Industrial mineral potential of andalusite and garnet in the Scottish Highlands

Mitchell, C.J. ORCID:; Harrison, D.J.. 1997 Industrial mineral potential of andalusite and garnet in the Scottish Highlands. Nottingham, UK, British Geological Survey, 70pp. (Mineral Reconnaissance Programme report 142) (Unpublished)

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Following the completion of a desk review of andalusite and garnet occurrences in the Scottish Highlands, a programme of field sampling, laboratory characterisation and mineral processing was undertaken to identify their potential for use as industrial raw materials. Bulk samples were collected from selected occurrences of andalusite schists in the Dalradian metasedimentary rocks of north-east Scotland, and from garnetiferous Lewisian, Moine and Dalradian rocks and beach sands at sites throughout the Highlands. Petrographic studies of the andalusite samples showed that andalusite mostly occurs as large or small, euhedral or anhedral grains. The euhedral grains have a well developed crystal structure and contain few inclusions, and it is these characteristics which influence their quality and suitability for mineral processing. Petrographic analyses of the garnet samples showed that garnets occur in a range of grain size, form and colour and contain variable amounts of inclusions. Some garnet samples were relatively free of inclusions, whereas others were so full of inclusions that they have a honeycomb appearance. The former type are the preferred raw material. Following petrographical analysis, five andalusite samples and nine garnet samples were chosen for laboratory mineral processing trials whose aim was to produce concentrates of the target minerals with high grades and recoveries. A range of techniques was employed on both andalusite and garnet samples using a combination of magnetic and gravity separation. The results enabled the most promising samples to be identified and these were subject to pilot-scale mineral processing trials using a mineral spiral to produce mineral concentrates. The andalusite concentrates produced by laboratory mineral processing showed that concentrates with high andalusite contents and moderate recoveries could be obtained from some samples. The results produced by pilot-scale processing were mostly not encouraging, yielding concentrates with low andalusite contents and recoveries. The concentrates produced by both laboratory and pilot-scale mineral processing do not match the chemical specifications required for industrial grade andalusite. The A1203 and Fe203 contents, which are particularly critical, are respectively too low and too high. The andalusite-bearing rocks of north-east Scotland are therefore considered to have a low potential and do not warrant further investigation. The garnet concentrates produced by laboratory mineral processing trials mostly contained high garnet contents and high recoveries. Pilot-scale processing also resulted in concentrates with high garnet contents and recoveries. Concentrates produced by both laboratory and pilot-scale processing gave densities of 4 g/cm3, indicating their suitability as industrial-grade garnet products. The more promising samples were collected from garnet-mica schists of Dalradian age at locations from near Huntly in north-east Scotland to Loch Fyne in the south-west Highlands. Further investigations are recommended to prove the scale of resources and to carry out more detailed mineral processing and quality assessments.

Item Type: Publication - Report
Programmes: BGS Programmes > Economic Minerals
Funders/Sponsors: Department of Trade and Industry
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed but not externally peer-reviewed
NORA Subject Terms: Earth Sciences
Date made live: 14 Jan 2010 11:56 +0 (UTC)

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