Altered layer

Compared with XPS and AES sputter depth profiling After achieving sputter equilibrium, and until a layer with different sputtering behavior is reached [3.59], the SN flux represents stoichiometry and not altered layer concentrations evolving because of preferential sputtering effects. 

A second set of examples deals with the analysis of near-surface regions of glasses which normally have so-called altered or leached layers. The altered layer is found for soda-lime glasses and for many glasses used for optical applications. The chan-... 

Fig. 4.52. SIMS and IBSCA depth profiles of the altered layer region of a lithium aluminosilicate (LAS) glass ceramic (conditions SkeVAr" ).

Micro-XRD and micro-XANES analyses showed that the earliest stage of sulfide alteration is marked by the progressive oxidation of sulfide-S to sulfate-S that is then rapidly leached out from the system. Sulfide oxidation starts from particle rims or from intra-grain microfractures and is accompanied by a progressive loss of sulfur sulfides are then pseudomorphically replaced by goethite and minor bemalite. In addition to sulfides, many gangue silicates are efficiently altered and only quartz is preserved within the altered layers. 

Chou and Wollast (23) challenged XPS studies which indicate that incongruent surface layers thicker than several Angstroms do not exist. They argue that material balance calculations require some sort of altered layer in order to account for the observed incongruency between alkalis, silica, and aluminum. Their material balance calculations suggest that the layer thickness must be on the order of only tens of nanometers, which, despite their arguments to the contrary, is not inconsistent with the surface chemistry observations (e.g., XPS) they seek to refute. 

NH4 1-200 ppm Forms pure component NH4Al3Si308 (buddingtonite), sporadically found in hydrothermally altered layers. 

The depth of the altered layer, in our opinion, can be estimated by the following procedure. The energy (momentum) pulse is likely to penetrate the lattice more deeply than does the particle itself. For example, 200 eV Xe ions collide with a tungsten lattice and are primarily reflected back into the gas phase. Nevertheless, atomic motion induced by the impinging ion probably occurs to a depth of 30-50 A beneath the surface. Therefore, a crude estimate (but probably the best available) of the depth of the altered layer can be obtained by assuming a constant value of 30 A at energies where... 

That these ideas have some merit is indicated by the work of Hart, Dunlap, and Marsh . These investigators deposited a fraction of a monolayer of copper onto a silicon wafer and then monitored the position and concentration of the copper using Rutherford backscattering. After deposition, the copper, which was then located on the immediate surface, was bombarded with 20 keV Ne ions to a fluence sufficient to sputter 90 A of Si from the surface. The Rutherford backscattering spectrum, which was taken after this bombardment, showed that the copper was uniformly distributed to a depth of 600 A which corresponds roughly to the projected range of the Ne" " ions, i.e., the depth of the altered layer was approximately equal to the projected range of the Ne. 

The fact that the constituents of a multicomponent system are not removed stoichio-metrically by sputtering not only influences the altered layer but has also been used to control the composition of sputtered films, which have been grown in a plasma environment. A quantitative model, which relates composition to sputtering coefficients, has been published . 

Fig. 2. Alteration layers observed on the surface of basaltic and nuclear glasses, comprising a porous zone and phyllosilicates (a) tholeiitic basalt glass altered 120 days at 50 C (Crovisier et al. 1987) (b) MW nuclear glass altered 5.7 years at 90 CC (Curti et al. in preparation).

Fig. 6. Outer region of the alteration layer formed after 600 days. Note that the outer hydrotalcite crystals are replaced by a silicate.

Feng, X., Cunnane, J. C. Bates, J. K. 1994. A literature review of surface alteration layer effects on waste glass behavior. Ceramics Transactions, 39, 341-352. 

Murakami, T., Banba, T., Jercinovic, M. J. Ewing, R. C. 1989. Foimation and evolution of alteration layers on borosilicate and basalt glasses Initial stage. In Lutze, W. Ewing, R. C. (eds) Scientific Basis for Nuclear Waste Management XII. Materials Research Society Symposia Proceedings, 127, 65-72. 

The recent microstructural evidence (Section 5.3.1) gives no indication that a membrane or other product distinct from that formed later is formed during the initial reaction in C3S pastes, though a gelatinous coating is formed in cement pastes, which show an induction period similar to that observed with C3S (Section 7.5.1). For C3S pastes, this evidence excludes hypothesis 2, and gives no positive support to hypothesis 1. It does not exclude the formation of an altered layer on the C3S surface, no more than a few nanometres thick. Tadros et al. (T28) postulated the formation of a SiOj-rich layer with chemisorbed Ca ", and Barret et al. (B63) that of a superficially hydroxylated C3S, formed by protonation of the 0 and SiO ions, balanced by loss of Ca ". ... 

Non-commensurability can be observed in the published diffraction patterns of tochilinite I from Cyprus It is of the SI type instead of the SC type which occurs in the original tochilinite I from the Mamonovo (U.S.S.R.) deposit. Small chemical differences, resulting in altered layer charges and periodicities, appear to be responsible for the changes in the interlayer fit - here (Table 3), and in the valleriites (Table 2). 

Although silicate dissolution is now not generally thought to be rate hmited by diffusion through an armoring alteration layer, much evidence has accumulated documenting the development of silicon-rich cation-depleted layers of varying... 

Altered surfaces have been inferred from solution chemistry measurements (e.g., Chou and Wollast, 1984, 1985) and from spectroscopic measurements of altered surfaces, using such techniques as secondary ion mass spectrometry (for altered layers that are several tens of nm thick (e.g., Schweda et al, 1997), Auger electron spectroscopy (layers <10 nm thick (e.g., Hochella, 1988), XPS (layers <10 nm thick (e.g., Hochella, 1988 Muir et al, 1990), transmission electron microscopy (TEM, e.g., Casey et al, 1989b), Raman spectroscopy (e.g.. Gout et al, 1997), Fourier transform infrared spectroscopy (e.g., Hamilton et al, 2001), in situ high-resolution X-ray reflectivity (Farquhar et al, 1999b Fenter et al, 2003), nuclear magnetic resonance (Tsomaia et al, 2003), and other spectroscopies (e.g., Hellmann et al, 1997). 

Thickness of altered layers on feldspars generally decreases with increasing dissolved cation content of the leaching solution (Nesbitt et al, 1991) and increases with decreasing pH... 

Figure 2 A high-resolution TEM photomicrograph of the amorphous altered layer (lower left) developed on crystalhne lahradorite (hulk material, upper right) after dissolution at pH 1. The hlurry lattice fringes at the interface reflect the varying boundary orientation with respect to the ultrathin section. Interface thickness is 0.5-2 nm. Energy filtered (EE) TEM was also used to chentically characterize the alteration zone, which was found to he depleted in Ca, Na, K, and Al, and enriched in H, O, and Si. The sharp structural interface shown here and the sharp chemical interface observed with EFTEM are interpreted by the authors to indicate that the alteration layer is formed by dissolution-precipitation. Such amorphous altered layers are often high in porosity and yield high BET surface areas (reproduced by permission of Springer from Phys.

Distinguishing the difference between these two mechanisms of formation of altered layers... 

Development of alteration layers on minerals dissolved under neutral and alkaline conditions has not been thoroughly investigated, but some work has been completed, especially on feldspar compositions (Chou and Wollast, 1984 Hellmann et al, 1989, 1990a Muir et al, 1990 Nesbitt et al, 1991 Hellmann, 1995, Hamilton et al, 2000). Under neutral conditions, the leached layer thickness (tens of angstroms to a few hundred angstroms) is generally less than that observed for more acid dissolution, with variability reported in the composition and thickness of the layer i.e., sodium depletion is generally observed, but both aluminum depletion and enrichment (with respect to silicon) have been reported. Variations in solution chemistry (see Section 5.03.7) and feldspar composition may explain some of these differences for... 

Hellmann R., Eggleston C. M., Hochella M. F., Jr. and Crerar D. A. (1989) Altered layers on dissolving albite I. Results. Sixth International Symposium on Water—Rock Interaction, WRI-6, 293-296. 

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