Study of Mineral Surfaces

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

In this chapter the surface chemistry of selected nonsulflde flotation systems, including soluble alkali halide salts, phyllosilicates, quartz, and some naturally hydrophobic minerals, were studied using MD simulation. Issues such as water structure and dynamics, solution chemistry, interfacial water structure, and adsorption states for surfactants and macromolecules were examined. It is clear that MD simulation has been validated as a very useful tool to study the surface chemistry of certain flotation systems. As a complement to experimental studies, MD simulation analysis provides further information and understanding at the atomic level to issues such as water structure, particle dynamics, solution viscosities, mineral surface wetting characteristics, surface charge, and adsorption states. A wide application of MD simulation in the study of mineral surface chemistry is expected to have a significant impact on further advances in flotation technology. 

Electron spectroscopy in its several modes has proved to be particularly powerful for determining the nature of species present on solid surfaces. These methods have been applied extensively to the characterization of SAMs, and they are finding an increasing application to mineral processing systems. Of the various available techniques. X-ray photoelectron spectroscopy (XPS) is particularly appropriate for the study of mineral surfaces because a knowledge of the chemical environment of atoms, in addition to elemental composition, is usually required. Such information is important in identifying surface thiol species on sulfide minerals be-... 

Pyrite is not only one of the key compounds in Wachtershauser s theory, but could also have fulfilled an important function for phosphate chemistry in prebiotic syntheses. A group in Rio de Janeiro studied the conditions for phosphate sorption and desorption under conditions which may have been present in the primeval ocean. In particular, the question arises as to the enrichment of free, soluble inorganic phosphate (Pi), which was probably present in low concentrations similar to those of today (10 7-10 8M) (Miller and Keffe, 1995). Experiments show that acid conditions favour sorption at FeS2, while a weakly alkaline milieu works in an opposite manner. Sorption of Pi can be favoured by various factors, such as hydrophobic coating of pyrite with molecules such as acetate, which could have been formed in the vicinity of hydrothermal systems, or the neutralisation of mineral surface charges by Na+ and K+. 

Knoop indenters are particularly useful for studies of the anisotropies of indentations on surfaces because of their elongated shape which gives an indenter two-fold symmetry. For studies of minerals this is quite useful and has been discussed in some detail by Winchell (1945). 

Applications of Surface Techniques to Chemical Bonding Studies of Minerals... 

Sanchez, V. M. and Hiskey, J. B., 1988. Electrochemical study of the surface oxidation of arsenopyrite in alkaline media. Metallurgical Transactions B, 19(6) 943 - 949 Sanchez, V. M. and Hiskey, J. B., 1991. Electrochemical behavior of arsenopyrite in alkaline media. Minerals Metallurgical Processing, 8(1) 1-6 Sasaki Takehiko, Goto Yoshio, Tero Ryugo, Fukui Ken-ichi, Iwasawa Yasuhiro, 2002. Oxygen adsorption states on Mo(l 12) surface studied by HREELS. Surface Science, (502 - 503) 136-143... 

S.A. (1980) Infrared study of the adsorption of carboxylic acids on hematite and goethite immersed in carbon tetrachloride. J. Chem. Soc. Faraday Trans. I. 76 302-313 Buerge, I.J. Hug, S.J. (1999) Influence of mineral surfaces on chromium(VI) reduction by iron(II). Environ. Sci. Techn. 33 4285-4291 Buerge-Weirich, D. Hard, R. Xue, H. Behra, P. Sigg, L. (2002) Adsorption of Cu, Cd and Ni on goethite in the presence of natural groundwater ligands. Environ. Sci. Techn. 36 328-336... 

Castro, S.H. and Baltierra, L. (2005) Study of the surface properties of enargite as a function of pH. International Journal of Mineral Processing, 77(2), 104-15. 

Ongwandee et al. studied the adsorption behavior of trimethylamine (TMA) on three surfaces ZrSi02 (representative of mineral surfaces) (Ongwandee, Bettinger... 

There are several major areas of interfacial phenomena to which infrared spectroscopy has been applied that are not treated extensively in this volume. Most of these areas have established bodies of literature of their own. In many of these areas, the replacement of dispersive spectrometers by FT instruments has resulted in continued improvement in sensitivity, and in the interpretation of phenomena at the molecular level. Among these areas are the characterization of polymer surfaces with ATR (127-129) and diffuse reflectance (130) sampling techniques transmission IR studies of the surfaces of powdered samples with adsorbed gases (131-136) alumina(137.138). silica (139). and catalyst (140) surfaces diffuse reflectance studies of organo- modified mineral and glass fiber surfaces (141-143) metal overlayer enhanced ATR (144) and spectroelectrochemistry (145-149). 

Special Considerations in Spectroscopic Studies of Mineral Structure and Energetics of Importance to Surface Chemistry... 

The method for the study of crystal structure, including mineral composition, is x-ray diffraction. It is a semiquantitative method (Klug and Alexander 1954 Warren 1999) for the study of mineral composition. Powder samples of rocks are studied, and the place of the peaks (in nanometer or diffraction angle) and their relative intensity are observed. The diffractogram is usually compared to that of known substances that can be found in databases (ICDD 2009). In order to accurately quantify the rock samples, x-ray diffraction must be complemented with other quantitative techniques, including chemical analysis (Section 4.1.1), thermal, infrared analyses (Chapter 2, Section 2.1.2) and surface area (Chapter 1, Section 1.3.4.1.5 Section 4.1.3) measurements. 

Oxides and sulfides with tetrahedrally coordinated cations such as those with the zincblende structure may undergo substantial surface reconstruction. There are consequently also substantial changes in electronic structure. Much of the work undertaken on surface characterization of oxides and sulfides has been concerned with materials of importance in semiconductors and catalysts, but a certain amount of work has also been prompted by the importance of surface studies in mineral technology. Examples include studies of the surface properties of chalcopyrite (Buckley and Woods, 1984), galena (Tossell and Vaughan, 1987), or pyrite during extraction by flotation (Brion et al., 1980). 

Burns, R. G. (1989). Spectral mineralogy of terrestrial planets scanning their surfaces remotely. In Spectroscopic Studies of Minerals Principles, Applications, and Advances (D. J. Vaughan, ed.) Mineral. Mag. 53, 135-51. 

A few researchers have investigated the rate of dissolution through microscopic examination of mineral surfaces (e.g., Hellmann et ai, 1992 Macinnis and Brantley, 1992, 1993 Dove and Platt, 1996 Mellott et al, 2002). For such studies, the dissolution rate (molm s ) is generally obtained from the expression... 

---