Optical microscopy - transmitted and reflected light

The GeoRessources Laboratory provides three optical microscopes for the observation and analysis of different geological specimens including thin sections, thick sections and polished sections.

Description and applications:

- 2 Olympus BX51microscopes, 1 Zeiss Axioscope A1microscope

- Transmitted and reflected light illumination

- High-resolution Zeiss Axiocam Icc cameras -1Megapixel and 3Megapixel resolutions

- Detailed petrographic and mineralogical studies of rocks and minerals

- Fluid inclusion studies

Zeiss AxioImager A1m optical microscope equipped with a UV lamp

The principle of this technique is based on emission of visible light by a specimen that has been excited using a higher-energy light (with a lower wavelength) such as UV light.

The microscope is equipped with an excitation filter (365nm), a dichroic mirror (400nm) and an emission filter to allow the collection of wavelengths above 400nm.

The wavelength emission varies with the density of the oils contained within inclusions. Light oils are blue to blue-green in colour while heavy oils are yellow to reddish-brown. The wavelength obtained therefore provides a good indication of the density of the petrol.

Description of the microscope:

- Stand equipped with 2.5x, 5x, 10x, 20x, 50x and 100x objectives

- High-resolution cooling camera AxioCam MRc - 5 Megapixels

- Sample illumination via transmitted white light

- Sample illumination with reflected UV light

Applications:

- Detailed petrological and mineralogical studies

- Investigating the fluorescence of organic matter

- Identifying hydrocarbon fluid inclusions, which are naturally fluorescent due to their aromatic molecule content

Inclusions fluides hydrocarbonées, Yemen

Inclusions fluides hydrocarbonées, Mexique

Keyence VHX – 1000 digital optical microscope

The Keyence VHX-1000 digital microscope is a multi-use instrument that provides a depth of field that is 20 times greater than that of a conventional optical microscope, enabling sharp observations of 3D objects. Its comprehensive set of objectives (20x-200x, 100x-1000x, 500x-5000x) and adjustable observation system (illumination by inclined reflected light or transmitted light) allow us to obtain high-quality images of a large variety of samples, including thin sections, polished sections and macroscopic samples.

Applications:

- 2D and Z-stack imagery

- High-resolution mapping of thin sections

- Fluid inclusion imaging 

Chlorite sur fluorine, Alpes

Plans nets d’inclusions fluides, Alpes

Microscope equipped with a cold-cathodoluminescence system

Cathodoluminescence is defined as the emission of light photons by a solid surface that has been bombarded with electrons (emitted from a cold cathode). The technique is non-destructive; however the impact of the electron beam can cause alteration of the minerals or paste. This can lead to the creation or ionization of defects, or may trigger their diffusion.

The wavelength of the photons depends on the nature of the minerals and is generally in the visible portion of the spectrum although can also be in the infrared (IR) or ultraviolet (UV) portions.

The spatial resolution of cathodoluminescence is largely determined by the photon emission volume. This in turn depends on a number of parameters and can vary in size from cubic nanometer (nm³) to cubic micrometer (μm³).

The system comprises a CITL 8200 MK4 cold cathode coupled to an Olympus BX50 microscope with a high-resolution Zeiss Axiocam MRc 5 camera to record the emission of luminescence. The sample-chamber pumping system is ensured by a rotary vane pump.

Applications:

- identification of crystalline defects and impurities responsible for the luminescence

- revealing crystalline growth-zones and fractures in minerals

- distinguishing between minerals that possess similar optical characteristics (e.g., calcite and dolomite)

- obtaining information about diagenetic transformations (cementation, dissolution, porosity loss or gain) that occur during the burial of sedimentary sequences

Dolomite et calcite, Pyrénées

Microthermometry of fluid inclusions

Olympus optical microscope equipped with a Linkam stage

Fluid inclusions are small, fluid-filled cavities inside a mineral. Fluids are trapped during growth of the crystal or after crystal formation. Fluid inclusions may contain several phases that can be observed under the microscope: a liquid phase (e.g., H₂O, petroleum, or a CO₂ or H₂S dense phase), gas phase (e.g., water vapour, CO₂, CH₄, N₂, H₂S, H₂ or O₂) or solid phase (e.g., chlorides, carbonates, silicates, sulphates).

Microthermometry is an analytical technique used to identify and characterise the fluids that have circulated in rocks by measuring the temperatures at which phase changes occur in the interior of the fluid inclusions.

These changes are observed in three analytical steps:

- Cooling (to -180°C): nucleation phase of ice and salt or gas hydrates.

- Reheating (to 35°C): eutectic, fusion of CO₂, fusion of ice, homogenisation of CO_2.

- Heating (up to 600°C): homogenisation step.

Description of the systems:

- Linkam MDS600 and THMS600 stages, heated and cooled (-196°C to 600°C) by automated temperature regulation.

- Olympus BX50 and BX51microscopes fitted with x4 to x100 objectives.

- Observation under transmitted light, infrared and UV fluorescence.

Applications:

- Study of aqueous and hydrocarbon inclusions in transparent minerals (quartz, feldspars, carbonates, fluorides, sulphates...)

- Inclusion studies in opaque minerals (sphalerite, pyrite, stibine...).

- Measuring homogenisation temperatures in order to estimate fluid-entrapment temperatures during crystal, and therefore petroleum- or ore-deposit, formation.

- Estimating fluid pressures in hydrocarbon deposits using the ‘double isochore’ method.

- Determining the nature of dissolved salts by measuring the eutectic.

- Estimating the salinity of an aqueous fluid, expressed as equivalent weight % NaCl, from the fusion temperature.

Platine Linkam THMS600

Inclusion fluide hydrocarbonée, Yemen

Confocal microscopy

Nikon TE2000-U inverted laser scanning confocal microscope

Confocal microscopy is a high-resolution imaging technique based on laser excitation of a self-fluorescent sample. The excitation produces the emission of fluorescent rays from the different planes of the specimen. A diaphragm is placed in front of a detector (confocal aperture) enabling fluorescence emitted by planes located outside the focal plane to be eliminated. The fluorescent rays then pass through filters that separate the excitation and emission wavelengths, before arriving at the detection system. Electron photomultipliers record the photon emission and transform the light signal into a 2D image that corresponds to the specimen section. A three-dimensional image is obtained by simultaneous displacement of the stage along the Z-axis.

With minimal alteration and preparation of the sample, we are therefore able to obtain extremely fine optical sections (from 0.5µm thickness) using a pinhole, and to achieve a true optical sample dissection.

Description of the system:

- 5  laser lines: Argon at 457, 477, 488 and 514nm, Helium-Neon at 543nm

- 2 laser diodes: blue at 405nm, red at 637nm

- Inverted microscope equipped with a range of objectives (x4, x20, x40 immersion, x60 immersion and x100)

- Confocal head with 3 photomultiplier detectors

- Illumination in visible transmitted light and fluorescence

Advantages and applications:

- High lateral (0.1 μm) and axial (0.3 μm) resolution

- Acquisition of a series of optical sections

- Three-dimensional observation of hydrocarbon fluid inclusions

- Measurement of volumes and gas-filled percentage of fluid inclusions

- Quantitative study of microstructures in minerals and rocks (fractures, cleavage, fission tracks...)

- Examination of surface state

Optical microscopy of organic matter

Zeiss AxioImager microscope with system for measuring fluorescence and vitrinite reflectance

Coals are classified according to their rank, which reflects their enrichment in carbon during the burial process. The variation is mainly determined by an increase in temperature, from peat, which marks the first stage of thermal maturation (diagenesis), up to graphite, which marks the first stages of metamorphism.

Macerals (vegetal matter), the main constituents of coal, are divided into three groups: vitrinite, leptinite and inertinite. Vitrinite is the most abundant of these.

The thermal maturity of coals (houillification) is assessed by measuring the reflectance of vitrinite on a polished surface in reflected in UV light. The technique is based on measuring the fraction of light reflected by particles of vitrinite relative to the incident light. PRV or Ro(%) increases progressively with coal rank, and therefore with the highest temperature reached by the coal, and varies from 0.2% for peat, 5% for anthracite, to 9% for graphite.

The vitrinite reflectance measurements are converted by a spectrometer and reported in the form of a histogram, the shape of which allows the degree of homogeneity of the coal to be assessed.

Description of the system:

- Zeiss AxioImager microscope equipped with 3 objectives: x10, x20 and x50 immersion

- CCD spectrometer with cooled sensor; range 220-1000 nm; resolution 0.8nm

Applications:

- Fluorescence spectra and reflectance histograms;

- Degree of thermal maturation of coals; coal ranking; determination of the carbon content;

- Estimation of the maximum temperature reached by an organic sample.

Histogramme de réflectance d’un alum shale, Suède

Spectre de réflectance d’un alum shale, Suède