Minerals and Other Detrital Silicates

CHAPTER 14 Clay Minerals and Other Detrital Silicates... 

The lithophile elements are those that generally occur in silicate phases and include among others Si, Al, Ti, K, Na, Zr, Be, and Y. These would be expected to occur in coals in some combination with the silicate minerals kaolinite, illite, other clay minerals, quartz, and stable heavy detrital minerals. 

There are a number of ways of obtaining U-Pb ages on zircons, but in the last two decades the use of the ion microprobe has become the method of choice. Zircon (zirconium silicate, ZrSi04) is a common mineral in granitic rocks, often abundant as a detrital mineral in clastic sediments and a rare accessory mineral in mafic rocks and some metamorphic rocks. The frequent occurrence of the mineral zircon, and its resilience to later thermal events that would reset other isotopic systems are the reasons for its frequent use in studies of the early Earth. 

Judging from the mineral composition of the shales in which such secondary transformations would be reduced to a minimum there could have been only 15-30% detrital illite as a primary constituent in the sandstones of the Hassi Messaoud field whereas the amount of illite detected in the Cambrian siltstones is 30-50%. Consequently, only 1% of secondary quartz could have resulted from the transformation of illite to kaolinite. Thus, widely developed kaolinization of illite and feldspars is accompanied by the liberation of silica in the form of silicic acid which is mostly consumed in the formation of overgrowth on quartz grains. The extent of this silicification, however, is controlled by other factors the chemical nature of the environment created and especially the permeability of the rocks. The lower the permeability, the more intense silicification will be. 

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