Experimental formation and significance of etch patterns on detrital garnets

Abstract

The patterns found on the surfaces of etched detrilal garnets are formed by facets that resemble a series of elongated overlapping sharp-edged rectangular or rhombohedral building blocks, mammilla, or rounded hummocks that are gradational to facets and rhombohedral or triangular pits.Opinion on the origin of these surface features is split between those who have suggested that they are due to etching and those who believe they are due to overgrowth. In order to resolve this controversy, crushed garnets were subjected to three sets of experiments. The first involved leaching the crushed garnets in various strengths of acid (HF) and alkaline (NaOH) solutions. The second involved thin-sectioning of naturally etched garnets to determine if a nucleus could be seen under a petrographic microscope and the third involved electron microprobe and SEM X-ray analyses across cut sections of naturally facetted garnets to determine compositional variations through individual grains and between grains from a single sample.The results of the first set of experiments showed that etching starts immediately in all concentrations of HF and that within the first 24 h, facets, mammilla, and pits are visible on the surface of the grains removed from a 40% solution of HF. The second and third sets of experiments showed that, first, the garnets are chemically homogeneous from the inside to the outside of the grains and, second, that there is a wide variation in the chemical composition of the edges of garnets from a single sample.The chemical homogeneity of the garnets demonstrates that the surface patterns did not develop from growth over a pre-existing nucleus. This observation, and the variation in chemical composition of the edges of garnets within a single sample, indicate that the surface patterns on the detrital garnets are the result of etching by intrastratal solutions.