Quartz overgrowths

Fig. 3.40 Histogram of 5 0-values of quartz in sandstone from 6-10 rm spots by ion microprobe. Mixed analyses are on the boundary of detrital quartz and quartz overgrowths (Kelly et al. 2007)...

Millot, 1964). However, some quartzites are reported to contain grains with quartz overgrowths due to deposition during weathering (Millot, 1964). These cases must be considered as a possible stable desposition of quartz although it could be just due to crystallization upon a preexisting structure. 

Franks and Forester (1984) have discussed this mechanism in detail for Gulf Coast sediments, with particular emphasis on pre- and post-secondary porosity mineral assemblages. For many localities they found strikingly similar mineral assemblages (Table 8.1). Early carbonate cements had precipitation temperatures in the range of 40° to 75°C. Quartz overgrowths were observed to precipitate... 

Figure 8.38. Summary of diagenetic events affecting Helderberg Group rocks in the central Appalachians. Hydrocarbon migration occurred during deep burial and development of quartz overgrowths in the oil-generative window of Figure 8.37, and at a temperature of about 180 to 220°C. (After Dorobek, 1987.)...

Fisher D. M. and Brantley S. L. (1992) Models of quartz overgrowth and vein formation deformation and episodic fluid flow in an ancient subduction zone. J. Geophys. Res. 97, 20043-20061. 

Makowitz A. and Sibley D. F. (2001) Crystal growth mechanisms of quartz overgrowths in a Cambrian quartz arenite. J. Sedim. Res. 71, 809-816. 

Reed R. M. and Laubach S. E. (1996) The role of microfractures in the development of quartz overgrowth cements in sandstones new evidence from cathodoluminescence smdies. Geolog. Soc. Am. Annual Meeting, 28(7),... 

GS-fabric Grain supported fabric - skeletal grains (i.e. grains >30 pm diameter) constitute a self-supporting framework. Subdivided by cement type (a) optically continuous quartz overgrowths (b) chalcedonic overgrowths (c) microquartz/cryptocrystalline/opaline silica in-fill... 

Eogenetic feldspar dissolution and kaolinization are potential sources for the early-burial quartz overgrowths. Determination of the source for quartz outgrowths and overgrowths is beyond the scope of this study. 

Fig. 4. Post-quartz late calcite (c) localized on K-feldspar remnants (k). Arrow indicates euhedral terminations of quartz overgrowths. Back-scattered electron image. Scale bar lOOftm.

Fig. 7. Thin-section scale localization of authigenic ferroan dolomite. Back-scattered electron images. Scale bars 100 tm. (A) Ferroan dolomite (f) localized on non-ferroan detrital dolomite (d) that has been fractured, partially dissolved, and partially replaced by the authigenic overgrowth. Note that the outer zones of the ferroan dolomite (f ) are brighter, reflecting their greater Fe content. (B) Ferroan dolomite (ankerite) (f) localized in the vicinity of a partially dissolved and replaced K-feldspar (k). The arrow indicates euhedral termination on a quartz overgrowth.

Fig. 14. (A) Scanning electron micrograph of prismatic quartz outgrowths covering and engulfing chlorite rims (B) optical photomicrograph of a sandstone extensively cemented by quartz overgrowths which contain bitumen inclusions crossed polarizers.

Calcite cement typically pre-dates quartz cementation, as indicated by the lack of quartz overgrowths within calcite-cemented zones (Table 1). As abundant fluid inclusion evidence (see review in Walderhaug, 1994) strongly indicates that quartz cementation typically becomes significant at temperatures of 70-80°C, this is strong confirmation that most calcite cementation was completed at temperatures below around 0 C. 

Up to 2% authigenic kaolinite and dickite occur in host rocks as loosely packed booklets of pseudo-hexagonal crystals (5-15 pm). They occur on authigenic quartz overgrowths and on partly dissolved feldspar grains. 

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