MINERAL REPLACEMENT CLARIFIES GEOCHEMICAL DYNAMICS: EXAMPLES FROM WEATHERING AND PETROLOGY

Richard Hay's deduction of the paleoclimate at Olduvai Gorge was based on identifying very early zeolitic replacements petrographically. His finding that Cambrian-Silurian rocks of the entire U.S. mid-continent have been replaced by K-feldspar - implying a huge metasomatism of potassium - also rested on petrography. Since replacement is characterized by spatial properties – preservation of mineral volume and of internal shapes – it can be detected only visually, not analytically. Evidence of replacement combined with a new theory of replacement physics clarifies geochemical reactions and feedbacks that remain hidden if viewed from the standpoint of equilibrium.

Brucite-for-periclase replacement in magnesian marbles is represented by MgO_(pericl) + H₂O = Mg(OH)_2(bruc), which assumes Mg is immobile and ignores that replacement preserves volume. The correct statement is 2.2MgO_(pericl) + 2.4H⁺ = Mg(OH)_2(bruc) + 1.2Mg⁺⁺ + 0.2H₂O, which preserves mineral volume and tells us what drives the dolomite-for-calcite replacement corona that occurs around the brucite: the released Mg⁺⁺ does. Only now do we see that the two replacements were simultaneous and coupled, one driving the other.

Students of weathering calculate mass loss and gain in the weathering profile by assuming that Al, Ti, etc., is immobile. But when, for example, gibbsite replaces K-feldspar in lateritization, it is the volume, not Al, that gets conserved. This requires approx. 3 gibbsite formulas in the replacement reaction: KAlSi₃O_8(fp) + 2Al³⁺_(aq) + 7H₂O = 3Al(OH)_3(gibb) + K⁺ + 3SiO₂ + 5H⁺. We now see that Al³⁺_(aq) must be supplied – from above – to the feldspar being replaced, and that it is not immobile. This leads us to find the feedback central to the weathering of aluminosilicates at a moving reaction front, a feedback which remains hidden if Al is assumed immobile.