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The kinetics of silica-water reactions

Authors:

JD Rimstidt HL Barnes

Institution:

Ore Deposit Research Section, The Pennsylvania State University, University Park, PA 16802, U.S.A.

Abstract:

A differential rate equation for silica-water reactions from 0–300°C has been derived based on stoichiometry and activities of the reactants in the reaction SiO₂(s) + 2H₂O(l) = H₄SiO₄(aq)

(?a_{H_{4}SiO_{4}} ?t)_{P.T.M.} = (A M)(γ_{H_{4}SiO_{4}})(k+a_{SiO_{2}} a^{2}_{H_{2}O} ? k_a_{H_{4}SiO_{4}})

where (

A M

) = (the relative interfacial area between the solid and aqueous phases/the relative mass of water in the system), and k₊ and k_? are the rate constants for, respectively, dissolution and precipitation. The rate constant for precipitation of all silica phases is

log k_{?} = ? 0.707 ? 2598 T (T, K)

and E_act for this reaction is 49.8 kJ mol^?1. Corresponding equilibrium constants for this reaction with quartz, cristobalite, or amorphous silica were expressed as

log K = a + bT + c T

. Using

K = k_{+} k_{?}

, k was expressed as

log k_{+} = a + bT + c T

and a corresponding activation energy calculated:

	a	b	c	E_act(kJ mol ^-1)
Quarts	1.174	-2.028 x 10³	-4158	67.4–76.6
α-Cristobalite	-0.739	0	-3586	68.7
β-Cristobalite	-0.936	0	-3392	65.0
Amorphous silica	-0.369	-7.890 x 10^-4	3438	60.9–64.9

Keywords:
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