| Summary: | The crystalline forms of quartz and that it is insoluble are well known. The hydrolysis of silica and formation of polymeric silicic acids in slightly alkaline (pH 8.1 - 8.3) sea water is less well known and not recognised by many geologists. Silica is one of the most abundant components of ordinary sediments and the amorphous forms, the silica gels, have been closely studied. Their properties and behaviour are perhaps better known than those of many other colloids. Quartz veins are abundant in all types of sediments and in rocks and mineral deposits derived from them. Clearly if we study how silica is mobilised into these veins and lodes, how it silicifies wall rocks, forms opal, replaces shells and tree trunks, etc. then we may have a better basis for understanding how sulphide particles might similarly be mobilised into veins and lodes, permeate shales, form framboids, replace fossils and plant fragments or fine shale bands, etc. This article briefly summarises recent developments in the aqueous chemistry of silica. It emphasises the particulate nature of the amorphous silica species in order to update rather simplistic views that quartz veins and many other natural forms of quartz “crystallise directly from solution”. A number of features of quartz veins which are due to the particulate nature of natural polymeric silica are illustrated.
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