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Mineral Replacement Reactions

Institut für Mineralogie, University of Münster, Münster, 48149 Germany, putnis@uni-muenster.de

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Whenever a mineral or mineral assemblage comes into contact with a fluid with which it is out of equilibrium, reequilibration will tend to take place to reduce the free energy of the whole system (i.e., of the solid + fluid). Such fluid-solid interactions span a very wide range of possible reactions, and are responsible for most of the mineral assemblages we see in the Earth’s crust. However, before discussing mineral replacement reactions, we will put them into the broader context of fluid-solid interactions by considering some examples of such reequilibration.

In the simplest case, we could consider the situation where a mineral, thermodynamically stable under some speciflc temperature and pressure conditions, comes into contact with pure water, such as quartz within its stability field (e.g., at T = 100 °C and 1 atmosphere pressure). Clearly, quartz will tend to dissolve until, at equilibrium, the aqueous silica solution, which at neutral pH is H₄SiO_4(aq), becomes saturated with respect to quartz. The reaction for this equilibration can be written:

(1)

The equilibrium solubility constant for reaction (1) is given by

where a(i)_aq stands for the activity of the parenthetical aqueous species. For the case of pure quartz and pure water where the activity is 1, K_sp ( qtz ) ~1.2 x 10^–3 at 100 °C and 1 atmosphere pressure. However, if under these conditions the solid silica phase in contact with pure water was cristobalite (the high temperature polymorph of SiO₂), the resulting aqueous solution, saturated with respect to cristobalite, would be supersaturated with respect to quartz, since the less stable phase is more soluble. The value of K_sp for cristobalite at 100 °C and 1 atmosphere pressure is ~5.1 x 10^–3. Thus the thermodynamics would indicate that quartz should precipitate from such a solution. On . . . [Full Text of this Article]