Plagioclase dissolution

The continental pattern for Na matches the pattern for total feldspar percentages, as Na values are primarily correlated with plagioclase (Eberl Smith 2009). Feldspars are much more susceptible to chemical dissolution than quartz and, with sufficient time and precipitation, will weather mainly to clay minerals. As a result, total feldspar contents and Na contents decrease with increasing precipitation from west to east (Fig. 3). 

Plagioclase dissolution is known to be incongruent, with Al and Ca being preferably dissolved at the surface (Blum... 

Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. 

State (i) partial melting controlled by diffusion in plagioclase, and (ii) dissolution and recrystallization. Below we discuss partial melting controlled by diffusion in plagioclase. 

Donaldson C.H. (1985) The rates of dissolution of olivine, plagioclase, and quartz in a basaltic melt. Mineral. Mag. 49, 683-693. 

Tsuchiyama A. (1985a) Dissolution kinetics of plagioclase in the melt system of diopside-albite-anorthite and origin of dusty plagioclase in andesites. Contrib. Mineral. Petrol. 89, 1-16. 

Huang, W. H., and Kiang, W. C. (1972). Laboratory dissolution of plagioclase feldspars in water and organic acids at room temperature. Am. Mineral. 57, 1849-1859. 

Obviously, the dissolution of the elements leads to change in the crystal lattice and the mineral composition. This can well be seen during the acidic treatment of montmorillonite or bentonite for catalytic purposes (Section 2.1). The treatment is done using concentrated hydrochloric, sulfuric, or phosphoric acid. X-ray diffraction studies show that a commercially available montmorillonite has low montmorillonite content (53%). The other constituents are illite 10%, kaolinite 6%, quartz 10%, plagioclase 5%, gypsum 1%, anhydrite 4%, and amorphous 7%. 

Welch, S. A. Ullman, W. I. (1993). The effect of organic-acids on plagioclase dissolution rates and stochiometry. Geochimica et Cosmochimica Acta, 51, 2725-36. 

Figure 8. Comparison of the regression coefficients for parabolic and linear rate laws for plagioclase dissolution at 25°C and a pH range of 3-8...

Figure 11 Measured dissolution rate for plagioclase from Panola granite, Georgia, USA compared to published dissolution rates of other plagioclase samples. Panola plagioclase was dissolved either as fresh unweathered samples, or as naturally pre-weathered samples (see text). Also plotted are dissolution rates for plagioclase under ambient conditions in the laboratory at near-neutral pH for freshly ground samples from other localities (solid symbols) and for samples weathered naturally in other field localities and then dissolved in the laboratory (open symbols). All rates were normalized by BET surface area. Dashed line is a fit to all field and laboratory data, including field data from systems weathering for periods of time 1 yr (data not shown). Figure adapted from White and Brantley (2003), and all data are attributed in that paper.

Panola granite in the field (10 mol m s ). In both the case of Panola and the CA spodsol, the duration of natural weathering in situ prior to collection was —500 kyr. The slow rates of dissolution of naturally weathered Panola and the plagioclase from the CA spodsol in the laboratory are therefore best explained by both the extensive natural weathering experienced by each sample and by the length of duration of weathering in the laboratory. 

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