Mean grain diameters from thin sections: matching the average to the problem
Farr, Robert S
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Farr, R. S., Honour, V., & Holness, M. (2016). Mean grain diameters from thin sections: matching the average to the problem. Mineralogical Magazine, 81 515-530. https://doi.org/10.1180/minmag.2016.080.107
It is common practice to estimate a mean diameter for spherical or sub-spherical particles or vesicles in a rock by multiplying the average diameter of the approximately circular cross-sections visible in thin-section by a factor of 1.273. This number-weighted average may be dominated by the hard-to-measure fine tail of the size distribution, and is unlikely to be representative of the average particle diameter of greatest interest for a wide range of geological problems or processes. Average particle size can be quantified in a variety of ways, based on the mass or surface area of the particles, and here we provide exact relations of these different average measures to straightforward measurements possible in thin-section, including an analysis of how many particles to measure to achieve a desired level of uncertainty. The use of average particle diameter is illustrated firstly with a consideration of the accumulation of olivine phenocrysts on the floor of the 135m thick picrodolerite/crinanite unit of the Shiant Isles Main Sill. We show that the 45m thick crystal pile on the sill floor could have formed by crystal settling within a few months. The second geological example is provided by an analysis of the sizes of exsolved Fe-rich droplets during unmixing of a basaltic melt in a suite of experimental charges. We show that the size distribution cannot be explained by sudden nucleation, followed by either Ostwald ripening or Brownian coalescence. We deduce that a continuous process of droplet nucleation during cooling is likely to have occurred.
petrology, microstructure, grain size
VCH is supported by a Natural Environment Research Council studentship. MBH acknowledges support from the Natural Environment Research Council [grant number NE/J021520/1].
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External DOI: https://doi.org/10.1180/minmag.2016.080.107
This record's URL: https://www.repository.cam.ac.uk/handle/1810/255916
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