High-quartz solid solution

C.T. Li The crystal structure of LiAlSi2 06lII (high-quartz solid solution) , Z. Kristallogr. 127, 327-348 (1968)... 

High-quartz solid solutions are stuffed derivates of the quartz modification of Si02 [3.13]. Because the Si02 lattice allows for many substitutions, high-quartz solid solutions in glasses can have rather complicated compositions which may vary with the ceramization conditions. According to Petzoldt the composition can be described in general by [3.14] ... 

Coming 9625d OC-quartz solid solution, Si02 spinel, MgGAl203 enstatite, MgOSi02 very high strength classified... 

In many cases, more than one crystalline phase forms. Often, phases that form initially transform into another phase as heat treatment progresses. For example, crystalline phases of /5-quartz solid solutions (e.g., (S-eucryptite), which are not stable at high temperatures, convert to more stable phases (e.g., 6-spodumene, cristobalite, sapphirine, or lithium disilicate). The stable high temperature phase may have different properties— in this case, higher thermal expansion. Therefore, glass ceramics with the same chemical composition may have very different properties, depending on the heat treatment. 

It must be noted, however, that other crystal phases apart from the P-quartz solid solution are also involved in the crystal phase formation of primary crystal phases up to a-cordierite. These crystal phases render the solid-state reaction highly complex. Hence, several unknown crystal phases with flat, almost two-dimensional habits were observed and could not be identified as Mg-petalite or osumilite as shown by Schreyer and Schairer (1961). 

P-quartz solid solution grows. In the high-resolution TEM image by Maier and Muller (1989) in Fig. 

The main crystal phase of the Zerodur glass-ceramic is composed of P-quartz solid solution This phase is produced by volume crystallization. Special thermal properties devdop as a result of the formation of very small crystallites and the simultaneous high crystallite count per unit of glass volume. 

Fig. 2.37. Phase diagram for Ca0-Na20 Si02-(Al203)-H20 system in equilibrium with quartz at 400°C and 400 bars. Plagioclase solid solution can be represented by the albite and anorthite fields, whereas epidote is represented by clinozoisite. Note that the clinozoisite field is adjacent to the anorthite field, suggesting that fluids with high Ca/(H+) might equilibrate with excess anorthite by replacing it with epidote. The location of the albite-anorthite-epidote equilibrium point is a function of epidote and plagioclase composition and depends on the model used for calculation of the thermodynamic properties of aqueous cations (Berndt et al., 1989).

The existence of tridymite as a distinct phase of pure crystalline silica has been questioned (42,58—63). According to this view, the only true crystalline phases of pure silica at atmospheric pressure are quartz and a highly ordered three-layer cristobalite having a transition temperature variously estimated from 806 250°C to about 1050°C (50,60). Tridymites are considered to be defect structures in which two-layer sequences predominate. The stability of tridymite as found in natural samples and in fired silica bricks has been attributed to the presence of foreign ions. This view is, however, disputed by those who cite evidence of the formation of tridymite from very pure silicon and water and of the conversion of tridymite M, but not tridymite S, to cristobalite below 1470°C (47). It has been suggested that the phase relations of silica are determined by the purity of the system (42), and that tridymite is not a true form of pure silica but rather a solid solution of mineralizer and silica (63). However, the assumption of the existence of tridymite phases is well established in the technical literature pertinent to practical work. 

Figure 34. Alkali zeolites projected into a portion of the Na-K-Si coordinates. Anal = analcite Ph = phillipsite solid solution Ze = alkali zeolites undifferentiated Alb = albite KF = potassium feldspar Q = quartz Si = amorphous silica, a) low, b) medium, and c) high temperature facies. Shaded areas are two-phase fields.

The observed uptake of water by quartz under conditions of high temperature, pressure, and water fugacity indicates that the diffusivity and/or solubility of water-related point defects in solid solution are much lower than are required by the Griggs model (Gerretsen et al. 1989 Kronenberg, Kirby, and Rossman 1986 Rovetta, Holloway, and Blacic 1986). 

A very low thermal expansion is also exhibited by high-temperature quartz which forms solid solutions with eucryptite by substitution Li" + The... 

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. 

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