Craignurite is another strange local term applied to a Scottish igneous rock. It seems to have a strictly Scottish provenance: Emeleus and Gyopari (1992, pp.124,179) refer to occurrences on Mull and nearby Ardnamurchan. The very definition is rather fuzzy: Le Maitre et al. (1989, p.58) rafer to "A local Hebridean name for a series of volcanic rocks occurring as cone sheets and ranging in composition from intermediate to acid. They are characterised by acicular and skeletal crystals of plagioclase, augite and hornblende in a felsic groundmass". The arrested development of the larger silicate crystals (microcrysts or microphenocrysts) suggest a rapid cooling (quenching) of the magma.

At Craignure, this rock extends over a wide area around the ferry pier. The cm-scale spots in a fine-grained matrix suggest devitrification of a glassy igneous rock (glass, of just about any composition, is metastable on even modest geological time scales --- in time, the atoms suspended in the glass reorganize themselves and form discrete crystals of one or more minerals). This recrystallization eventually causes the rock to lose the lustre and translucency that we expect of a glass. The pale colour is consistent with a felsic to intermediate protolith. The outcrop shows flow banding roughly parallel to the shore. Apparent grain size, distribution of spots and bands vary widely. Genuine xenoliths (microdiorite?) are rare. The craignurite outcrop is cut by later dolerite (diabase) dykes. The craignurite exposure is large, and the original fresh rock, presumed to be pitchstone, could have been a flow (as seen in the spectacular Sgurr of Eigg, found to the north in the "Small Isles" between Mull and Skye) or a minor intrusion.

This Craignure outcrop and another by the shore to the northwest, near Altcrich Cottage, are described as examples of composite intrusions, composed of relatively thin basic (basaltic) margins and a more felsic (felsite / craignurite) core. Actually, the upper basic margin at Craignure is obscured by a raised beach (Skelhorn, 1969, in Skelhorn et al., pp.32-35) but this does not invalidate the interpretation, though it does postpone a definite proof. Skelhorn's work on these rocks suggests that the rounded inclusions in craignurite are remnants of a now largely devitrified pitchstone. Skelhorn noted the prior existence of the high-temperature polymorph of silica, tridymite, now inverted to quartz. It appears that the glassy rock known as pitchstone may be emplaced as shallow, rapidly-cooling intrusions or as volcanic flows, whether as lava or welded tuff. The example in the Sgurr of Eigg is hard to beat, and shows columnar jointing (Gillen, 2013, pp.141,144,145), further evidence of rapid cooling of magma, as seen elsewhere in the world, including on nearby Staffa and the famous Giant's Causeway, near Bushmills on the Antrim coast of Ireland. You can read more of pitchstones from the Isle of Arran and of younger samples from Popocatepetl volcano in Mexico.

In prep.: petrographic notes and photomicrograph(s) from a new thin section.