ABSTRACT

The heavy mineral fraction of sediments can be identified with a scanning electron microprobe. Sample preparation is relatively simple. The "heavies" after extraction with bromoform are sprinkled on a specimen support stub and covered with a thin film (150AO) of vacuum deposited gold.

Identification is accomplished by comparing the surface morphology and chemical composition of the unknown mineral grains with standards. Fine details of the particle surfaces are observed with a standard scanning electron microscope. An energy dispersive X-ray analyzer attached to the SEM provides qualitative and semi-quantitative elemental analysis. For example, zircon is readily recognized by its euhedral crystal habit and the presence of zirconium and silicon.

This technique is particularly suited to the study of heavy minerals with particle sizes less than 64 microns. The scanning electron microscope provides higher magnifications and considerably greater depths of field than conventional light microscopes. The microprobe can be used to determine bulk chemical composition, identify adsorbed materials, and analyze inclusions. The combined techniques permit a more complete examination of minerals in sediments

INTRODUCTION

In any study of heavy mineral assemblages it is customary to use light optical techniques. This paper describes a new technique for heavy mineral analysis.

A scanning electron microscope (SEM) with an energy dispersive X-ray analyzer can be used to identify and characterize heavy minerals. (Heavy minerals are those with a specific gravity greater than bromoform, 2.89.) The scanning electron microprobe (SEMP) technique provides an analysis with more data than can be obtained by conventional light microscope techniques.

Opaque and non-opaque fractions of a heavy mineral assemblage can be studied by using the SEMP. Minimal sample preparation and/or size fractionation is necessary. The greater depth of field associated with the SEM optical system provides a complete in focus surface examination of each particle. X-ray analysis yields a chemical composition of the particle. Light microscopy on the contrary requires special techniques to study opaque and non-opaque minerals. Only a portion of the particle surface is in focus at one time and very small particles cannot be examined. Chemical composition can only be intered.

EXPERIMENTAL METHOD

The heavy mineral fraction of the sediment is segregated from the light fraction using bromoform in a standard heavy liquid apparatus. The heavy fraction is dried and a representative sample sprinkled on a small brass specimen holder which has been covered with alcohol. When the alcohol dries, the heavies adhere firmly to the brass stub.

The specimens are placed in a vacuum evaporator and coated with a thin layer (approximately 150AO) of gold evaporated at vacuum conditions of 2 X 10−5 Torr. While the gold is being evaporated slowly, the samples are rotated and tilted. This two directional motion insures a gold layer of uniform thickness on the sample surface. The gold provides a conductive path to ground to avoid a charge buildup, and it forms a surface layer which is an excellent producer of secondary electrons.

In SEMP analysis numerous phenomena result from the interaction of the electron beam and the sample.

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