Farr, Robert S. and Honour, Victoria C. and Holness, Marian B. (2016) Mean grain diameters from thin sections: matching the average to the problem. Mineralogical Magazine. ISSN 0026-461X, Online ISSN: 1471-8022 DOI 10.1180/minmag.2016.080.107
Farr_et_al-2016-Mineralogical_Magazine-AM.pdf - Accepted Version
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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.
|Uncontrolled Keywords:||2016AREP; IA70;|
|Subjects:||05 - Petrology - Igneous, Metamorphic and Volcanic Studies|
|Divisions:||05 - Petrology - Igneous, Metamorphic and Volcanic Studies
08 - Green Open Access
|Journal or Publication Title:||Mineralogical Magazine|
|Depositing User:||Library Administrator|
|Date Deposited:||09 May 2016 16:24|
|Last Modified:||20 May 2016 17:14|
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