Colloid Geochemistry of Silica

Abstract

Silica, Si0 2 , is common in nature and occurs as seven distinct minerals, of which five show crystalline structures (quartz, tridimite, cristobalite, coesite, and stishovite) and two are amorphous (opal-A and lechatelierite). Lechatelierite is a silka glass and is very rare. Coesite and stishovite are also very rare and are found only in meteoritic craters, where they have been formed from quartz because of the high pressure resulting from the meteoritic impact. Opal is obtained by deposition from aqueous silica solutions at low temperatures. It is deposited by thermal waters associated with igneous activity and is also secreted by sponges, radiolaria, and diatoms. The most common form of silica is quartz.Amorphous silica is an important component of soils, coating particles of quartz, feldspar, and hornblende, as well as clay minerals (McKyes et aI., 1974). It also plays an important role as a cementation agent in many sediments. Monosilicic acid is an important constituent of natural aqueous solutions. In the pH range of normal natural water the monosilicic acid occurs predominantly as the uncharged Si(OH)4 species. However, suspended species of silicates are important and, according to Garrels and Mackenzie (1972), the dominant transfer of silicon in the sedimentary cycle occurs in the form of suspended quartz or suspended clay minerals. In their model the ratio of silica in the suspended flux to silica in the dissolved flux is about 4 to 1. From the Cambrian period (some 500-600 x 10 6 years ago) and in the present period most of the dissolved silica is removed from the oceans in the tests of organisms as opaline silica. Eventually, it is transformed to quartz as chert and/or overgrowth on detrital quartz grains or reincorporated into silicate minerals. According to Wise and Weaver (1974) chert formation is a "maturation" process, i.e., the aging of colloid silica system occurs as follows: The amorphous precipitate of biogenous silica is transformed into opal-CT, a poorly ordered crystalline form of silica that contains cristobalite and tridymite, and on further aging, into quartz.During the Precambrian period, prior to the advent of siliceous organisms, amorphous silica probably precipitated from marine waters by non-biogenous mechanisms. According to Oehler (1976) the diagenetic reactions culminating in biogenous and nonbiogenous chert formation may differ. Some nonbiogenous cherts were formed through an intermediate opal-CT stage and some crystallized directly from silica gel without the intervention of an intermediate crystalline phase such as opal-CT. The quartz crystallized primarily in the form of radially fibrous, chalcedonic spherulites, which later recrystallized to form anhedral grains, producing the interlocking mosaic texture typical of most cherts. The following diagenetic sequences were suggested by Oehler:S. Yariv et al., Geochemistry of Colloid Systems © Springer-Verlag Berlin Heidelberg 1979 248 Amorphous biogenous silica