Petrography Tools
As with any discipline, the worker needs to know the tools, what they’re good for and how they work. Below is a brief summary of the tools at the disposal of the petrographer. Each has unique strengths for a given circumstance and for deriving certain types of information.
Hand Lens:
A handlens is a fine companion at home or in the field, almost effortless to carry even on long treks to remote areas, and no batteries required.
Low-power stereo microscope:
This technique is really just one step up from a handlens. But, for a view of an entire thin section, there are a variety of methods (high-resolution scanning, mosaic mapping, etc), but no technology is faster and cheaper and more effective than low-power stereo microscopy. Adjusting whatever light source you have available, so that it shines brightly on the object of interest is the key to success. Some of these ‘scopes have built-in light sources, others work with independent light sources. Either way, spend some time optimizing the illumination on the object of interest. One of the advantages of this method is that it can be used with thin sections, rock slabs, whole rocks, and loose sediments.
Transmitted polarized light microscopy:
There is an emphasis on the usage of this tool. The reasons for this are practical (it’s a relatively cheap and
widely available method), historical (it was the earliest method after stereomicroscopy and we’ve learned
most of what we know about sandstones this way); and scientific (it’s a darn good method).On a practical
basis, the various optical properties (index of refraction, birefringence, extinction) are used collectively and
in combination with other clues (color, crystal form and appearance, general geological knowledge) to
identify minerals in thin sections. For the common minerals in sandstones as emphasized in this tutorial, a
general knowledge of the range of index (high or low?), birefringence (e.g., low for quartz and feldspar,
higher for micas, very high for carbonates), the extinction behaviors (parallel to the cross hairs or not?) may
suffice as you learn to make basic observations (what are the grains? the cements? What is the IGV?).
Reflected light microscopy
This technique is a more refined application of the use of reflected light (compared to low-power stereo
microscopy). At its most basic, you can simply drag a fiber optic light over to the polarizing microscope,
arrange the light carefully to illuminate the field of view, put you hand over the transmitted light source, and
look down the microscope! This is very revealing for opaque minerals. A more sophisticated form of
reflected light microscopy is applied to the study of rocks that are dominated by opaque minerals,
specifically, metallic ore specimens. Special microscopes for this technique illuminate the polished specimen
with polarized reflected light. Interactions of the polarized light with crystal surfaces can be interpreted with
respect to crystallography and mineralogy, just as with transmitted polarized light.
Fluorescence microscopy
Electron beam techniques
Scanning electron microscopy
Back-scattered electron microscopy
The highly accelerated electrons from the primary beam may penetrate deeply into the sample
where they are ultimately absorbed (thereby generating a variety of stimulated emissions, or, cause
heating), or, they may undergo collisions with atoms that deflect their path, in some cases leading to
deflections large enough to allow their escape from the surface (whereupon they are called back-
scattered electrons or BSE).
Cathodoluminescence microscopy
Interactions of the beam electrons with the outer-shell electrons in crystals can induce emission of
visible light, a phenomenon called cathodoluminescence (CL). The physics of CL is complex and
the intensity and color of CL can be related to the intrinsic properties of the crystal lattice, to trace
elements that have substituted for the ‘normal’ elements in the crystal, and to crystal defects.
Cold-Cathode
Hot-Cathode
X-ray mapping
EDS
WDS