Quartz structural properties

For example, carbon and silicon are found within the same group in the periodic table. Considering the trends in a group, we would expect the oxides of these two elements, C02 and Si02, to display similar properties. However, Si02 is a solid with a quartz structure while C02 is a gas that has great importance in the life cycle. What can be the reason for these two compounds being so different ... 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

Normal ice is analogous to tridymite water has the quartz structure with a greater density. The occurrence of the density maximum at 4°, a unique property of water, must be attributed to the gradual transition of the tridymite structure into the quartz-like structure, while at higher temperature the normal thermal expansion again gets the upper hand. 

Covalent network solids Atoms such as carbon and silicon, which can form multiple covalent bonds, are able to form covalent network solids. In Chapter 7, you learned how the structures of graphite and diamond give those solid allotropes of carbon different properties. Figure 13-20 shows the covalent network structure of quartz. Based on its structure, will quartz have properties similar to diamond or graphite ... 

Quartz Properties Microcrystalline quartz, structure confirmed by XRD, BET specific surface area 6 m /g [429], external specific surface area used in calculations was 1.7 mVg [428,429],... 

Chelikowsky, J.R., King, H.E., Troullier, B., Martins, J.L. and Glinnemann, J. (1990) Structural properties of a-quartz near the amorphous transition Phys. Rev. Lett. 65,3309-3312. 

BertheviUe B, Herrmannsdorfer T, Yvon K (2001) Structure data for K2MgH4 and Rb2CaH4 and comparison with hydride and fluoride analogues. J Alloys Compd 325 L13-L16 Hines J, Cambon O, Astier R et al (2004) Crystal structures of a-quartz homeotypes boron phosphate and boron arsenate structure-property relationships. Z Krist 219 32-37 Feldmann C, Jansen M (1995) Zurkristallchemischen Ahnlichkeit von Aurid- und Halogenid-lonen. Z anorg allgem Chem 621 1907-1912... 

The Cs structure and dimensions (Fig. 17.26b) were established by microwave spectroscopy which also yielded a value for the molecular dipole moment p. 1.72D. Other physical properties of this colourless gas are mp -115° (or -123°), bp -6°, A//f(g,298K) —34 10kJmol [or — 273kJmol when corrected for A//f(HF, g) ]. FCIO2 is thermally stable at room temperature in dry passivated metal containers and quartz. Thermal decomposition of the gas (first-order kinetics) only becomes measurable above 300° in quartz and above 200° in Monel metal ... 

Ceramic materials are typically noncrystalline inorganic oxides prepared by heat-treatment of a powder and have a network structure. They include many silicate minerals, such as quartz (silicon dioxide, which has the empirical formula SiO,), and high-temperature superconductors (Box 5.2). Ceramic materials have great strength and stability, because covalent bonds must be broken to cause any deformation in the crystal. As a result, ceramic materials under physical stress tend to shatter rather than bend. Section 14.22 contains further information on the properties of ceramic materials. 

The most characteristic feature of any crystal is its symmetry. It not only serves to describe important aspects of a structure, but is also related to essential properties of a solid. For example, quartz crystals could not exhibit the piezoelectric effect if quartz did not have the appropriate symmetry this effect is the basis for the application of quartz in watches and electronic devices. Knowledge of the crystal symmetry is also of fundamental importance in crystal stmcture analysis. 

Any two samples of a particular mineral, whatever their source or place of origin, have the same basic composition and characteristic crystal structure moreover, no two different minerals have identical chemical composition and crystal structure (see Textboxes 8 and 21). Quartz, for example, is a common and abundant mineral composed of silicon dioxide, a compound that occurs naturally not only as quartz but also in other crystal structures, known as polymorphs (polymorphs are minerals that have the same chemical composition but different crystal structure), some of which, listed in Table 23, have been used for a variety of purposes. The crystal structure, which is essential for the characterization of solid materials, is just one of a wide range of physical properties, that is, properties not involving chemical differences, which provide convenient criteria for characterizing and identifying solids. 

Flint is a hard and easily split variety of the mineral quartz (composed of silicon dioxide), which occurs not only as flint but also in a wide range of other varieties. Some of these exhibit different colors and colored patterns and have characteristic crystalline structures, while others are amorphous (see Textbox 21). In all its varieties, nevertheless, the hardness of quartz is very high, being graded as 7 on the Mohs scale (see Textbox 23). Almost all varieties are either transparent or translucent and display a distinctive luster. These properties made quartz an attractive material for making ornamental... 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

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