The paper by Flinn and Moffat (1985) on which Nesbitt and Hartmann comment reiterates geological evidence from the Cunningsburgh area of Shetland published in this journal sometime ago (Flinn 1967) and adds to it some previously unpublished details of a spinifex-like texture in the Cunningsburgh serpentinite. The paper then proposes a komatiite origin for the serpentinite. According to Nesbitt and Hartmann this proposal depends on the validity of the spinifex texture in the Shetland serpentinite. This is incorrect. Flinn and Moffat (1985) cite evidence from Flinn (1967) pointing to a volcanic origin for the serpentinite, which was published at a time when volcanic ultrabasic rocks were not considered possible by most petrologists and komatiites had not been invented. Certainly if the texture is true spinifex texture then the volcanic origin of the serpentinite is proved, but if it is a jackstraw or metamorphic texture then the protolith of the serpentinite is unknown and could still be a komatiite.
Nesbitt and Hartmann state that the texture in the Cunningsburgh serpentinite is jackstraw and not spinifex on the grounds that the serpentinized olivine grains are not skeletal and that they 'overlap'. Donaldson (1982) states that although these two textures bear 'an uncanny resemblance' to one another, with care, they are distinguishable because the spinifex olivines are skeletal and have a certain specified crystallographically controlled habit, while jackstraw olivines are not skeletal, have a different habit and cut shears and banding in the rocks. The coarse nature of the serpentization and steatitization of the Cunningsburgh serpentinite destroys any possibility of determining the crystallographic habit, the presence or absence of a skeletal habit and often even the outline of the grains.
Nesbitt and Hartmann specify a difference not mentioned by Donaldson (1982). They state that in a true spinifex texture the olivines do not 'overlap'. It is difficult to understand how 'overlap' of crystals can be observed in thin section and the significance it would have if it occurs. Perhaps they mean 'interpenetrate'. However, olivines or serpentinized olivines can be seen to interpenetrate in komatiites in figs. 16.6a, 16.5, 16.4 (Donaldson 1982) and in Figure 1c (Nesbitt and Hartmann 1986) and in igneous rocks primacrysts interpenetrate when they