Structures seen on the surface of a rock (e.g. with fossils) are frequently difficult to illustrate photographically due to the low contrast between different rock constituents, particularly when the general colour and texture are uniform. The method described below is based on the chemical properties of the constituent minerals and achieves in many cases a higher degree of contrast between structure and matrix. Rosenqvist (1950), investigating the adsorptive properties of clay minerals, quartz, and feldspar, suspended in a diluted radium solution, showed that radium was preferentially adsorbed by the clay minerals. This property is utilized in the technique described below. Radium is usually less readily adsorbed on the surface of a fossil and the contrast with the matrix is accentuated.
An autoradiographic technique for illustration of structures in sedimentary rocks.