Mineral Surfaces
Chemical interactions at crystal-water interfaces are crucial to a broad range of scientific and technological topics, including corrosion, heterogeneous catalysts, chemical sensors, teeth and bones, and a host of everyday products from paints and glues to solvents and cleaners. Geochemists pay special attention to reactions that occur between mineral surfaces and aqueous species – interactions central to weathering and soil formation, hydrothermal ore deposition, pH buffering, biomineralization and biofilm formation, uptake and release of chemicals that affect water quality, and many other natural processes.
Adsorption of organic molecules on mineral surfaces has long been studied in connection with scenarios for life’s origin. Prebiotic synthesis of relevant biomolecules (the first step in biogenesis) has been demonstrated for numerous plausible environments, including Earth’s Hadean surface, dense interstellar clouds and hydrothermal systems. A central challenge in origins research is elucidation of the next step – efficient mechanisms for selection, concentration and organization of monomers into life’s macromolecules. Many minerals have thus been studied as possible templates for organic adsorption.
Our studies of mineral surfaces focus on several complementary aspects, including (1) identification and characterization of common faces of rock-forming minerals; (2) experimental study of molecular adsorption for individual mineral-molecule pairs by several methods; (3) theoretical studies of atomic-scale interactions between small organic molecules and mineral surfaces; and (4) applications of microarray technology to the study of numerous mineral-molecule pairs in a single experiment.
1.Characterization of surface structures, especially chiral surfaces, of common rock-forming minerals. We find that most rock-forming minerals display at least one common chiral surface. We have also developed a “chiral index” to estimate the enantioselective potential of different surfaces.
Hazen, R.M. (2004) Chiral crystal faces of common rock-forming minerals. In G. Palyi, C. Zucchi and L Cagglioti, Eds. Progress in Biological Chirality. New York: Elsevier, Chapter 11, pp.137-151.
Churchill, H., H. Teng and R.M.Hazen (2004) Measurements of pH-dependent surface charge with atomic force microscopy: Implications for amino acid adsorption and the origin of life. American Mineralogist 89, 1048-1055.
Downs, R.T. and R.M.Hazen (2004) Chiral indices of crystalline surfaces as a measure of enantioselective potential. Journal of Molecular Catalysis 216, 273-285.
Hazen, R. M. (2006) Mineral surfaces and the prebiotic selection and organization of biomolecules (Presidential Address to the Mineralogical Society of America). American Mineralogist 91, 1715-1729.
Common crystal forms of quartz include the hexagonal prism m (100), the dominant rhombohedron r (101) and the secondary rhombohedron z (011). Left- and right-handed quartz (a and b, respectively) may be distinguished by two additional forms, denoted s (111) and x (511). Most crystals, such as the 3.2-cm diameter specimen from Montgomery County, Arkansas (c), display only the m, r, and z faces.
The (100), (101) and (011) surface structures of quartz (SiO2), viewed from above (a, c, and e, respectively) and tilted 3Ëš from horizontal (b, d, and f, respectively). Oxygen and silicon atoms are shown in red and blue, respectively. Positions of terminal oxygen atoms are indicated by yellow Xs. In each drawing the c-axis projection is vertical and each drawing presents an area 15 x 15 Ã….
Hematite (Fe2O3) preferentially deposits on (101) faces of quartz, while (011) faces remain largely uncoated (~1-mm diameter crystals from Paterson, New Jersey). This phenomenon results from significant differences in the surface structures of these two rhombohedral faces.
Common crystal faces of feldspar include the chiral form m (110), which is often well developed in orthoclase (a) and albite (b). The 7- x 7-cm specimen of alkali feldspar (c) from Ethiopia displays these faces.
One possible (110) chiral surface structure of orthoclase, which is a member of the alkali feldspar group. Silicon, oxygen and potassium atoms are shown in blue, red and turquoise, respectively. Terminal oxygen atoms are marked with yellow Xs. The [001] axis is vertical and the area is 15 x 15 Ã…. Note that terminal oxygen atoms are chosen in this model so that potassium is fully coordinated, which effectively shields potassium atoms from the surface.
Calcite, CaCO3, frequently occurs with (a) the chiral scalenohedral form [designated v; (211) or (214) in the hexagonal morphological or structural setting, respectively], as well as (b) the rhombohedral form [designated r; (101) or (104), respectively], which is also the common cleavage plane. (c) A doubly-terminated crystal from Elmwood Mine, Tennessee, displays a well-formed scalenohedron.
(a) The calcite rhombohedral cleavage [(101) or (104) in the hexagonal morphological or structural settings, respectively] presents a surface in which Ca cations and rigid CO3 anions alternate. The surface has glide plane symmetry (vertical yellow lines) and so is achiral. (b) The cleavage surface topology is revealed in a view that is tilted 6Ëš from the horizontal. Ca, C and O atoms are turquoise, blue and red, respectively. Each drawing is approximately 15 x 15 Ã…, and the c-axis projections are vertical.
(a) The structure of the scalenohedral face of calcite [(211) or (214) in the hexagonal morphological or structural settings, respectively] features a chiral arrangement of positive (+) and negative (X) charge centers near the crystal termination. Ca, C and O atoms are turquoise, blue and red, respectively. In this 20 x 20 Å view the (01 ) axis in the hexagonal structural setting [equivalent to the (01 ) axis in the hexagonal morphological setting] is vertical – an orientation that provides a useful image of the surface structure. (b) A view of this surface tilted 3˚ from horizontal (projected almost down the [01 ] axis) reveals the irregular surface topology, including 2-Ǻ-deep steps (yellow arrow) that result from the oblique intersection of layers of Ca and rigid CO3 groups with the surface (yellow line).
2. Experimental demonstration of molecular selection on mineral surfaces: Recent and ongoing experiments are designed to measured the relative adsorption of different sugars and amino acids on different mineral surfaces. For example, we demonstrated the chiral selection of D- and L-ASP on calcite (214) by immersing a carefully cleaned calcite crystal in a racemic ASP solution for 24 hours, washing the crystal, removing any adsorbed ASP with dilute HCl, and analyzing the desorbed amino acid with sensitive chiral GC techniques.
Hazen, R.M., T.R.Filley and G.A.Goodfriend (2001) Selective adsorption of L- and D-amino acids on calcite: implications for biochemical homochirality. Proceedings of the National Academy of Sciences (US), 98: 5487-5490.
Hazen, R.M. (2001) Life’s rocky start. Scientific American 284, #4, 76-85.
This graph shows six vertical columns representing the six (214)-type terminal faces of a calcite crystal. Data points indicate D:L ratios of desorbed amino acids, which show a statistically significant D excess on “right-hand” calcite faces, and a similar L excess on “left-hand” calcite faces.
3. Theoretical modeling of mineral-molecule interactions: We have used density functional theory to study possible bonding configurations and relative energies for left- and right-handed amino acids adsorbed onto the calcite (214) surface, one of the commonest chiral surfaces in nature.
Asthagiri, A. and R. M. Hazen (2007) An ab initio study of adsorption of alanine on the chiral calcite (2131) surface. Molecular Simulation 33, 343-351.
Our models start with unrelaxed surfaces and the amino acid in a plausible starting configuration (left). All atom positions ae unconstrained in our models. After convergence we find significant surface relaxation of the calcite and shifts in the molecular position of the amino acid, but relatively little change in the molecular conformation.
In the case of alanine adsorbed onto the calcite (214) surface, we find two strong points of interaction – one Ca-O bond and one O-H bond forms. However, there is no significant difference in adsorption energy between the case of L-ALA (left) versus D-ALA (right). Note that two points of bonding are insufficient to produce chiral selection.
In the case of aspartic acid adsorbed onto calcite (214)-type surfaces, we find a significant difference in the energy of the minimized solution depending on the molecular handedness. L-ASP forms two strong bonds on the (214) surface, but D-ASP forms three strong bonds (two Ca-O and one O-H) and consequently has ~8 kcal/mol lower energy. This fortuitous geometrical match explains the experimentally observed chiral selective adsorption of ASP on calcite.
4. Applications of microarray technology: The number of possible mineral-molecule pairs that we would like to study is much too great for separate experiments on each combination. We have thus been studying applications of microarray technology to document semi-quanitatively the extent of adsorption of sugars and amino acids on various mineral surfaces.
Hazen, R. M. (2006) Mineral surfaces and the prebiotic selection and organization of biomolecules (Presidential Address to the Mineralogical Society of America). American Mineralogist 91, 1715-1729. [pdf]
Microarray of fluorescent-tagged L lysine on a glass slide.
A ToF-SIMS mass spectrum showing the difference between a clean calcite surface and one with lysine.
ToF-SIMS analyses of pentose sugars on feldspar.
A feldspar crystal in the ToF-SIMS.
For more information on our NASA-NSF project see our project website.