Diagenesis: Replacement & Recrystallization
2015. "Diagenesis: Replacement & Recrystallization", A Color Guide to the Petrography of Sandstones, Siltstones, Shales and Associated Rocks, Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Replacement refers to the process of precipitation of minerals that take the place of preexisting minerals, either in detrital grains (monomineralic grains or rock fragments) or previously formed cements. It can even affect nonminerals, as in the case of petrified wood. Replacement, as strictly defined, proceeds by dissolution of the precursor mineral and essentially simultaneous in-situ precipitation of the replacement mineral. Typically, that takes place along a solution front or film that can be extremely thin (micrometer to nanometer scales) and replacement can start at one or many small, mineralogically susceptible nucleation sites throughout the host crystal. The general concept of replacement, however, also includes mineral dissolution that is followed by temporally unrelated cementation of the newly-created secondary pores perhaps thousands to many millions of years later. This situation, sometimes termed “solution-cavity fill” (SCF)(see Folk, 1965), is clearly not precisely synonymous with “replacement” as used in its strictest sense. Nonetheless, it should be noted that it commonly is not possible to unequivocally distinguish strictly-defined replacement from SCF, especially where the replacement is complete, with essentially no remnants of the original mineral (but it is possible to tell them apart if relicts of the host mineral are present or other features of the host are preserved within the new phase). Therefore, in this book we use the term replacement in its general sense for either type of alteration. We show examples in this chapter of both near-simultaneous replacements, as narrowly defined, and SCF replacements, despite the fact that the separate processes of dissolution and cementation have already been covered in other chapters.