Mineralogy today
Modern methods of analysis of minerals
Fluid inclusions an outstanding result of modern mineralogy
At the beginning of the Sixties, a major technological revolution occurred, which can only be compared to the polarising microscope introduction one century and half earlier: the electron microprobe, invented by J.D, Castaing in Orsay’s University (France). It was the first of a numerous series of analysis instruments, which today enable scientists to know the composition and structure of mineral phases with a degree of precision and resolution completely impossible to consider few years ago.
These analysis instruments arrive on the market at such a rate that it becomes difficult to control all the analysis parameters. The majority of them use sourve having energies higher than X-rays, in particular neutrons, electrons and synchroton radiation. Moreover, many ancient optical techniques in the visible spectrum, were completely renewed by the use of coherent sources (laser). Laser are also used for plasma formation, surface layers ablation, or in molecular spectroscopy (Raman effect).
Beam particles (electrons, ions, protons) based analysis devices
Bombardment of a polished mineral surface by an electron beam leads to the emission of secondary X-rays characteristic of the chemical elements constituting the mineral. Thus it becomes possible to analyse in a specific (few square microns analysed areas) and non-destructive way inside any mineral phase the composition variations with an extremely fine resolution (exsolutions, crystalline defects agregates), even any unknown phase observed under the electron microscope can then be determined (solid inclusions or zoning in minerals). The analysis instrument can be optimised, either for the element analysis (electron microprobe), or to complement the observation at very high magnification (electron scanning microscopes equipped with an to energy dispersion analysis system = EDS). The most recent precision analysis devices are highly automated and sensitive (limit of detection reaching p.p.m. for a large majority of the natural elements.
However, a certain number of parameters cannot be analysed:
- the oxidation degree for elements having various valences (e.g. : Fe2+ and Fe3+),
- isotopes,
- most of the volatile elements,
- elements lighter than boron or beryllium.
Moreover electrons possess a very weak penetration capacity, which authorises only surface analysis of the minerals.
The electron microscope is known since the Thirties, but it is only since the Seventies that it was used in mineralogy. Its first uses specially explored the possibilities to perform observation at higher magnification than with the optical microscope. The usefulness of energy dispersion analytical devices became quite comparable to the one of the electron microprobe. In fact they are complementary in their use, while the E.D.S. is optimised for imaging the microprobe is more devoted to in analysis.
The scanning electron microscope (S.E.M.) is in its rough principle comparable to a reflection microscope while in the transmission electron microscope (T.E.M.) electrons pass through the studied preparation. In the most powerful devices (Very High Resolution ElectronMicroscope), the very high resolution allows practically to observe the atomic structure of the mineral.
The transmission electron microscopes allow also electron to perform diffraction analysis of the sample. Compared to the unit cell dimension the electron’s equivalent wavelength is small therefore the analysis possess a very high resolution, but also a strong interaction with the atoms occurs. Thus, sections must be very thin (some Angstroms), to be prepared by very specialised techniques such as surface ablation by ionic bombardment. The TEM is the only instrument making possible to obtain information on the mineral structure on the scale of the unit cell or even the atom scale so useful for the study of crystal defects (dislocation type), stoichiometry lacks, microtwinnings, etc. It is often used by the specialists in crystal growth or minerals strain, in particular to study creep within the terrestrial mantle.
A major disadvantage of the electron beam devices is due to the weak capacity of electrons to penetrate deep inside the analysed specimen. This leads to limit their use to the study of surface phenomena, or not very deep layers (some Angstroms deep). Some of these disadvantages are somewhat compensated by the recent introduction (during the Eighties) of the ionic microprobe, which use an ions beam instead of an electrons beam. The much larger penetration capacity of ions authorises to obtain analysis at even the core of the mineral, and theoretical more significant are their possibilities to analyse light elements (down to hydrogen), and also to measure in-situ isotopic ratios (and thus to measure the age of minute crystals like zircons in igneous rocks). However, those instruments remain very expensive, complex to use, with a difficult maintenance and cannot equip most of the laboratories.
Ionic microprobe is only the first example of a whole series of devices build around protons sources. In the case of the PIXE (Proton induced X-Ray emission) or PIGE (Proton induced gamma-ray emission), the principle remains the same than for the electron microprobe, but the incidental beam consists of protons instead of electrons. The penetration capacity is much larger, at least comparable with that of the ionic microprobes, the energy of the incident particles is known in a more precise way according to the characteristics of the source (in general a linear or Van de Graff accelerator). The emitted signal (X-rays in the case of the PIXE, gamma in the case of the PIGE) is analysed by crystal detectors, in a roughly comparable way than with the electron microprobe.
One used these techniques for the analysis of noble metals in trace in sulphides, like for the determination of certain elements non-detectable with the electron microprobe (such as carbon in the chondres of the oldest meteorites). The method’s sensitivity is very good, but difficult is the necessary calibration.
Modern methods of minerals analysis
Mineralogy during the last years was very aware of new analytical methods derived from Physics, which one make use of high-energy sources other than X-rays, namely neutrons, electrons and synchrotron radiation.
Neutron diffraction was experimentally proven by W.M. Elsasser in 1936, and the first neutron diffractometer built by W.H. Zinn in 1947. However, there are serious limitations in its use, both intrinsic (the equivalent wavelength of the neutron beam is much longer than the diameter of the atom which is expected to diffract; the interaction with matter is rather weak; the power diffusion of atoms varies irregularly, and irrespectively to the atomic number; etc.) and practical (reactor sources are rare and weak, specimens need to be large; etc.).
Consequently, neutron diffraction began to give useful data only in the late '60s, especially after H.M. Rietveld who conceived (1967) and improved (1969) his structure refinement method based upon spectrum fitting to the full diffraction profile that render possible to work on powders. The first significant results, obtained on zeolites, were brought are as late as the years '70s. Neutron diffraction is complementary to X-rays diffraction because it is more adapted to material having a large content in light elements; thus, it is usually applied to locate the H atom in OH-bearing minerals (occasionally also after deuterisation), or to differentially locate the distribution in a structure of two atoms having close atomic weights (e.g. : Al vs. Si, Fe vs. Mn or Ni, Ti vs. Mg or Nb, etc.). Furthermore, neutrons possess a dipole momentum and interact with atoms having unpaired electrons; consequently, they can be used to define magnetic ordering in minerals showing the ferromagnetic to antiferromagnetic transition, such as in the magnesioilmenites, which are important on Earth because they register residual paleomagnetism.
Electron diffraction is widespread because it does not require a reactor as a source, but an electron microscope working in transmission mode (T.E.M.). In its basic principles it completely differs from both X-rays and neutron diffraction; the accelerated electron beam has an extremely short wavelength (<0.03 Å) but it is strongly attenuated by collided atoms: thus specimens have to be prepared as very thin sections.
Nevertheless, this is the only currently available method to collect information on crystals in extremely small areas, down to few unit-cells in diameter, and to evaluate defects such as stacking faults, dislocations, microtwins, etc. This is why electron diffraction is so important for crystal growers. When TEM is implemented with a few accessories, a spectroscopic study can be made either on the surface or deeper in the material.
Since G. Bathow, E. Freytag and R. Haensel (1966) demonstrated that a portion of the synchrotron radiation spectrum is emitted in the same energy range as X-rays while maintaining its outstanding special properties (very high intensity and polarisation, very low divergence, pulsed structure), synchrotron radiation is being used to gather diffraction interactions information in a very short time (milliseconds) or from very small crystals (microns) with a resolution and signal over noise ratio that make crystal structure solving a very precise procedure.
Synchrotron radiation is useful, specially in:
* very high pressure crystallography under the diamond anvil cell: e.g. : good diffraction effects have been obtained from ice that permitted to detect a new ice high-pressure polymorph via the Rietveld method;
* high-resolution diffraction studies, in which reflections angle modification of few hundredths of degree are used as an index of deviation from the theoretical structure due to the presence of impurities or internal tensions in the sample;
* following the reaction kinetics, when a phase changes into another with a rapid re-ordering of the structure.
The "white" character of synchrotron light permitted a new diffraction strategy to be followed in determining crystal structures: as the "anomalous diffusion" method (L.M. Maroney, P. Thompson & D.E. Cox, 1988) consisting in three or more recordings of the full diffraction spectrum with the impinging radiation monochromatised at or near the characteristic wavelenghts of the atoms present in the sample. This information makes it easy to separately refine the thermal factors of the various atoms, and reduces their positional disorder in the crystal structure.
The "white" character of synchrotron radiation also re-actualised the old Laue's method: it is now possible to record thousands of diffraction spots from very small crystals (<0.05 mm) in a very short time (min) on position-sensitive image plates (PSD) and to solve structures containing few hundreds of atoms only. The increased power of modern computers has given a definite contribution in helping to reach this goal.
However synchrotron radiation is more easy to study minerals by spectroscopic methods, which are complementary to diffraction. But those spectroscopic methods did not play a major role in Mineralogy for many years because the available X-rays sources were too weak to allow meaningful signals. Thus, for a long time spectroscopy was limited to spotting foreign atoms present in a sample, mostly REE. Synchrotron radiation render X-rays absorption spectroscopy (XAS) an efficient method only in the late '60s. Oscillations above the absorption limit of an atom, which were discovered by L.D. Kroning in 1922, can now be recorded independently upon those of other present atoms, they can then be interpreted (F.W. Lytle & E.A. Stern, 1979) to determine the oxidation state, coordination number, and even the shape and orientation of the coordination polyhedron surrounding the absorbing atom. Thus an atomic short-range order distribution can be determined that compares usefully with long-range order, best determined by X-rays diffraction. Local lattice defects with dimension down to the one of an individual site in the unit cell can be appreciated and, in particular, all this can be obtained even for atoms in small content in the sample, since the very high intensity of the synchrotron beam enhances them enough even in the presence of abundant other components. Total content (SR-XRF), local environment (EXAFS), valence and coordination (XANES) can be separately determined for all elements present in a compound, and used to reconstruct its bulk crystal-chemistry. A special technique (SEXAFS) allows studying the surface of the specimen, where most reactions with the external environment occur. This contributes to understand the effect of the industrial and/or natural pollution through the study of minerals.
Gamma-rays resonance spectroscopy is based on an effect discovered by R.L. Mössbauer in 1958 and became the standard method of Mineralogy (G.M. Bancroft & R.G. Burns, 1967) to determine the oxidation state of Fe and to locate its distribution among the various sites of a structure if these have different crystal field energies. However, atoms displaying the Mössbauer effect are few (Fe, Sb, Sn, Dy etc., among which Au), and the relevant minerals have been today fully investigated. Recently, however, this method received a new impulse by the development of a microprobe (C. McCammon, V. Chaskar & G. Richards, 1991) suitable to extract information from very small samples such as those in the diamond anvil cell.
Nuclear magnetic resonance (NMR) spectroscopy was discovered by E. Zavoisky in 1945, but entered Mineralogy only when the magic angle spinning (MAS) technique was developed and permitted recording effects on solids that are as well resolved as those previously obtained on liquids. E. Lippmaa. M. Mägi, A. Samoson, M. Tarmak & G. Engelhardt (1980) showed the importance of NMR by systematically surveying the isotope 29Si behaviour in silicates, and J. Klinowski, S. Ramdas, J.M. Thomas, C.A. Fyfe and J.S. Hartman (1982) demonstrated how important could be the selectiv study of 27Al and 29Si in aluminosilicate minerals such as zeolites, where NMR is the only possible method producing meaningful information. NMR is used also for very light elements such as H, B, P, F and O. All of this makes NMR one of the most promising methods to study real minerals, in contrast with the average distribution determined by XRD, and to kinetically study the transition from solid to liquid, since MNR, like all other spectroscopic methods, gives good content analysis whatever the state of the sample is.
IR spectroscopy and Raman spectroscopy
J Lecomte and Cl Duval published in 1943 the first work relating to the absorption spectra IR obtained on goethite, manganite, brucite, diaspore and hydrargilite. J Lecomte build in addition the first spectrograph equipped with photographic recording . The IR spectrums allow for a good characterisation of volatile element (OH-, CO32-, etc.) contained in the mineral structure. They become an essential method for the study of minerals too small to be studied under the optical microscope, like most of the main minerals in sedimentary rock (clay minerals). The interpretation of molecular spectroscopy spectrums most often requires to apply complex computing methods like the Inverse Fourier Transform used to perform the spectrum deconvolution (F.T.I.R.).
Near to I.R. spectrometry is the Raman spectrometry, another molecular spectrometry, which currently knows a great development in certain specialised sectors of mineralogy, in particular gemmology and study of fluid inclusions. The Raman effect, which corresponds to a certain light wavelength drift by molecular interaction, is known since the years ‘30s, but it became possible to use it in mineralogy only when sufficiently powerful and coherent light sources (laser) became available. The advantages of Raman are numerous (fast and nondestructive analysis performed on samples having very small size), but analysis is only possible on " active " substances. Are excluded, purely ionic compounds (e.g; native elements, chlorides), as well as the fluorescent substances, which mask the Raman effect and forbid any measurement.
Ablation methods: Laser Ablation, S.I.M.S.
In these techniques, the beam of particles or a sufficiently powerful laser radiation is used to remove ions or surface atoms and to analyse them with the mass spectrometer (SIMS = Secondary Ions Mass Spectrometry). The method can analyse elements that are considered as very useful to understand the geochemical exchanges in crustal rocks: rare earths elements (R.E.E.) , isotopes, etc. or to analyse thin layers, in order to follow the dissolution or the alteration of mineral surfaces. Mass spectrometer analysis is particularly adapted to the measurement of isotopic ratios, therefore the method allows to measure radiogenic ages on specific samples (like zircons).
During the last century, mineralogy produced an abundant theorical work, a large part of which is related to solid state physics. Let us quote two selected examples of such works both related to crystal associations : twins and epitaxy.
Twins
Twins are complex objects resulting from the juxtaposition or interpenetration of several individual crystals according to a well defined law of orientation. Many mineralogists studied the genesis of twins : mechanical origin, by crystallographic planes slip or by growth defect juxtaposition. In 1904, Friedel states a general law on the prolongation of the periodic lattice through various crystals constituting the twinning. Excepted merihedral twins, twins possess generally a lower symmetry than the symetry of the separated single crystals.
In 1928, J Drugman publishes significant work concerning feldspars, associations of different twins in the same crystals group in orthoclase as well as the Zinnwald’s twins in bipyramidal quartz. Ungemach observed the twins in the realgar from Matra in Corsica (France),while similar studies were carried out sphalerite and tetrahedrite from Saintt- Etienne de Baigorry. In 1965, Curien and Bondot extended this field of research to the chemical compounds, and they study the roses twins, knee twins and 3 branches star twins of the artificial potassium chromate
In 1970, Dussausoy and Wandji described the twinning of iron disiliciure.
Other work were carried out to describe perfect twins following the reticular merihedry law. From these data, Waintal and Sivardière could carry out the rigorous notation of twins using the real representations of dimensions within the specific groups. This notation render possible to consider all possible shapes of twinning in a given holohedral system
Epitaxy
L Royer defined in 1953 by the term epitaxy, the orientation of a crystal which settles on another crystal belonging to a different mineral species. This orientation, consequence of certain specific lattice characteristics common to both crystals, requires two conditions :
1) - the existence, in the two lattices, of simple or multiple cells with almost identical shapes and dimensions,
2) - the ions of the epitaxic crystal, replacing the ions of the host crystal possess all the same orientation.
In laboratory experiment were also produced many epitaxic groupings and it was noted that the phenomenon was more easy to produce on crystals the hardness of which is lower than 4. In 1978, R. Kern publishes a summary of his twenty years work on the subject, highlighting the need for thermodynamic calculation to envisage certain modes of epitaxic growth.
The ambiguous relationship between mineralogy and the other domains of the Earth Sciences: the role of mineralogical museums
To a certain extent, Mineralogy was victim of its success. It is today at the boundary of many scientific disciplines :
* physics, for the study of the imperfections of the crystals at the atomic scale and the development and handling of the large modern analysis devices ;
* chemistry, for the crystallographic study of the artificial compounds and especially, the thermodynamic calculation of inter-minerals equilibriums ;
* petrography, for the analysis of the different groups of mineral forming the rocks: on the 3000 recognised species, a few tens may be are sufficient to explain more than 99% of all the rocks, sedimentary, metamorphic or magmatic ;
* geochemistry, for the study of the chemical tracers (trace elements, radioisotopes, fluid inclusions) to emphasise the conditions of formation, the origin and the future of various components of a mineral phase.
In almost all the countries, these disciplines, are more powerful, better regarded and more codified than mineralogy. This one is therefore reduced to be considered as a tool at the service of a group of scientists which, sometimes, is even unaware of its essence. To measure the importance of the risk this problematic situation represents, it is only to consider the dramatic rate to which university chairs of mineralogy disappears at University. Mineralogy chair are almost systematically transformed into chair of petrography or geochemistry, when they do not disappear purely and simply.. Because this risk obviously exists: the physicist, who knows only the atomic scale, the geophysicist, which is interested only in the physical properties of minerals, the geochemist, often unable to identify, on field, the commonest mineral species, are not true mineralogists. They know only one aspect, sometimes deeply deformed, of a complex unit which does take its plain value only when it is apprehended in its totality.
Let us recognise that this evolution is irreversible: major problems arise at the planet scale, and the mineral represents only one small elements of Earth’s complexity. In Earth Sciences, petrography or geochemistry possess, for this goal, a decisive advantage, since their object corresponds precisely to the desired scale. But it matters that they recognise all the importance of mineralogy, without underestimate the technical difficulties which make that, very often teaching mineralogy is very unpopular with the first degrees students.
In this respect, museums of Mineralogy have a primordial importance. They constitute not only the ideal place to preserve the heritage bequeathed to us by the preceding generations. Many of the beautiful ancient mineral specimens came from deposit, now disappeared, but Museums are also the place where systematic mineralogy can and must keep all its importance, and not only for the aesthetic aspect and the museologic value of the large collections. Obviously, this aspect is probably the most highly regarded by the public, and one cannot underestimate its importance. But mineralogical museums must also be active research centres, to ensure the fundamental studies which disappear gradually from the universities. If this goal is not kept in mind, we should observe rapidly the disappearance of treasure of knowledge and expertise of which we will only measure the importance when it will be lost.
Fluid inclusions an outstanding result of modern mineralogy
An example of the result of modern mineralogy: informations given by the fluid inclusions.
Evolution versus temperature of a pure C02 in a fluid inclusion garnet crystal of granulite rock from Hakututak ( SRI LANKA)
Hight-density CO2-rich fluid inclusions in garnet from Hakurutale, Sri Lanka, were stuied by microthermometry, Raman spectrometry and SEM analysis.Negative crystal shaped inclusions contain magnesite as a daughter mineral, occurring in relatively a contant solid/inclusion volume ratio.
Evolution versus temperature of pure CO2 in a fluid inclution garnet crystal of granulite rock of granulite rock from Hakurutak (Sri Lanka)
From the primary character of those inclutions, a calulated mixed mixed magnesite/CO2 isochore and thermodynamic calculations of magnesite stability in the presence of the granulite mineral assemblage it is concluded that the metamorphic fluid trapped in the inclusions at peak metamorphic conditions did consist of a homogeneous mixture of CO2 and magnsite, later dissociated within the inclusion.
