LAUSR

Abstract Iron-rich duricrust profiles are commonly located on old flat surfaces. They promote landscape stability by the protective action of ferricrete, providing important records of multiple soil and geomorphic processes, as well as paleoclimatic events. In the Southern Serra do Espinhaco (SSE) in Brazil, iron-rich duricrust profiles occur on flat to slightly convex hills of quartzite at elevations of mostly 1200–1400 m a.s.l. Some of these profiles may be remnants of old paleosurfaces. Here we describe an image analysis technique to measure morphological attributes of quartz grains in thin sections of iron-rich duricrusts; we present a way to investigate the genesis of these materials. Size and shape variations of quartz grains were measured for iron-rich duricrust profiles from four different locations (GS2, PF1, PF2 and PF3). Thin sections of 5 × 7.5 cm from surface horizons and iron-rich duricrusts were studied. Distinct morphological regions (iron concretion and matrix) of iron-rich duricrusts were separately analyzed to identify variations in regolith morphology. A TESCAN VEGA3 scanning electron microscope (SEM) was used to produce backscattered electron (BSE) images for regions of sufficient area to enable the use of image analysis software (ImageJ 1.50i). BSE images provide a clear contrast between quartz grains and iron-rich duricrust matrix allowing resolution of quartz grains for specific investigation. The size, circularity and roundness of many quartz grains were easily determined. This procedure enables the quantitative analysis of soil materials allowing comparisons of soils and the testing of hypotheses for soil genesis. Quartz grains within iron concretions of PF3 had different morphological properties from quartz in the surrounding ferricrete indicating that different processes had occurred during the genesis of the two materials. In contrast, PF1 which showed no evidence of parent material diversity, contained quartz grains that are indicative of a polycyclic character. We observed fragments of iron concretions with pores previously occupied by quartz grains indicating that extensive quartz dissolution had occurred during duricrust evolution.