The analysis of minerals and rocks

Early mineralogists and petrologists identified minerals by their more obvious physical properties (hardness, density, cleavage, crystal form and so on), and they determined the chemical compositions of minerals and rocks by so-called wet' gravimetric methods, which involved dissolution of the sample followed by selective precipitation and weighing of individual constituents.

With the discovery of the polarization of light early in the nineteenth century the observation of physical properties of minerals could be extended to a study of their behaviour in polarized light. The application of X-rays to analysis of crystal structure in the early twentieth century was followed by the development of several indirect and generally non-destructive methods of chemical analysis. These new techniques have supplemented rather than supplanted the earlier well-tried methods, which are still widely used. All students of mineralogy learn how to identify minerals on the basis of their 'external' properties, and gravimetric analysis, though somewhat laborious despite modern improvements, still provides the standards against which indirect physical methods of analysis are calibrated.

Three main kinds of physical method exist for the determination of the nature and chemical composition of minerals and rocks : ( 1 ) optical methods, involving the use of polarizing microscopes; (2) diffraction methods using either X-rays (see above) or electron beams to elucidate the internal structure of minerals; and (3) analysis of emission or absorption spectra, which reflect the relative abundances of elements. Of these three, the first is the longest established. It requires the simplest and least costly equipment and it remains the most powerful tool for the laboratory study of minerals and rocks.

The great advantage of the polarizing microscope is its versatility. All transparent minerals, save those crystallizing in the cubic system, are themselves capable of polarizing light, and they produce complicated but characteristic combinations of absorption and interferenc colours when polarized light is passed through thin sections of rocks or mineral grains. The identification of minerals can also be refined by the measurement of their refractive indexes using carefully calibrated immersion oils. Some minerals are opaque, however, even in thin sections, and these can be examined by polishing their surfaces and examining them in reflected polarized light.

Because the resolution that can be obtained in any microscope system is limited by the wavelength of the radiation used, there is a lower limit to the size of objects that can be distinguished by optical microscopy. Under the most favourable conditions optical microscopes cannot resolve objects less than half a micron or so in size (and few petrological microscopes are capable of this). The electron microscope is capable of far greater resolution.

Although electrons are generally thought of as charged particles they also possess wavelike properties, and electron beams can be reflected, refracted and diffracted, just as beams of light can. However, their wavelength is much less (in the order of a few pm (1 pm = 10^-12 m) which is smaller than atomic dimensions) than that of light (which is around 10 ^-7 m). Moreover, because electrons are charged particles electron beams can be focused using electromagnetic 'lenses', the magnification of which can be controlled by changing the strength of the current in the lenses.

The electrons are produced by a heated filament, and the beam is controlled by the various lenses in the system, which are analogous to the condenser, objective and ocular lenses in optical microscopes. The whole system must, however, be maintained under high-vacuum conditions, and the preparation of specimens is a complicated process, because it is not possible simply to transmit electrons through, or reflect them from, the object to be studied.

Optical emission and X-ray fluorescence (XRF) spectrometry are methods for determining the chemical composition of minerals and rocks. They therefore contrast with the techniques just described, which are concerned with physical properties and characteristics. Optical emission spectrography depends upon spectral analysis of radiation produced by the excitation of atoms by strong heating. When the radiation is passed through a prism, it is refracted and dispersed according to wavelength. The optical emission spectrum of an element consists of a series of lines, corresponding to radiation of different specific and characteristic wavelengths. The intensity of the lines in the spectrum can be measured, and it is related to the concentration of the corresponding element in the sample.

XRF spectrography combines the principles of X-ray diffraction and emission spectrography. It provides a rapid method of analysis and is nowadays widely used. The atoms are excited by a primary X-ray beam, rather than by heating, and the greater energy input causes them to emit energy at X-ray wavelengths (a few tens of pm). Just as with optical emission spectrography, the X-radiation for each element is of specific and characteristic wavelength, and the intensity of the radiation can be measured and is proportional to the concentration of the element in the sample.

The electron microprobe, whose development was pioneered in the 1950s by J. V. P. Long of Cambridge, England, embodies perhaps the most important new technique of any for the chemical analysis of minerals. Previously, it was only possible to measure the bulk composition of minerals separated from rocks, a method that could not always eliminate microscopic impurities adhering to or enclosed within mineral grains. The electron microprobe enables the composition of minerals to be measured in situ within the rock, and furthermore it enables variations of composition within individual mineral grains and between different grains of the same mineral in a rock to be determined.

The technique is based upon principles applied in reflected light and electron microscopy and XRF analysis: a high-energy beam of electrons is focused upon the polished and carbon-coated specimen surface. The electrons excite the atoms in the sample to energy levels at which they emit their characteristic X-radiation. The X-rays from the sample are then diffracted and detected in just the same way as they are for XRF analysis. Because the specimen is polished, it can be viewed in the specimen chamber by an optical (rellecting) microscope system, and grains or parts of grains can be selected for analysis; and because the electron beam can be focused down to about 1 micron diameter, selected spot analyses or traverses can be made across individual grains. In addition, whole grains can be scanned to determine the overall distribution within them.

Most of these methods (the notable exception is transmitted light microscopy) require sophisticated equipment, power sources with stabilized voltage, cooling systems of constant water pressure and (for electron microscopy and microprobe) high-vacuum chambers. Developments and extensions of the methods are constantly being made and are applied in a wide variety of fields, not just to those of mineralogy and petrology. It is well known, for instance, that the electron microscope is widely used in biological studies, and XRF and the electron microprobe has obvious applications in the study of alloys and ceramics.