Frio clay

Figure 7 Replacement driven by force-of-crystal-Uzation is characterized by authigenic phases that develop euhedral faces that are not plausibly constmed as crystal growth within pore spaces (a) sphalerite replaces albitized detrital feldspar and adjacent portions of clay-rich matrix, Frio Formation, Oligocene, South Texas and (b) siderite crystal (s) attacks a detrital K-feldspar (K) in sandstone, Breathitt Formation, Pennsylvanian, eastern Kentucky.

The difficulties imposed by clay coatings on quartz nucleation probably explain why there are few reports of quartz cementation in shales. The oxygen in quartz silt is isotopically heavy (8 0 = 19%o) compared to sand-size quartz (5 0 = 14%o) (Blatt, 1987) suggesting that a substantial component of relatively low-temperature quartz resides in shales. The application of cathodoluminescence microscopy to shales has been quite limited. In Frio Formation shales of south Texas no convincing quartz cement is observed across a range of depths in which quartz cementation becomes important in associated... 

Freed R. L. (1982) Clay mineralogy and depositional history of the Frio Formation in two geopressured wells, Brazoria County, Texas. Gulf Coast Assoc. Geol. Soc. Transac. 32, 459-463. 

Lynch F. L. (1997) Frio shale mineralogy and the stoichiometry of the smectite-to-illite reaction— the most important reaction in clastic sedimentary diagenesis. Clays Clay Mire 45, 618-631. 

---