Isometric system

Tesseral-kies, m. skutterudite smaltite. -system, n. (Cryst.) isometric system. 

A naturally occurring alloy of silver with mercuiy, also referred to as mercurian silver, silver amalgam, and argental mercuiy. The natural amalgam crystallizes in the isometric system hardness. 3-3.5 sp gr, 13.75-14.1 luster, metallic, color, silver-white streak, silver-white opaque, Amalgam is found in Bavaria. British Columbia. Chile, the Czech Republic and Slovakia, France, Norway, and Spain. In some areas, it is found in the oxidation zone of silver deposits and as scattered grains in cinnabar ores. 

ANALCIME. A common zeolite mineral, NaAlSi2C>6 H20, a hydrous soda-aluminum silicate. It crystallizes in the isometric system, hairiness, 5-5.5 specific gravity, 2.2. vitreous luster colorless to white but may be grayish, greenish, yellowish, or reddish. Its trapezohedral crystal resembles garnet but is softer it is distinguished from lcucitc only by chemical tests. 

FLUORITE. Fluorite is a calcium fluoride mineral CaFi crystallizing in the isometric system, often in superb cubic crystals. Twinned crystals are common, usually as cubic penetration twins. It is found in many diverse... 

HESSITE. A mineral telluride of silver. AgyTe. with some gold, crystallizing in the monoclinic syslem at normal temperatures isometric system above I49.5F (65.3 C). Crystalline form not ohvious at normal temperatures. Hardness. 2-3 specific gravity. 8.24-8.45 color, gray with metallic luster opaque. Named after G.H. Hess (1802— 1850). 

The three Miller indices for a cryslal face in all systems except the hexagonal, which requires four indices, arc always given in die same order as their crystallographic axes. a. I>. c, respectively o, cr. a in the isometric system a1. < c. in the tetragonal system and a1, o. r. in the hexagonal. 

SPERRYLITE. A mineral diarsenide of platinum. PtAs2. Crystallizes in the isometric system. Hardness, 6-7 specific gravity, 10.58 color, white opaque. Named after Francis L, Sperry, Sndbury, Ontario. 

SPHALERITE BLENDE. Also known as zinc blende, this mineral is zinc sulfide, tZn, Fc)S, practically always containing some iron, crystallizing in the isometric system frequently as tetrahedrons, sometimes as cubes or dodecahedrons, but usually massive with easy cleavage, which is dodecahedral. It is a brittle mineral with a conchoidal fracture hardness, 2.5-4 specific gravity, 3.9-4.1 luster, adamantine to resinous, commonly the latter. It is usually some shade of yellow brown or brownish-black, less often red, green, whitish, or colorless streak, yellowish or brownish, sometimes white transparent to translucent. Certain varieties... 

Butyrylamino-4-hydroxyphenylarsinic acid, CH3.CH3.CH2. CO.NH(OH)CgH3AsO(OH)2, yields microscopic hexahedrons of the isometric system, melting with decomposition at 218° to 219° C. its sodium salt crystallises in clumps of colourless needles containing 10 molecules of water of crystallisation. 

For each crystal system, three axes (four in the hexagonal system) are assigned that coincide with the symmetry axes or are perpendicular to mirror planes, ha the isometric system, these axes coincide with the four-fold rotation or rotoinversion axes or the two-fold rotational axes. They are mutually perpendicular and are labeled ai, a.2, and a3, rather than the conventional labels of a, b, and c, because they are identical in every respect other than orientation. By convention, the positive end of the ai axis is toward the reader, the positive end of the aa axis is to the right in the plane of the paper, and the positive end of the 3 axis is up in the vertical direction. The axes are shown for the octahedron in Figure 27. 

Figure 27 The crystallographic axes for the isometric system shown for an octahedron.

We have previously discussed the crystallographic axes. In the isometric system, the axes extend through the centers of each face of a crystal and intersect in the center as shown in Figure 44. 

Figure 44 The perpendicular axes for a crystal in the isometric 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.

Figure 93 The dodecahedron formed by symmetry operations in the isometric system on the (101) face.

The goethite pseudomorph is an easy one—the original pyrite is in the isometric system. Incidentally, these particular pseudomorphs are sometimes referred to as limonite. The term limonite is not an accepted mineral name and is used when the composition of the iron oxide is uncertain. The names of minerals are... 

Nonopaque materials, those that allow the transmission of light, can be optically classified as "isotropic" or "anisotropic." Isotropic materials (e.g., oil, water, epoxy, crystals in the isometric system) transmit light in all directions at the same velocity. Anisotropic materials (e.g., alite, calcite, quartz, all crystals in systems other than the isometric) divide the light into... 

Figure 2 shows the sensitivity of Debye correlation function to the change of parameter a which is defined in Eq. (10). As we explained in Section on Theory of scattering, a = 0 corresponds to an isometric two-component system and x 0 a non-isometric system. When x = 0.1 a two-component system becomes a non-isometric system with volume fractions, cpi = 0.46 and q>2 = 0.54. The Debye correlation functions for two cases, a = 0 and X = 0.1, were calculated with a set of representative values of a, b and c. Figure 2 shows that a small deviation from isometry does not affect shape of the Debye correlation function. All our samples were prepared so that they are isometric at a reference salinity, and the change of an effective volume fraction as a function of salinity is expected less than 10%. Therefore, we treat the parameter a as effectively zero in all data analysis. 

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