Early development of Al, Ca, and Na compositional gradients in labradoriteleached in pH 2 HCl solutions

Labradorite reacted with HCl solution (pH = 2.0) develops leached layers ex tending to about 500 A depth after 12 h leaching, and to 1500 Angstrom dept h after 143 h leaching. Accurate Al, Ca, and Na compositional depth profile s were measured using x-ray photoelectron spectroscopy (XPS), with composit ional accuracy of about 10% and depth resolution of about 50 Angstrom. XPS analyses of pristine K-feldspar and labradorite surfaces yield Ca, Al, Si, and O analyses within about 5% of electron microprobe results. Alkali eleme nt analyses are inaccurate due to preferential sputtering or mobility induc ed by fracture. The accurate compositional depth profiles yield well constr ained diffusion coefficients and moving boundary velocities for Ca and Al. Na, Ca, and Al compositional gradients change character after about 2 days of leaching, from a convex upward hyperboloid to a sigmoid shape. Thereafte r. the feldspar diffusion front is clearly separated from the surface (wher e silica dissolution occurs), with the diffusion front migrating into the f eldspar at about 4 x 10(-11) cm/s. Al diffuses down the compositional gradi ent at about 2.5 x 10(-17) cm(2)/s and Ca diffuses almost twice as fast (4. 0 x 10-17 cm(2)/s). The solution-solid interface and active leaching zone a re separated (after 2 days) by a Si-rich zone virtually devoid of Na, Ca, a nd Al. Diffusion rates through this Si-rich overlayer may be very rapid and approach rates observed in aqueous solutions. Diffusive release of Ca and Al from labradorite cannot be modelled accurately with mathematical solutio ns where diffusion through homogeneous media is assumed. During leaching, C a and Al apparently diffuse by "jumping" to, and residing on, previously va cated structural sites of the feldspar. The probability of Ca and Al migrat ing towards solution consequently is greater than their probability of migr ating towards pristine plagioclase, primarily because there are many more " vacant" sites in the leached zone than in pristine plagioclase. Diffusion i s inhomogeneous and "impeded" in the direction of the pristine feldspar. Co pyright (C) 2001 Elsevier Science Ltd.