Surfaces of natural pyrite crystals have been extensively studied and an intrinsic dependence of their structure on morphology and growth mechanisms has been demonstrated. Apparently, the main factors controlling crystal morphology and growth are supersaturation, temperature, and, incorporated impurities. At conditions close to equilibrium, spiral growth dominates the growth rate, while at higher supersaturation, two-dimensional nucleation on pre-existing surfaces controls the growth mechanism. Technological advancements (e.g., development of special phase-contrast interferometry, scanning tunneling- and atomic force- microscopy) have improved the resolution of surface structure measurements. Although intensive studies on surface structures of natural pyrite have been carried out, in situ growth experiments have been lacking. For various other minerals experimentally grown from solution, in-situ growth rates (R), hillock slopes (p) and step advancing rates (v) have been measured as a function of known saturation state (s) and, in general, a linear relation between R, p, v, and (s) has been established. Evaluating these parameters on natural samples makes this function much less tractable because saturation states are unknown. However, surface topography permits the precise measurement of step height / width and slopes of spiral hillocks. Therefore, in this study, striation densities, step dimensions and hillock slopes for the {100} face of pyrite have been measured in an attempt to further explore pyrite growth mechanism.
Macroscopic smooth pyrite crystals from Navajun, Spain, were initially studied using Scanning Electron Microscopy (SEM, Phillips, SX-20) and a thorough surface survey (including relative height and width measurements) revealed a dual distribution of the height and width of striations for cubic {100} faces. To numerically assess hillock slopes, topographic and higher resolution techniques were required. Therefore, several samples, were evaluated on a 10-1000 mm scale with a Surface Profilometer (SP, Alpha-500, Tencor) (Fig. 1) and on a micro to nano scale with an Atomic Force Microscope (AFM, DI-3100) (Fig. 2). The measurements of step height, width, and angle of profiles traces taken over hillocks were used to calculate the hillock slopes (p). Neighboring hillock growth - centers caused the growth of asymmetric pyramids with slopes differing by a factor of 2-7 on the two sides. On the other hand, elongated pyramids indicate highly anisotropic growth conditions, and the slopes vary by a factor of up to 10. An estimate of growth rates requires an independent estimate of the saturation state. Additional measurements are in progress, both to improve methods of characterizing the shapes of pyrite surfaces and to better understand the causes that produce these shapes.
Fig. 2. AFM trace on the rise of a hillock on a {100} face.
In: LPI Contribution No. 971, p. 25-26, Lunar and Planetary Institut, Houston, 1999