Earth Observation in the frame of EO-MINERS - Earth Observation methods
The mineralogical composition can be an important factor determining the environmental behaviour and geotechnical properties of rocks and sediments. Some of the methods described below, therefore, straddle the fields of geochemistry and geotechnics/soil mechanics.
Solid rocks are usually mineralogically characterised intact, while the mineralogical compounds of loose sediments or less consolidated rocks (e.g. mudrocks) often can be separated for analysis.
Mineral separation methods - For analytical (and commercial) purposes mineral grains in loose sediments can be separated according to their physical properties, namely specific density and magnetic properties. 'Panning' for gold is a typical method to separate out the heavy gold. For analytical purposes the sediment can be suspended in liquids of different densities to separate 'heavy minerals' from common quartz or clay grains. Flotation is an industrial process based on a similar principle to separate ore grains from gangue, e.g. quartz. The mineral density-dependent settling velocity in long (>10 m) vertical tubes can also be used for mineral separation, though this method is mainly used for granulometric purposes. Ferromagnetic minerals, e.g. magnetite and other iron minerals, can be separated by exposing them to a magnetic field.
Optical microscopy - is the classical method of mineralogy. While some minerals can be identified with the naked eye in rock specimens, their exact identification and quantification may require inspection under a microscope. Two types of microcopic methods are used, namely under transmitted light and under reflected light (mostly for opaque ore minerals). In the first instance samples are prepared as thin-sections, i.e. slices of rock are glued to a glass carrier and ground down to a thickness of less than 0.03 mm. In the second case small specimens are imbedded in special carriers and their surface is ground flat and then polished. Inspections are carried out under normal light, polarised light (some minerals polarise light and can be identified in this way) or UV (some minerals and hydrocarbons exhibit a specific fluorescence under UV). Traditionally optical microscopy requires a good deal of experience and good colour identification ability on the side of the operator, computer-supported image analysis today aids in routine analyses.
Thin-section of basalt under normal light (left) and under polarised light (right) (from www.zeiss.de).
X-ray crystallography - is the classical method of crystallography. Atoms in crystalline substances deflect X-ray beams in patterns that are characteristic for their spatial arrangement in the substance. The diffraction patterns vary according to the direction from which the beam penetrates the crystal lattice. A crystal of sufficient size is mounted in so-called goniometer and slowly rotated in the X-ray beam. The resulting diffraction patterns were originally recorded on photographic films and analysed manually, while today photomultiplier tubes and analog-digital recording simplified data capturing for processing in a digital computer. The latter greatly facilitates the Fourier-transformation that are needed to convert the two-dimensional diffraction patterns into three-dimensional patterns of electron densities, representing the atoms in the crystal lattice.
Configuration for x-ray crystallography (from www.district87.org)
Powder-diffractometry - Often a mineral to be identified is not available as an isolated single crystal of sufficient size and mixtures of minerals have to be used. Powder (x-ray) diffractometry allows the rapid identification of crystalline substances. Unlike in x-ray crystallography, the two-dimensional diffraction patterns are not points, but rings on a flat detector, as the multiple crystals are ordered randomly in the sample. Location and intensity of the rings can be compared against standard patterns that are collated in international standardisation libraries. The presence of several mineral species results in the superimposition of pattern. Their analysis today is greatly facilitated by analog-digital data capturing and computer-aided optical pattern recognition methods. Various chemical compounds may undergo (re-)crystallisation processes, e.g. during diagenesis, and these processes can be observed in variations of the standard diffraction pattern for a given mineral.
Configuration for x-ray powder-diffractometry (from www.physik.tu-dresden.de).