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A process for reducing rocks and concentrating heavy minerals

March 30, 2016

To obtain minerals suitable for age-dating and other analyses, it is necessary to first reduce the mineral-bearing rock to a fine, sand-like consistency. Reducing whole rock requires crushing, grinding, and sieving. Ideally, the reduced material should range in size from 80- to 270-mesh (an opening between wires in a sieve). The openings in an 80-mesh sieve are equal to 0.007 inches, 0.177 millimeters, or 177 micrometers. This size range ensures that compound grains are mostly disaggregated and that grains, in general, are dimensionally similar. This range also improves the segregation rate of conspicuous to extremely small individual heavy mineral grains.

Once the rock is reduced to grains, it is necessary to separate the grains into paramagnetic and nonparamagnetic and heavy and light mineral fractions. In separating grains by property, those minerals chemically suited for radiometric dating are abundantly concentrated. Grams of mineralogical material can then be analyzed and characterized by multiple methods including trace element chemistry, laser ablation, and in particular, ion geochronology.

Publication Year 2016
Title A process for reducing rocks and concentrating heavy minerals
DOI 10.3133/ofr20161022
Authors Thomas R. Strong, Rhonda L. Driscoll
Publication Type Report
Publication Subtype USGS Numbered Series
Series Title Open-File Report
Series Number 2016-1022
Index ID ofr20161022
Record Source USGS Publications Warehouse
USGS Organization Central Mineral and Environmental Resources Science Center