Introduction

Grain shape is something that is easy to express qualitatively, but difficult to quantify precisely (Costa & Cesar, 2001, Verrecchia, 2003). In earth sciences we do not generally need extremely precise measures of shape, because the objects we deal with are commonly not perfectly regular. However, quantification of aspects of shape can help in understanding the petrogenesis of rocks. The subject naturally falls into two domains. For crystalline rocks (igneous and metamorphic rocks, chemical sediments and hydrothermal ore deposits) we are concerned with the shape or habit of crystals, whereas in clastic rocks we want to quantify the shape of clasts and grains. The methodology of these two fields is partly shared but could benefit from more exchange.

The overall shape of crystals reflects growth, solution and deformation. Crystals that grew unimpeded from a fluid may have many different forms, but all are ultimately controlled by the crystal structure. We usually think that faces bounding such grains should be flat, but in some minerals growth faces are curved. In thin section crystal outlines are qualitatively classified as euhedral, subhedral or anhedral.

The habit of euhedral crystals has been studied for a long time (see the introduction in Sunagawa, 1987a): in 1669 Nicolaus Steno proposed that the interfacial angles of a crystal were constant and that the external form of a crystal growing in a fluid depends on the relative growth rates of the different faces.