Gypsum crystal characteristics

Optical properties of twinned crystals. Each individual in a twin exhibits its own optical characteristics. If a gypsum twin is seen along its b axis and examined between crossed polarizers, it can be seen that each individual extinguishes independently. The twin plane (100) is a plane of symmetry of the composite whole, and the vibration directions of the two individuals, like all the other properties, are related to each other by this plane of symmetry (see Rig. 57 a). 

Although the chemical composition of the calcium sulfate compounds is relatively well known, the physical characteristics of these compounds received little attention despite their great influence on the manufacturing processes and uses of the gypsum products. Table 3 illustrates the diverse nature of the physical characteristics of the known phases of calcium sulfate as indicated by their widely varying crystal modification, aggregation state, and internal structure. 

The population distribution in the combined product is strongly bimodal, with one peak at about the same size as with MSMPR operation. The second peak size may be several times as large as the first one, and the average size two to three times that obtainable from an MSMPR crystallizer. This may dramatically change the filtration characteristics of the product In the disposal of waste gypsum, for example, ordinary crystallization produced an unfilterable slime, but the product from a DDO crystallizer, despite its high content of small crystals, filtered like sand. - ... 

Conflicting results on tlie effect of flow rate or fluid velocity on scale deposition have been reported by Andritsos and Karabelas [21]. They showed that the flow rate either increased or decreased the mass of scale deposited over a certain period of time. Clearly, this is due to the nature of the solid-fluid interface layer and the shear sti ess induced to the scale by the flow. These two incompatible forces are further complicated by other parameters such as the presence of additives, which may exert a significant effect on the nature of the interface layer as well as on the characteristics of the scale deposited [26]. Fluid velocity also affects the orientation of the growing scale or crystals [2, 27, 28]. The tendency is that the scale to orient to the direction of the fluid flow [2]. A rather recent study on gypsum and calcium carbonate scaling of membrane desalination by He and co-workers [29] shows that a faster flow rate increases more deposition of gypsum scale. 

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