The effect of the matrix on reflectivity spectra of nanophase (superparamagnetic) hematite (np-Hm) dispersed within it was studied over the 350--2200-nm wavelength range. Np-Hm is characterized by particle diameters less than ~10 nm and has properties distinctly different from larger diameter hematite particles (bulk-Hm). Data were obtained for four series of powder samples which have different matrix properties and variable concentrations of np-Hm dispersed within discrete powder particles. The matrix materials are two size ranges of each of two compositions which have different internal scattering characteristics. Reflectivity data show that matrix properties have a large influences on the reflectivity spectra of pigmentary np-Hm. Samples with the same Fe2O3, content can have np-Hm absorption edges characterized by very different positions (color) and consequently, very different slope (R600/R450) and curvature ((R600)¿(R450)/ (R550)2) indices, where, for example R600 is the reflectivity at 600 nm. Conversely, samples with equivalent absorption edges can have very different Fe2O3 concentrations. For all samples ranges of slope and curvature indices are 0.99--8.55 and 0.50--2.38 respectively. Although it may be possible to place limits, quatitative relationships between positions of ferric absorption edges and Fe2O3 concentrations are unreliable without knowledge of matrix properties of the system under consideration.

As an example, the curvature index is a measure of relative crystallinity (relative proportion of bulk-Hm) for a series of samples containing relatively increasing proportions of bulk-Hm over np-Hm only at constant Fe2O3 concentration and constant matrix effects. In the absence of the stated conditions, variations in the curvature index can result from any combination of crystallinity, matrix effects, and pigment (hematite) concentration. It is possible to match the Fe2O3 concentration, magnetic properties, and spectral data for Martian surface material with a laboratory mixture whose only ferric-bearing phase is hematite. What made this possible is a matrix material with suitable optical scattering properties and differences in optical and magnetic properties of np-Hm and bulk-Hm. Most of the hematite is present as strongly magnetic np-Hm, but some bulk-Hm is required to give a shallow ~860-nm band. Hematite is thus very likely present on Mars and may even be the dominant ferric mineralogy. ¿ American Geophysical Union 1990