The usual identification tests given in most mineralogy books include colour, hardness and streak. Older texts may even describe a range of chemical tests such as reactions on charcoal, blowpipe analysis and bead colour. Whilst most of these methods are useful in establishing the identity of relatively simple or common minerals, including their basic chemical composition, they are not very helpful for species which are complex or where the there is significant similarly between them. For example it is almost impossible to distinguish between the various copper carbonates using these methods. Also, apart from the observed crystal form and habit, these methods tell us almost nothing about the internal arrangement of the atoms in a given mineral’s crystal structure.

Here in an outline of the main identification methods developed during the past fifty years in an ongoing effort to make minerals give up some of their secrets. Regrettably, all depend on large and expensive items of specialist equipment invariably putting them beyond the means of ordinary collectors. Nevertheless, it is important to know about these methods because their development goes hand in hand with our growing knowledge of mineral structure and composition. Also, many geology departments will allow fee-paying submission of specimens for analysis so it helps knowing what to ask for.

In essence, most modern techniques depend on some form of radiation or particles being projected at the mineral sample. These interact with the mineral and by examining the nature and pattern of what emerges mineralogists can determine the composition of the sample and how the atoms within are arranged. The article looks at these methods roughly in chronological order which also tends to correspond with advances in our knowledge of minerals and their properties.

Optical Methods

These methods depends on examining a very thin slice of a mineral through a specially adapted microscope. Typically the slices are ground to less than 0.2mm thick where most minerals become at least partially transparent. The microscope is equipped with a filter which only allows light polarised in one plane to pass through the optics rather than the usual random orientation of ordinary light. Most minerals have the property of rotating light and therefore by measuring the angle by which a passing beam of polarised light is rotated, and by comparing this against reference values, it is possible to establish the exact species under observation. Polarising microscopy is particularly useful for identifying the constituent minerals of different kinds of rock, allowing these to be classified according to their mineral composition.

Birefringence works on the principle that many crystalline materials have different indices of refraction depending on different crystallographic directions. Putting a birefringent material between crossed polarisers in the microscope can give rise to interference colours. Minerals are identified by examining the pattern of colours and the angles at which the colours are produced in respect to the polarising plates.

X-Ray Diffraction (XRD)

Whilst optical methods are useful tools for identifying many types individual minerals they tell nothing of their internal structure. This remained the case until the use of X-rays to probe the internal structure of crystals by the Braggs in the 1930s.

X-rays have the useful property of having a wavelength similar to the size of individual atoms. Consequently when X-rays are fired at a crystalline sample placed in their path, a proportion get deflected by the regularly spaced atoms and fly off in different directions. This is known diffraction. Where two such beams come together, if they are in phase they strengthen each other whilst if they are out of phase they will cancel each other out. This process is known as interference and if the sample is surrounded by a photographic film the diffracted X-rays will consequently produce a pattern of lighter and darker lines on the film. The pattern of the lines on the film depends on what is in the sample and by reference to standard values, this pattern can be used as a 'fingerprint' to identify a wide variety of minerals.

Unfortunately this still tells us nothing about the internal arrangement of atoms in a mineral. However, by measuring the angles between the lines, comparing their relative intensities and a clever mathematical technique called Fourier Transform, this information can be converted into electron density maps. These show the contour lines of electron density in a material and since electrons more or less surround atoms uniformly, it is possible to determine where atoms are located. To get a three dimensional picture, the crystal is rotated while a computerized detector produces two dimensional electron density maps for each angle of rotation. The internal structure of the mineral is obtained by “stacking” the two dimensional “slices” together. This is the basis of structure determination for all minerals, and countless other materials and substances.

Energy Dispersive X-ray Fluorescence (ED-XRF)

In addition to being highly penetrative, the high energy nature of X-rays means that they have the ability to knock electrons in and out of orbits around individual atoms. By doing so, these X-rays excite the atoms to a higher energy state, much as a flame or UV light does. When the atoms return to their normal state, energy is given off as secondary (fluorescent) X-rays. Each element in the sample produces very characteristic X-rays with different energies. These X-rays can be detected and displayed as a spectrum of intensity against energy. This means that the positions of the peaks identify which elements are present and the peak heights identify how much of each element is present.

ED-XRF has the major advantage of being non-destructive and by being accurate and fast. A result can be obtained in a few minutes. However it is not sensitive enough to measure very low concentrations of elements and also only a thin layer, less than 0.1mm, is actually analysed. This can sometimes give misleading results on minerals coated by other species unless the surface is cleaned before analysis takes place.

Electron Probe Micro Analysis (EPMA)

The technique works on the same principles as ED-XRF, except that electrons, rather than x-rays, are used to bombard the sample. The X-rays which are produced are similarly characteristic of the various elements present. Again the intensity of the peaks are indicative of the amount present in the specimen. The key advantages of the method are that the equipment is much smaller, because electrons are generally much easier to produce, and the EPMA is more sensitive, allowing much smaller element concentrations to be determined. The technique is often referred to simply as Microprobe.

Atomic Absorption Spectrophotometry (AAS)

Atomic absorption spectrophotometry (AAS) is an analytical technique used to measure a wide range of elements. Although it is a destructive technique the sample size needed is very small, typically about 10 milligrams, and the removal usually causes little damage. The sample is accurately weighed and then dissolved by strong acids. The resulting solution is sprayed into the flame of the instrument and atomised. Light of a suitable wavelength for a particular element is shone through the flame, and some of this light is absorbed by the atoms of the sample. The amount of light absorbed is proportional to the concentration of the element in the solution, and hence in the original mineral. Measurements are made separately for each element of interest in turn to achieve a complete analysis of the sample. Thus the technique is relatively slow to use. However, it is very sensitive and it can measure trace elements down to the part per million level.

Extended X-ray Absorption Fine Structure (EXAFS)

The method is used to determine the localized structure within a material. It relies on directing a single-wavelength X-ray beam at the sample. The energy is gradually increased until it is sufficiently high to traverse a threshold above which the photons can excite the electrons of a particular element. Below this level almost all the X-rays emerge through the other side as absorption is low. However above the threshold absorption rises sharply the energy being transferred to an atom’s electrons. The point at which this happens is measured and is characteristic of a particular element present in the sample. What makes the technique particularly useful is the fact that the excitation level of the electrons for a given element is not always the same but is affected by the electron clouds of the surrounding atoms. Con-sequently the change in the level can be used to determine the atomic number, distance and coordination number of the atoms surrounding the element whose absorption is being examined. Regrettably the method cannot be used widely because of the difficulties of creating a high-intensity beam of single-wave X-rays. For this reason EXAFS is normally only available at large facilities which can produce synchrotron radiation.

Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS)

ICP-MS is a recent technique with the enormous advantage of being able to detect individual elements at parts per trillion concentrations. Consequently it is rapidly becoming the method of choice not only for mineral identification but also for the identification of trace elements present as their contaminants, which allows the identification of individual locations from which the specimens originate. It is extremely fast to use and allows the analysis of the entire spectrum of elements in one go.

In a typical procedure, the specimen is mounted and fired on by a laser. The laser strips off atoms and molecules from the surface which are carried off by a stream of argon gas. The mixture enters a torch which is heated inductively to approximately 10,000 degrees C. At this temperature, the gas and everything in it ionises, forming a plasma which is injected into a mass spectrometer. This uses magnets to separate the positively charged ions according to their weight, thus identifying all the constituent elements in the mineral. The size of the peaks identifies how much of each element is present.

The downside of the method is that it is destructive, but since such small amounts of sample are required in the first place, this hardly matters. It is perhaps the most useful tool in use by mineralogists today.