Peter J. Heaney and Donald M. Fisher are to be congratulated for their detailed mineralogical characterization of hawk's-eye and tiger'seye. However, their interpretation of mineral formation based on mineralogical and crystallographic information requires discussion. They conclude that the formation of hawk's-eye involved “cracking of the crocidolite host rock followed by the antitaxial deposition of columnar quartz crystals from silica-saturated fluids” (Heaney and Fisher, 2003, p. 325), implying that crocidolite asbestos and quartz originated during the same geological event. This conclusion is based on textural evidence and the presumption that pseudomorphous hawk's-eye and tiger's-eye after crocidolite should contain chalcedony or quartzine, and not well-crystallized columnar quartz. However, Heaney and Fisher's interpretation ignores the geological setting of tiger's-eye and hawk'seye.
South African tiger's-eye and hawk's-eye come from an area near Griquatown and Niekerkshoop, Northern Cape Province. They are restricted to a well-defined ancient planation surface that drapes the Asbestos Hills iron formations. This surface corresponds to the late Mesozoic African land surface (Partridge and Maud, 1987) (Fig. 1). Where this planation surface intersects the Asbestos Hills iron formations, it is marked by a 2–4-m-thick zone of massive silicification and goethitization. This altered zone is related to a specific period of silicification whereas goethitization appears to be an ongoing process that locally crosscuts the zone of silicification (Fig. 1).
The association of tiger's-eye and hawk's-eye with surficial silicification is evident in field outcrops, mining-related exposures, and drill core intersections. Tiger's-eye and hawk's-eye occur only where the planation surface cuts bedding-parallel vein systems filled by asbesti-form crocidolite (Fig. 1). Crocidolite mineralization in these vein systems is explained by a bedding-parallel crack-seal vein-filling process with fiber growth in minimum stress direction during a period of mild EW- and NS-directed compression (Dreyer and Soehnge, 1992). If the episodic crack-seal vein process and cogenetic formation of quartz and crocidolite proposed in the model by Heaney and Fisher applied, we would expect to see hawk's-eye, i.e., silicificed crocidolite developed at depth. However, this has never been observed in any of the extensive underground workings developed to exploit the crocidolite asbestos (Dreyer and Soehnge, 1992). In fact, tiger's-eye and hawk's-eye are restricted to the silicified surficial zone. A rapid but gradual transition of tiger's-eye at surface through hawk's-eye into unaltered crocidolite fibers is observed (Dreyer, 2003, personal commun.); this transition is obvious even on a hand-specimen scale (Fig. 2).
The intimate and systematic relation between hawk's-eye and tiger's-eye provides evidence for the transformation of crocidolite asbestos first into hawk's-eye by silicification and then into tiger's-eye by partial oxidation of the Fe2+ contained in crocidolite to goethite. Depending on the current depth of oxidation relative to past silicification, and the rate of mechanical weathering, tiger's-eye, hawk's-eye, and oxidized but unsilicified crocidolite occur in surface outcrops (Fig. 1). Field evidence thus leads to the conclusion that hawk's-eye and tiger's-eye, whilst not pseudomorphs sensu strictu, nevertheless originate as alteration products of pre-existing crocidolite veins in a replacement process that is marked by an exceptional preservation of textural detail.