INSIGHTS FROM ZIRCON CRYSTALS FOUND IN HISTORICAL TIN SLAGS OF DEVON, UK

Over his career, David Veblen spread a wide mineralogical net that delved deep into the structures of minerals to better understand geological and material sciences processes and to inspire his students about the relevancy of mineralogy. As a modern-day N.L. Bowen, Veblen even inspired research into the mineralogical nature of metallurgical slag, a material that is found globally and has environmental and archaeological significance.

In the 17^th century, tin smelting was an important industry for the southwest region of the UK. Tin was extracted from cassiterite in granitic blast furnaces and after the smelting was complete, material known as slag was left as a waste product. Pieces of slag are now found in piles or in stream sediments throughout Devon. Slag from 3 sites, Merrivale, Eggworthy, and Week Ford, were studied using light microscopy, XRD, and an SEM equipped with BSE and EDS capabilities, laying the foundation for future in-depth atomic-scale investigations. The typical slag from these 3 sites is glassy but also contains accessory amounts of small (~100 µm) zircon crystals. Many of the zircons display original zoning and some even retain a sub-euhedral shape. The Merrivale zircons are sharply banded and have rounded edges, whereas the zoned zircons from Eggworthy are rimmed by small dendrites of zircon. The zircon crystals from Week Ford appear to be annealed and are surrounded by thick blocky dendritic zircon extensions.

If the dendritic zircons from Eggworthy and Week Ford are associated with partial melting, it indicates that the furnaces reached very high temperatures. These temperature would have caused significant volatilization of tin, rendering the furnace very inefficient. These temperatures would also have degraded the granite walls of the furnace faster than lower temperature smelting operations. Another possibility is that the dendritic halos formed as precipitates after the molten glass partially dissolved the zircons. In the case of the Week Ford zircons, the toothed rim might not be dendritic and secondary, but rather the last vestiges of the outer zircon during dissolution.