Mean grain diameters from thin sections: matching the average to the problem

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

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Abstract

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.

Item Type:	Article
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
Identification Number:	10.1180/minmag.2016.080.107
Depositing User:	Library Administrator
Date Deposited:	09 May 2016 16:24
Last Modified:	20 May 2016 17:14
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3656

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