The dissolution of quartz in soils is reviewed following brief consideration of the mineral’s structure and chemistry, particularly emphasizing the effects of defects and dislocations at the crystal surface. Experimental dissolution investigations, usually under hydrothermal conditions, suggest that the principal factors controlling or inhibiting quartz dissolution are pH, and the concentrations of dissolved silica, alkali and alkaline earth cations, organic acids and aluminium and iron species in the external solution. The application of these factors to natural pedogenic environments is reviewed in the context of the orders of Soil Taxonomy, as revealed mainly by SEM observations of surface etching features in different horizons of the soil profile. For soils of a non-residual, alluvial nature it is possible that quartz grains will have been through several weathering cycles, leading to difficulties in interpretation, as both fresh and highly weathered quartz grains may occur in intimate admixture. However, where the soil is residual and directly related to the underlying parent rock, then it may be possible to relate the nature and extent of the dissolution features on quartz to the prevailing pedogenic regime and the principal chemical factors that have been explored experimentally. The formation of quartz in soils is controversial, although it is well-embedded in the soil science literature. In most soils, quartz dominates in the sand and silt fractions and is undoubtedly of inherited origin deriving ultimately from igneous and metamorphic rocks. Criteria that could be used to confirm the pedogenic formation of quartz in soils include separation of perfect unetched euhedral quartz crystals with an appropriate isotope composition confirming their low temperature origin. Experimental low temperature synthesis of quartz spans a time scale ranging from five years to millions of years and a critical analysis of the literature suggests that the latter is a more realistic assessment.

Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) and X-ray fluorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is the predominant phyllosilicate present, 48% to 71.7% bulk rock mineralogy (excluding carbonate cemented and highly bio siliceous samples) and 70% to 98% of the <2 μm clay fraction. Opal CT and less so cristobalite concentrations cause the main deviations from smectite dominance. Opal A is common only in the Upper Kreyenhagen. In the <2 μm fraction, the Moreno Fm is significantly more smectite-rich than the Kreyenhagen Fm. Smectite in the Moreno Fm was derived from the alteration of volcaniclastic debris from contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff is preserved. Smectite in the Kreyenhagen Fm was derived from intense sub-tropical weathering of granitoid-dioritic terrane during the hypothermal period in the early to mid-Eocene; the derivation from local volcanism is unlikely. All samples had chemical indices of alteration (CIA) indicative of intense weathering of source terrane. Ferriferous enrichment and the occurrence of locally common kaolinite are contributory evidence for the intensity of weathering. Low concentration (max. 7.5%) of clinoptilolite in the Lower Kreyenhagen is possibly indicative of more open marine conditions than in the Upper Kreyenhagen. There is no evidence of volumetrically significant silicate diagenesis. The main diagenetic mineralisation is restricted to low-temperature silica phase transitions.