New field, analytical data and melting temperature determinations from three vitrified forts in Lochaber, Western Highlands, Scotland

Abstract New field data is presented for the recently recognised vitrified fort at Eileanan Comhlach in Loch Shiel and the little studied fort on Eilean Loch Airceig in Loch Arkaig. New analytical data are presented from the melts and newly formed minerals in the vitrified forts at Eileanan Comhlach and Eilean Loch Airceig, together with the previously excavated Dun Rahoy in Morvern. Dun Rahoy is significant as it was the location of one of the first experiments to investigate melt production by setting fire to a reconstruction of a proposal of the fort wall. Samples from this early experiment were sought but not found for analysis in this work. The melts are demonstrated to be locally derived within the structures and to have been produced in batches that essentially did not mix, as boundaries between batches with different cooling histories may be found. New minerals found include Fe-olivine, clinopyroxene, orthopyroxene, spinels, Fe-Ti oxides and iron sulphide spherules. One sample from each fort contained newly crystallised Fe-rich olivine, which allowed the application of geothermometry based on the distribution of Mg and Fe in olivine - glass pairs, the first time this has been achieved in Scotland. The concordance of the determinations from the Mg and Fe data suggest that it was likely that a quasi-equilibrium was reached and indicate a crystallisation temperature between 800 and 950 °C. These new data corroborate earlier suggestions that melting was attained at low temperatures through the breakdown of biotite and muscovite. It is concluded that different melt batches were produced locally within an edifice due to irregular T variations across it. The batches did not move very far due to the high viscosity of the siliceous melt. This suggests melting was likely to have been uncontrolled, rather than a controlled process such as could be expected within a bloomery or furnace.