has the full isometric symmetry of 4/m32/m, while sphalerite has a slightly lower symmetry of 43 m. Both belong to the isometric crystal system. 

Figure 89 shows the (111) face in two different crystals. The shape of the face of the isometric crystal is clearly different from the (111) face of the tetragonal crystal. Thus, the same designation of the face does not indicate that the shape of the face is the same in different crystal systems. Indeed, the different shapes are a result of the different shaped unit cells in the isometric system the unit cells are cubes, in the tetragonal system the unit cells are shortened (in this case) in the c-direction. When the unit cell parameters for a substance are known, the length of the cell axes can be used as unit distances to derive the Miller indices. 

Figure 90 Several faces of a crystal in the isometric system. As drawn the crystal does not have isometric symmetry. A real isometric crystal containing face a would have seven other faces, one at each comer of the cube. These other faces are not shown.

Now let us try a slight variation on our isometric crystal by adding a third type of face. This face e is shown in Figure 91. This face clearly intersects the axes ai and 83, but runs parallel to axis aa. It therefore has intercepts of lai and las and infinity 82. The reciprocals of these are 1, 0, and 1, listing them in order of the axes. Face e then is given the indices (101). 

Now let us try an experiment with the (111) face in an isometric crystal in the point group 4/m32/m. The object of our experiment will be to see what set of faces is formed by operating on the (111) face with the symmetry elements of this point group. 

Figure 103 Rate of growth of different faces in an isometric crystal. Strong attractions for ions in solution increases the rate of growth of the (111) face (a) until eventually it disappears (b).

To obtain isometric crystals as described in Fig. 5.41, one must restrict the width of the thin lamellae. This is illustrated in Fig. 5.74 with single-molecule single crystals grown from droplets of a very dilute solution of poly(oxyethylene) in benzene. Each of the crystals contains only one molecule [27]. The poly(oxyethylene) was of a broad... 

Reducing the cooling rate (0.5-l°C/h) in the flux method may give block-like crystals of several millimeters thickness in the (001) direction. Below 0.5°C/h, thick isometric crystals with aspect ratios between 1 and 10 can be grown (Wolf et al. 1989), Under those... 

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