Abstract
IN the case of silica and its hydrates the physical and chemical phases of the Liesegang phenomenon manifest themselves in a great variety of rhythmic mineral deposits. Among these formations, hyalite and opal appear to be based upon a common colloidal substrate identical with the prototype, gelatinous silicic acid1. There is no such agreement, however, regarding the differences between these two minerals. Thus, the existing theories of opalescence ascribe the display of colours to (1) internal fissures (Hauy2), (2) air-pockets or microscopic cavities (Brewster3), and (3) incorporated curved opal lamellae (Behrens4). These theories appear to enumerate contrasting possibilities rather than elucidate realities, and certainly bear no relation to the conditions underlying the formation of the minerals. The absence of clear-cut differences (other than optical) between hyalite and opal in their state of hydration, specific gravity, and tensile strength, as well as the combined effect of environmental pressure and syneresis, are opposed to Hauy's and Brewster's theories. Any cavities or fissures which may occur would be merely adventitious and a source of weakness.
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