A Process for Reducing Rocks and Concentrating Heavy Minerals

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.

Introduction

In the field of geochronology—the geology discipline concerned with dating events in the Earth’s history—minerals containing certain elements are crucial to radiometric dating. Isotopes of potassium, uranium, lead, thorium, and some rare earth elements are commonly measured to determine the approximate dates of events in the formation of the Earth. Rare earth elements of particular interest include yttrium, lanthanum, and cerium. Not all minerals contain these elements; those that do must be isolated, or separated, from other incidental mineral constituents in rocks. Separation requires a method.

Necessary safety procedures and a general equipment guide will also be discussed.

Safety is paramount for the operator of rock reduction equipment. Before choosing a sample to reduce for the purpose of geochronology and other analytical tests, a few simple steps should be taken to minimize operator injury. These steps include the use of safety glasses, dust masks, gloves, and hearing protection. At the very least, a dust mask and disposable gloves should be worn when handling geologic material. Access to a vacuum hood is as important as access to safety wear. A vacuum hood prevents inhalation of airborne particles, including those invisible to the naked eye. Once protection is assured, the operator is ready to begin preparing the work area that will be used during comminution of the chosen geologic sample.

The jaw crusher and work area should be cleaned thoroughly before work begins. Clean equipment and work area minimize the possibility of cross contamination. Safety glasses, a dust mask, hearing protection, a stiff brush, a collection bin, a removable cover for the feed opening, and sample bags or containers will be needed for the crushing procedure. Note that the operator’s hands and clothing should never go past the opening at the top of the jaw crusher. Keeping limbs and sleeves well away from moving parts will prevent injury. If the laboratory facility is equipped with a dust collection system, it should be turned on before any processing begins. Installation of a dust collection system is recommended to remove rock dust particles from the air and prevent inhalation.