Bond Strength and the Relative Weathering Rates of Simple Orthosilicates

Common naturally occurring orthosilicates can be grouped into those in which the major non-tetrahedral cations are (1) FeII and/or Mg, plus or minus Al (olivine, pyralspite garnets, staurolite); (2) Al (Al2SiO5 polymorphs), or (3) Zr. Feand Mg-bearing orthosilicates have the weakest bonds between non-tetrahedral cations and structural oxygen (M-O bonds), and zircon the strongest, with Al-O bonds intermediate; thus, the relative vulnerability to weathering of the three groups decreases in the order Fe-Mg( Al) orthosilicates G Al2SiO5 polymorphs G zircon. Among Fe( Mg Al)-orthosilicates, Fe-O bond lengths vary with Fe coordination number (CN), and Fe-site energies decrease in the order fayalite olivine G almandine garnet GG staurolite. If other factors did not intervene, almandine (CNFe 8) could weather approximately as fast as olivine (CNFe 6). However, reactant-product molar-volume ratios involving almandine and common weathering products of oxidizing environments favor the formation of transport-limiting, rate-limiting protective surface layers on almandine and not on olivine; thus, almandine’s weathering rate is commonly suppressed by these protective surface layers to a sufficient extent that almandine weathers more slowly than olivine. Short, strong Fe-O bonds involving tetrahedral Fe render staurolite more resistant to weathering than either common olivines or common pyralspite garnets. Relative weathering rates within the Al2SiO5 group vary with the coordination number of Al. When formation of protective surface layers on garnet is taken into account, relative weathering rates of common orthosilicates decrease in the same order as the M-O bond energies for the dominant non-tetrahedral cations, giving rise to the commonly observed natural orthosilicate weathering series: Olivine G Garnet G Staurolite G Kyanite G Andalusite G Sillimanite G Zircon. The lengths and strengths of bonds between major non-tetrahedral cations and structural oxygen are a fundamental control of relative orthosilicate weathering rates in nature as in laboratory experiments.