In nature, silica (Si02) occurs as seven distinct polymorphs: quartz, cristobalite, tridymite, coesite, stishovite, lechatelierite (silica glass), and opal; the latter two are amorphous. Of these minerals, quartz is most abundant in soil environments, while disordered cristobalite commonly occurs in soils. Tridymite is rare in soils and is usually associated with siliceous volcanic rocks. Coesite, stishovite, and lechatelierite are rare polymorphic forms produced at high pressures, but found under atmospheric temperature and pressure at sites of meteor impact. Opal is a hydrated "amorphous" silica that comprises the bulk of diatomaceous rocks of biogenic origin and silica of inorganic origin (Frondel, 1962; Deer et aI., 1978). This chapter emphasizes quartz and the disordered polymorphs of silica. The disordered polymorphs have been referred to by a number of conflicting and sometimes ambiguous names, often resulting in confusion. The definitions of the disordered silica polymorphs as proposed by J.B. Jones and Segnit (1971) appear to be more widely adopted and will be utilized, where possible, throughout this chapter. Each of these distinct and unique polymorphs of silica expresses a definite degree of structural order. The structural order defines the individual polymorph and results in unique physical and chemical properties which are briefly summarized below.

Silica in Soils: Quartz and Disordered Silica Polymorphs

The microtextural analysis of sand grain surfaces has long been regarded as an important indicator of the environment through which a sediment has passed (Marsland and Woodruff, 1937; Bond, 1954). Until the advent of the electron microscope it had not been possible to examine the surfaces of the sand grains in any detail. Using light microscopy grains could be designated a roundness parameter and detail of frosted or clear surface textures could be recognized. Despite these limitations the significance of such an investigation had not gone unnoticed

Environmental reconstruction by electron microscopy

The relative abundance of thirty-two surface features identified on the surfaces of quartz sand grains from eight coded samples was noted independently by five scanning electron microscopists. These data (reduced to binary form) were the subject of canonical variate analyses which discriminated clearly between all samples. Thirteen variables were important in distinguishing between the eight samples. This indicates that there are no single key surface features which, on their own, would allow rapid environmental discrimination. The contention that a multiplicity of surface features should be recorded for the purpose of environmental discrimination is, therefore, upheld.



Operator variance, although considerable in the recognition of individual surface features, is negligible in discrimination of samples based on binary data. The SEM technique of analysis of quartz grain surface textures is a reliable and statistically valid means of discriminating between samples from different environments.

Environmental discrimination based on quartz grain surface textures: a statistical investigation

THE use of the electron microscope to determine the depositional histories of sand grains is a relatively new but well acclaimed technique1–6. The underlying principle is that a detrital quartz grain will bear surface textures that may be related to processes that have operated on it during transportation and deposition.

Depositional Histories of Sand Grains from Surface Textures

Micromorphology and dissolution of quartz sand in some exceptionally ancient soils

SUMMARY

In a historical review of the development of quartz-grain morphoscopy some unsolved problems such as the real aspects of the details on frosted grain surfaces and the interpretation of the origin of different surface textures are elaborated. Discrepancies in the presentation of quartz-grain types as observed by electron microscopy are mentioned. It is proposed to relate such investigations to CAILLEUX's (1942) classical morphoscopic method; this could be facilitated by the modern scanning electron microscope. The possibilities and advantages of this instrument for morphoscopic research are presented by first results concerning the surface textures of original quartz grains derived from a weathered granite and of experimentally and naturally etched grain surfaces. For generalized conclusions a greater number of further observations must be made.

Horst E. Schneider

Papers

Problems of quartz grain morphoscopy