Beta-quartz

Table IV. Comparison of X-Ray Data for Magnesium Aluminum Silicates Found in this Study with High (beta) Quartz ...

Alpha quartz (low quartz) is most often used for piezoelectric crystal detectors. It crystallizes at temperatures below 573 °C, and transforms to beta quartz (high quartz) between 573 and 870 °C. Generally, AT- and ST-cut quartz plates are used in BAW and SAW, respectively. The term cut designates the direction of the normal to the major faces. A standard AT cut, for example, describes a plate with thickness in the y direction rotated 35° 15 counterclockwise about the X axis (figure 19.1). 

Zeolite beta was obtained as a pure ptiase from all experiments. A complete crystallization, as indicated by XRD, was attained after 24 hours for experiments from i to 4, whereas 72 hour.s were needed for experiments 6 and 6. On longer crystallization time other phases appeared beside zeolite beta Quartz was formed after 6 days in the conditions of expserlment 1, zeolite ZSM-12 after 3 days in the conditions of experiment 4, and zeolites ZSM-5 and ZSM-12 after 6 days in the conditions of the experiments 5 and 6. 

There are seven crystal modifications of natural quartz alpha- and beta-quartz alpha-, beta-, and go/w/wfl-tridimite and alpha- and /)eto-crystoballite. Below in Scheme 1.1. are shovm the temperatures of transfers between the different quartz modifications. 

Quartz exists as two polymorphic forms alpha-quartz (or low quartz), stable at room temperature and pressure, and beta-quartz (or high quartz), stable only at temperatures above about 600°C. The structure of alpha-quartz is shown below ... 

Alpha quartz belongs to the point group 32, whereas the more synunetric beta quartz has 622 symmetry. Alpha quartz has a variety of different crystal habits, several of which are shown below ... 

The two forms on the left are formed by beta quartz, but are also formed, although not commonly, by alpha quartz. Quartz forms a large number of colored varieties, including rose quartz, amethyst, smoky quartz, citrine (Ught-yellow), and milky quartz. Inclusions such as rutile and hematite are fairly common. It also exists in a large number of varieties that exhibit no macroscopic crystalline structure. These include chalcedony, agate, flint, chert, and jasper. 

Silica, with the chemical formula Si02 and relative molar mass of 60.084, exhibits a complex polymorphism characterized by a large number of reversible and irreversible phase transformations (Figure 10.1) usually associated with important relative volume changes (AV/V). At low temperature and pressure beta-quartz (P-quartz) [14808-60-7] predominates, but above 573°C, it transforms reversibly into the high-temperature alpha-quartz (a-quartz) [14808-60-7] with a small volume change (0.8 to 1.3 vol.%) ... 

T.Shibuya, K. Matsui, M. Matsumoto Infrared transparent glass ceramic articles with beta-quartz solid solution crystals without any other crystals , U.S. Pat. No. 4,835,121, May 30, 1989. 

K. Chyung Transparent beta-quartz glass-ceramics , US Patent 4,018,612, April 19, 1977... 

Siiicon dioxide (a-quartz) SiOj 14808-60-7 60.086 coi hex cry trans to beta quartz 673 2960 2.648 HjO, acid sHF... 

At 573°C, the transformation of alpha to beta quartz occurs with an abrupt increase in volume. 

Inversions and conversions of free silica (Si02) phases that involve potentially destructive volume changes. The well-known alpha-to-beta quartz inversion is seen at 573°C. 

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