Note on a new method of using hydrofluoric acid for the study of conodonts in cherts in the Torlesse terrane, New Zealand

Abstract. A number of papers have described procedures for the recovery of conodonts from cherts by dissolution in hydrofluoric acid (Ethington & Austin, 1993; Stone, 1987), but, in our experience, the techniques have generally led to fragmentation of the conodont specimens.

A new method of etching by hydrofluoric acid allows observation of whole conodonts in chert samples. The method was developed on material of Permian age from New Zealand. Although this technique does not, at present, enable entire conodonts to be extracted from the rock, it makes it possible to photograph conodonts in even highly microbrecciated cherts, and the technique should be applicable to conodont-bearing cherts of any age.

The new technique involves HF etching of the surface of small samples of chert, preferably split along bedding planes. Samples weighing about 5 kg were collected in situ from siliceous pelagites (cherts) from the Torlesse terrane, near Meyers Pass, South Canterbury (Ford, 1995). The blocks were split or chipped along bedding planes to produce a large number of thin, 5 cm square samples. These wafer-sized chips were placed in a 1 litre polypropylene (NOT glass) beaker, and covered with 35% HF.

The chips slowly turned white over a period of 15–30 min as a reaction gel formed on the surface of each chip. When whitened, the HF was carefully decanted off and replaced with water. Taking care not to abrade the now soft skin of reaction gel, the samples were gently rinsed with water until the HF was neutralized sufficiently for . . .