Ardbeg, Islay
Geological Conservation Review site | GCR #3289 | Structural and Metamorphic Geology | Dalradian
Scotland's geosites are chosen because of their local, national or international importance. Take only photos, leave only footprints: avoid causing any damage to this site. You can walk almost anywhere in Scotland without the need to ask permission or keep to paths, but you have a responsibility to care for your own safety, to respect people's privacy and peace of mind and to cause no damage.
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Summary
The Ardbeg GCR site is notable for the occurrence of the metamorphic mineral stilpnomelane within a metadolerite sill.
Although stilpnomelane occurs sporadically elsewhere in the Scottish Dalradian, it is particularly easy to find here as it is relatively abundant and the crystals are up to 1 mm in size, which is quite large for stilpnomelane. There is little published on the mineral, and although the crystal structure is well known, its crystal chemistry and thermodynamic properties are not well understood. For this reason it is important to preserve a site where fresh, relatively large grains are preserved in a host rock that has not been significantly retrogressed. Here its growth is reasonably well constrained with respect to the regional deformation and metamorphism, and the host-rock mineral assemblage can be used theoretically to quantify the temperature and pressure of metamorphism under which this particular stilpnomelane formed. An important constraint on the formation of stilpnomelane is the chemical composition of the host rock, which can be reliably established at this site as the rock is relatively fresh.
This small site also exhibits good examples of sedimentary structures such as cross-bedding, dewatering structures and scours, which are found within the Laphroaig Quartzite. Three different small-scale tectonic fabrics are easily distinguished here and these could prove important in establishing tectonic relationships in south-east Islay.
David Webster
Oct. 18, 2018
Metabasite sill near the stilpnomelane locality
David Webster
(PDF) extract from gcr publication