AVS 47th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP1
Oriented Growth of Rod-like Second Phase Precipitate on the (1014) Calcite Surface

Tuesday, October 3, 2000, 5:30 pm, Room Exhibit Hall C & D

Session: Poster Session
Presenter: A.S. Lea, Pacific Northwest National Laboratory
Authors: A.S. Lea, Pacific Northwest National Laboratory
D.R. Baer, Pacific Northwest National Laboratory
T.T. Hurt, Pacific Northwest National Laboratory
J.E. Amonette, Pacific Northwest National Laboratory
Correspondent: Click to Email
An apparent epitaxial growth from solution of a Mn-rich phase on calcite (CaCO@sub 3@) has been observed by atomic force microscopy (AFM). Calcite, which is the most widely distributed carbonate mineral, interacts with and incorporates contaminants from the environment and serves as a major reservoir in the global carbon cycle. We have used an AFM equipped with a fluid cell to examine the influence of various solution contaminant ions (Mn, Sr, PO@sub 4@) on the rates of dissolution of pure calcite. Here, we report on the influence of a soluble Mn@super 2+@ impurity on the dissolution behavior of calcite. At Mn@super 2+@ concentrations well below saturation for bulk MnCO@sub 3@, the Mn@super 2+@ inhibited dissolution uniformly. However, at Mn@super 2+@ concentrations near the MnCO@sub 3@ saturation level, we observed the formation and oriented growth of rod-like precipitates on the surface. These rods grew along the [221] direction of the surface until they reached a step edge where they could grow no further. While these rods could grow many microns in length, their width was limited to 120 to 180 nm and their thickness was measured consistently at 25 Å, which is indicative of a critical thickness of growth. XPS analysis of the rod covered surfaces showed the presence of Mn with a +2 or +3 valence. EPR analysis of the bulk sample after rod formation shows an intense broad signal that is indicative of dipole-dipole alignment between neighboring Mn(II) ions and distinct from the sextuplet (commonly seen for isolated Mn(II) ions in bulk calcite) observed in the untreated sample. Together these analyses suggest formation of an epitaxial Mn(II) phase on the surface of calcite.