Inversion of quartz

Microstresses arise inside the material as a result of the unequal thermal expansion coefficients of the glass and of the crystals. This phenomenon is characteristic of polyphase ceramics. In porcelains, the stresses arise in particular at the boundary between large quartz crystals and glass. Their formation is also contributed to by the modification inversion of quartz. The stresses sometimes bring about formation of microscopically visible cracks inside the quartz grains or around them. The structure of porcelain thus contains weak points which enhance the development of fracture on loading. 

Keith ML, Tuttle OF (1952) Significance of variation in the high-low inversion of quartz. Am J Sci Bowen Volume, p 203-280... 

The DTA technique has been applied to examine the fire clay products for underfiring. An example has been provided by Honeybome. In Fig. 28, thermograms of a brick from a certain locality are given. (A) refers to the locality, (B), (C), and (D) show the thermograms of the same clay fired to 600, 800, and 1000°C. The increase in the firing temperature modifies the thermograms, and at 1000°C, only the peak due to the inversion of quartz (570°C) remains. Curve (E) refers to a sample of facing... 

It should be noted that, whereas ferroelectrics are necessarily piezoelectrics, the converse need not apply. The necessary condition for a crystal to be piezoelectric is that it must lack a centre of inversion symmetry. Of the 32 point groups, 20 qualify for piezoelectricity on this criterion, but for ferroelectric behaviour a further criterion is required (the possession of a single non-equivalent direction) and only 10 space groups meet this additional requirement. An example of a crystal that is piezoelectric but not ferroelectric is quartz, and ind this is a particularly important example since the use of quartz for oscillator stabilization has permitted the development of extremely accurate clocks (I in 10 ) and has also made possible the whole of modern radio and television broadcasting including mobile radio communications with aircraft and ground vehicles. 

The progress of this reaction on the surface of quartz was studied by Bodenstein and Ohlmer.f The velocity was found to vary in direct proportion to the pressure of oxygen, and in inverse proportion to the pressure of the carbon monoxide itself. 

This scattering mode occurs only in piezoelectric materials, i.e., in crystals without inversion symmetry, and is caused by the electric field associated with acoustical phonons. Zinc oxide exhibits very high electro-mechanical coupling coefficients P (see later), exceeding that of quartz [42,59], which is one of the mostly used piezoelectric materials. Zook [60] has calculated the piezoelectrically limited mobility on the basis of the elastic and piezoelectric constants as (see also Rode [54]) ... 

The acidic surfaces of quartz and tungsten oxide are noted in this table, as well as, the basic surfaces of alumina and magnesium oxide. It should also be noted that the method of powder fabrication is important in establishing the structure of the surface and, therefore, the isoelectric point of the powder surfiice. The lEP for a simple oxide is inversely proportional to the ratio of the valence, z, to the radius, Rcaom > of the metal cation making up the oxide. The regression equation for lEP data from Parks [53] is as follows ... 

High-temperature electrolysis of tridymite brings about migration of impurities towards the cathode, while tridymite in the vicinity of the anode is converted to quartz below 1050 °C and to cristobalite above 1050 °C this temperature is approximately equal to that of quartz-cristobalite inversion, according to data of other authors. 

Figure 8 indicates that the conversion of quartz into cristobalite proceeds very slowly even at 1500 °C, i.e. almost 500 °C above the equilibrium inversion point, so that it is not surprising that quartz is present in ceramics even after high-temperature firing. 

The following sequence of inversions takes place during gradual heating up of low-temperature quartz at 573 °C, jS-quartz is very rapidly inverted to a-quartz, which is stable up to about 1025 °C when it has high purity (content of impurities < < 10 %). Itis then converted to cristobalite. If the quartz contains more impurities (solid solutions), then at about 870 °C a-quartz is converted to a-tridymite which in turn is transformed to a-cristobalite above 1470 °C, Non-equilibrium fusion of quartz at temperatures above 1400 —1450 °C was often observed cristobalite is then formed secondarily from this melt. 

All the displacive inversions of silica exhibit well discernible enthalpic effects on DTA (differential thermal analysis) curves. Pure quartz produces a minor but... 

Tuttle OF (1949) The variable inversion temperature of quartz as a possible geologic thermometer. Am Mineral 34 723-730... 

Silica crystallizes from sodium silicates in three forms, cristobalite, tridymite, and quartz. The inversion temperatures are 1470 and 870°C. Cristobalite, the high-temperature modification, melts at 1713°C. The cristobalite liquidus decreases from the melting point of SiOz to the inversion point (located at 88.7 wt % Si02) between cristobalite and tridymite. The tridymite liquidus then descends from this point and meets the liquidus curve of quartz at 75.5 wt % Si02 (870 10°C). The tridymite liquidus extends metastable below 870 to 793°C ending at the disilicate-... 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

The classical sensing application of quartz crystal resonators is microgravimetry [1,5]. Many commercial instruments are around. These devices exploit the Sauerbrey relation (Eq. 28). For thin films, the resonance frequency is—by and large—inversely proportional to the total thickness of the plate. The latter increases when a film is deposited onto the crystal surface. Monolayer sensitivity is easily reached. Flowever, when the film thickness increases, viscoelastic effects come into play, as was for instance recognized by Lu and Lewis, who derived a refined version of the Sauerbrey equation [6]. These authors mainly intended to improve the microweighing procedure. Actually measuring viscoelastic properties with the QCM was not a major issue... 

Since the force across the contact only weakly perturbs the motion of the crystal surface, the displacement, i/(f), is mainly governed by the dynamics of the quartz crystal. u t) is sinusoidal with time, and the force F(f) is a function of the displacement and the direction of motion. F(f) can be calculated by inversion of Eq. 25 as ... 

Influence of Clay Mineral Content. A number of authors like Thompson (1959), Heald and Renton (1966) and Sibley and Blatt (1976) have shown quantitatively that thin isolated sand intercalations or continuous sandstones possessing abundant clay bridges on their quartz grains have suffered more pressure solution and eventual removal of SiOj than sandstones with a smaller amount or total absence of clay bridges in the form of a film. This was also observed in the Saharan sandstones (Plate 13). It is, however, difficult to establish a quantitative statistical correlation between the amount of clay components and pressure solution. There is at the same time in the Saharan sandstones a clear inverse statistical correlation between the amount of clay minerals in the form of a film and the quartz cement (Fig. 4.12, Plates 13,14). This is in agreement with observations showing that the clay cement impedes in the sandstones the formation of overgrowth rims of quartz (Millot 1964 Heald and Larese 1974). 

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