Kinetic-rheological insights uncovered by the self-accelerating brucite-for-periclase replacement - but only if adjusted on volume. The blind spot of geochemists

Geochemists of weathering, metamorphism, dolomitization, etc., have long adjusted mineral reactions conserving one or another component among the minerals, but paying no attention to whether the volumetric consequences of such adjustments are consistent with independent petrographic evidence. In fact the widespread occurrence of replacement in all types of water-rock interaction implies that the mineral reactions involved did conserve solid volume, not a component. The conflict has been hidden in a blind spot that geochemists appear to be unaware of having. Characteristically, replacement preserves both volume and morphological details of the host. It originates not by dissolution-precipitation, but by guest-growth-driven pressure solution of host by guest (Merino/Dewers 1998; Merino/Canals 2011) The replacement of periclase by brucite common in magnesian marbles is attributed (Turner 1965; Ferry 2000) to: MgO(per) + H2O = Mg(OH)2(bruc), (EQ 1), which conserves Mg between the two minerals. But with formula volumes of pericl & bruc ~11 & 25 cm3, this reaction cannot preserve mineral volume; it conflicts with excellent petrographic evidence that mineral volume was preserved. However, by adjusting the local mass balance on volume, as observed, 2.2MgO(per) + 2.4H+ = Mg(OH)2(bruc) + 1.2Mg++ + 0.2H2O (EQ 2), we gain striking dynamic/geochemical insights: (I) Seeing that Mg++ is locally released to the pore fluid, we suddenly grasp why the brucite is typically surrounded by a rim of dolomite replacing matrix calcite. As it is released, the Mg++ reacts with calcite and produces dolomite that replaces calcite, also isovolumetrically, as observed. (II) The second replacement (dolo-for-calc) was simultaneous and thus adjacent to the first, and must have happened at the same T,P, and at the same rate, as the first. (In the conventional view, the dolomite rim is attributed, ad hoc, to the reaction bruc + calc + CO2 = dolo + water, which necessarily happens later and at different T,P; which doesn't preserve volume; and which tells us nothing about rates.) (III) The Mg++ released by each increment of bruc-for-pericl replacement must increase the local ion-activity product for brucite growth from its ions, and thus the growth rate of the next increment of replacement, and so on: The bruc-for-pericl replacement is self-accelerating. This is why it always reaches completion - the brucite keeps replacing periclase faster and faster until all the periclase is gone. (Also self-accelerating are the dolo-for-calc replacement in dolomitization [Merino/Canals, 2011], and the serp-for-oliv replacement in serpentinization.) (IV) A dramatic feedback arises: As a self-accelerating replacement takes place in a non-newtonian rock of the strain-rate-softening kind (as crystalline carbonates and dunites are), it must reduce the local rock viscosity more and more, and, unless a needed reactant is used up first, the viscosity may become low enough for the growth of the guest mineral to gradually pass by itself from replacive to displacive (both régimes being driven by the guest-growth induced stress). This is how thin, displacive zebra veins form routinely in burial dolomitization (Merino/Canals 2006, 2011) and in serpentinization of dunite.