Ab-initio studies of the kinetics and mechanisms of feldspar dissolution: Ion exchange, H[sub 3][sup +]O catalysis, and the formation of leached layers

Conference · Wed Jan 01 00:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5690848

Xiao, Yitian; Lasaga, A C ^[1]

---

1. Yale Univ., New Haven, CT (United States). Dept. of Geology and Geophysics

Mineral dissolution is a complex process involving many reaction steps such as hydration, ion exchange, acid-base catalysis, and hydrolysis. Each of these elementary reactions is determined by the chemistry and structure of the interface between solid and solution. It is generally agreed that the selective hydrolysis of Si-O-Al and Si-O-Si linkages in feldspar leads to the formation of leached layers. To truly understand the detailed mechanisms of feldspar dissolution, the authors carried out high level ab-initio molecular orbital calculations for the following reactions: [triple bond]Si-O(Na[sup +])-Al[triple bond] + H[sub 3]O[sup +] [r arrow] [triple bond]Si-O(H[sub 3]O[sup +])-Al[triple bond] + Na[sup +] and [triple bond]Si-O(H[sub 3]O[sup +])-Al[triple bond] [r arrow] [triple bond]Si-OH + [triple bond]Al-OH[sup +][sub 2]. The ab-initio results successfully explain the pH dependence of feldspar dissolution, the formation of leached layers, and recent experimental data on the kinetic isotope effect. The calculated structures and energetics indicate that: (1) The substitution of Na[sup +] by H[sub 3]O[sup +] at a bridging oxygen site significantly weakens the Si-O-Al and Si-O-Si linkages; (2) The activation energies for the hydrolysis of Si-O-Al and Si-O-Si bonds by H[sub 2]O are 27 kcal/mol and 32 kcal/mol, respectively, while under H[sub 3]O[sup +] catalysis they are 16 kcal/mol and 22 kcal/mol based on calculations at the MP2/6-31G level. Therefore the ab-initio results provide direct evidence of H[sub 3]O[sup +] catalysis and the preferred hydrolysis of Si-O-Al linkages over Si-O-Si linkages; and (3) Pure H[sub 2]O hydrolysis predicts a rather big kinetic isotope effect (k[sub H][sub 2]O/k[sub D][sub 2]O [equals] 4) while under H[sub 3]O[sup +] catalysis k[sub H][sub 3][sub O][sup +]/k[sub D][sub 3][sub O][sup +] [equals] 0.9.

• OSTI ID: 5690848
• Report Number(s): CONF-921058-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
FELDSPARS
DISSOLUTION
ACTIVATION ENERGY
CATALYSIS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
HEAVY WATER
HYDROLYSIS
ION EXCHANGE
ISOTOPE EFFECTS
MOLECULAR ORBITAL METHOD
ROCK-FLUID INTERACTIONS
CALCULATION METHODS
DECOMPOSITION
ENERGY
HYDROGEN COMPOUNDS
KINETICS
LYSIS
MINERALS
OXYGEN COMPOUNDS
REACTION KINETICS
SILICATE MINERALS
SOLVOLYSIS
WATER
152003* - Geothermal Data & Theory- Rock-Water-Gas Interactions