Preparation of Material

Choice of material

Click hereAlthough visual inspection of ores using a hand lens or binocular microscope yields valuable information about the minerals present and their textures, the most useful technique for the mineralogical and petrographical study of ores is reflected light microscopy. A well polished surface is essential for good petrography and is also needed for examination or analysis using the scanning electron microscope (SEM) or electron microprobe.

Click hereUltimately, the information obtained will only be as good as the sampling procedures allow. Therefore, as wide a range of material as possible should be chosen. In addition to representative ore samples, oxidized and altered ores, weakly mineralized wallrocks and country rocks should be collected. Often, weakly mineralized material yields more and clearer information than massive ores.

Click hereHand specimens should be sliced in a number of different orientations, especially if the rock/ore has a distinct fabric. In addition to there being good geological reasons for this, there is a practical one, for many minerals are markedly structurally anisotropic and in some orientations will polish very badly, whereas in others they polish well. This problem is especially tiresome in harder minerals like magnetite, haematite, pyrrhotite and pyrite, for those with good cleavages like galena, or those that have a sheet-like structure like molybdenite and graphite.

Choice of sections

The material can be prepared in a number of ways:

Click herePolished sections (polished blocks) This has been the standard method, for they are simple to prepare and are robust. Friable rocks are impregnated with resin before cutting into blocks. Polished slabs are similarly prepared. However, only limited information about the gangue mineralogy can be obtained from them. If made, or cut, to a suitable size they can be used for subsequent SEM or electron microprobe analysis.

Click herePolished thin sections: These are the usual alternative to a polished section and have the advantage that all the mineralogy can be described, as the section can be examined using both transmitted and reflected light techniques. They are especially useful for unmineralized rocks, ores with an extensive and variable gangue content, those containing low-reflectance ore minerals (sphalerite and cassiterite) and for altered and oxidized ores. They are slower to produce and slightly more difficult to use in reflected light, particularly in oil at high magnifications, when strong internal reflections from the non-opaque phases can mask the properties of small opaque minerals.

Click hereDoubly polished thin sections These are similar to polished thin sections, but both surfaces are polished in order to reduce the amount of light scattered from the bottom surface of the slice. This allows detailed internal structures, like twinning and zoning, to be seen in semi-opaque minerals, notably in sphalerite and cassiterite. Doubly polished wafers, 3-4mm thick, are extensively used in fluid inclusion studies. Polished thin sections are used for cathodoluminescence, SEM and electron microprobe studies.

Click hereGrain mounts These are prepared in a similar manner to polished sections and are used for unconsolidated natural materials, heavy mineral concentrates and extensively in beneficiation studies. Much textural information is lost, so, wherever possible, grain mounts are used together with sections of the original rock. Grains are cleaned of any heavy liquids used in their separation, washed and dried before mounting.

Preparation of polished sections

Click hereThere are many methods that can be employed to produce polished sections. Although these methods vary in detail, they all share a common sequence of mounting, grinding and polishing stages. Slices are cut to a suitable thickness, < 0.5 cm and trimmed to remove any sharp irregularities. With friable material this is done after impregnation with resin. The surface to be polished is flattened and its edges are smoothed on a grinding wheel or on a glass plate. Specimens too small to be cut have one surface flattened. This flattening and smoothing of the rims of the slice is to avoid areas where grinding grit can lodge and later be plucked out to scratch the polished surface.If reusable moulds are used, the inner surface is smeared with an even coating of petroleum jelly to allow easy removal of the section. The trimmed slice is placed face down in the centre of the mould.

Click hereThe mould is filled with epoxy resin, ensuring that all air bubbles (which also collect grit) are removed by pressing down onto the slice, repeating this after about half an hour. Cold setting resins are better than hot-setting resins, as the latter can alter the mineralogy of low-temperature or metastable opaque phases. A resin that will not volatilize extensively under an electron beam is best and only small batches of resin are made (50-100g), as some resins have a strong exothermic reaction when the hardener is added. A specimen label is inserted into the resin; this is better than writing on the base of the slice as the resin can become cloudy and obscure the number.

Click hereThe resin is left overnight to harden and checked that it has set sufficiently by pressing a thumbnail into it; when only a slight indentation is made, then the resin is hard. At room temperature, 12-24 hours is enough, but at cold temperatures it will take longer. If, after a few days, the resin is still soft, it is better to remove the slice and start again. Where this is impossible (grain mounts), heating the section and allowing it to cool again may harden it. The section is removed from the mould, its edges are beveled and a hole is drilled into the bottom of the section (if this is required by the polishing machines).

Click hereA cheap alternative to reusable moulds is clear Perspex tubing, cut to a suitable length. The tubing is flattened to avoid leakage of the resin and then placed onto cling film stretched tightly over a glass plate. The Perspex tubing then becomes the permanent outer casing to the section.

Grain mounts

Click hereSome material is always kept in hand against mishaps, either non-setting of the resin or the possibility of grinding and polishing away the material. The grains are sprinkled onto the base of the mould and the resin is carefully poured over them; better, the resin is poured in, any air bubbles allowed to rise to the surface before adding the grains which sink through the resin. The grains are sprinkled around the edge of the mould; the centre will be covered automatically. With time even fine-grained particles sink; breaking the surface of the resin with a needle is helpful. If settling is very slow, then the resin-grain mixture is gently stirred until some of the grains reach the bottom of the mould. However, most grains will remain entrained in the body of the resin.

Polished thin sections and polished wafers

Click hereIt is best to impregnate the rock slice with resin. The thin section is made in the usual way but is thicker (50-100µm) than standard thin sections. Naturally, a cover slip is not used. If doubly polished sections are to be made, they are initially mounted onto a glass slide with Canada Basalm or Lakeside Cement so that they can be demounted and polished on both sides.


Click hereHand grinding using a circulatory motion on glass plates gives the best results, although initial grinding using a grinding wheel is faster. If this is used, it is followed by hand grinding. Cleaning between different grades of grit is essential and a separate glass plate for each grade is helpful. The more time spent grinding, the better will be the final section.

Click herePolished blocks are initially ground with 400 grade alumina/carborundum only if the section contains hard phases or if there is a thick coating of resin to remove before eruption of the slice. Once the surface has erupted, the section is cleaned in an ultrasonic cleaner.Most sections are initially ground using 600 or 800 grit, especially sections containing soft minerals (chalcopyrite and softer), or gangue phases like carbonates and clays, and for all grain mounts. The sections are ground until a uniform, matt, scratch-free surface is obtained. The section is cleaned. Using 1200 grade alumina paste, the section is finally ground until the surface is flat (when it should become difficult to move over the glass plate) and the surface is scratch-free.


Click hereThe object is to obtain a mirror-like, topographically relief-free, scratch-free surface. Most polishing is done mechanically and a wide variety of machines and methods are available. Polishing on soft metal, cloth or paper laps using colour-coded diamond paste is the most common method. An oil-based lubricant is added to assist the process. Polishing either takes place from the centre of the section to the edges or vice versa (this depends on the polishing machine), hence the need to centre the rock slice in the mould so that the slice is uniformly polished. Smaller sections polish faster than the large ones, and centrally placed slices better than eccentrically placed ones.

Click hereThe best results are obtained by polishing at slow speeds, using the minimum of lubrication, but taking care not to burnish the section (this takes place if the section dries out). Fast speeds and weights cut polishing times, but cause relief, burnishing and plucking out of phases. Too much lubrication allows the section to glide over the lap without effective polishing.

Click hereUsing 6µm diamond paste (by introducing the paste onto the surface of the section), the section is polished until there are no significant scratches on the hardest phases. Many sections contain quartz, pyrite, marcasite, magnetite or haematite and these can be used. Softer minerals and the resin still are scratched, but the resin should no longer be matt but clear. If scratches are not removed from the hardest phases at this stage, they are unlikely to be removed subsequently and the use of scratch density, as a guide to relative hardness of the minerals, gives erroneous answers. If too much topographical relief is produced at this stage, then adjacent softer minerals may not polish. This polishing stage is the longest and the most important.

Click herePetrographical information can be obtained after polishing at 6µm, especially textural information. In particular, the nature of grain boundaries and the grain sizes of isotropic minerals (seen because of their differential polishing behaviour), data which are lost in later polishing, can be described and measured. This is useful for 'massive' galena, pyrite, sphalerite and magnetite-bearing ores.

Click hereAfter polishing at 6µm, the section is cleaned and polished at 1µm until no scratches are visible. For most purposes this is the final polishing stage, but for sections containing very soft minerals like sulphosalts, stibnite, molybdenite, clays and chlorites, or for sections to be photographed at high magnification, the section is cleaned and polished using ¼µm diamond paste. The procedure is identical for polished thin sections which are usually held in metal holders. After final cleaning, the polished surface is ready for use. Hand polishing, using techniques employed by metallurgists, can produce excellent results in very short times, but often the sections show excessive relief.

Polishing problems

Click hereSome minerals take a poor polish, due to their hardness (magnetite) or structural anisotropy. Other minerals pluck out because of their perfect cleavages; the triangular pits in galena are a classical example of this. Smearing of very soft minerals with a layered structure can also happen, for example with molybdenite. Some of the native metals are too soft to take a good polish and also smear, notably native gold and silver.

Click hereOverpolishing leads to burnishing and alters the surface colour and reflectance of strongly coloured materials by lessening the colour and increasing the reflectance. Chalcopyrite burnishes to a pale yellow more characteristic of pyrite; magnetite and ilmenite burnish to colours more reminiscent of haematite. With less intensely coloured minerals, the effect, although present, is less obvious. Burnishing is detected by looking at less well polished areas of the section, namely along fractures or edges of mineral grains (where the true surface colour is present), and comparing this with the centre of thegrains.

Click hereThe presence of adventitious materials also causes problems. These can be introduced at all stages of sample preparation and even after the section has been made. Tramp metals, notably steel, iron and copper used during mining and milling, are introduced into mine concentrates and mill products. Often they have rounded or splinter-like shapes which aid their identification. During preparation of polished sections, adventitious metals or alloys are introduced in a number of ways, from the cutting saw (phosphor bronze) and by smearing from metal laps or the 'metal holders containing polished thin sections (lead alloys, brass). Metal lap alloys can be mistaken for soft white native metals and small amounts of brass for native gold, especially under oil immersion at high magnifications.

Click hereCarborundum grits resemble TiO2, minerals, although their deep blue internal reflections, angular shape and restricted size range are characteristic. Diamond paste of ¼ µm grade is dyed white and resembles poorly polished disseminated TiO2. The problem of natural phases picked up from the polishing laps and introduced into the section is more difficult. Effectively this is avoided by polishing batches of mineralogically similar material and using new laps for each type of ore. To check whether any of these materials have been introduced during polishing, the following areas are inspected: the resin and soft gangue phases (especially clays) to see what is embedded in them, the resin-polished slice junction, the resin-perspex outer casing junction, erupted air bubbles and along fractures within the section or voids within its surface. A medium-power air lens is used, for in oil it is difficult to tell if the material is part of the section or not. Generally, adventitious mater is seen as a loose aggregate, comprising grinding grit, diamond pastes and angular minerals lying in imperfections in the polished surface.

Click herePetrographical studies and photography ideally are done prior to preparing the sections for scanning electron microscope (SEM) and electron probe microanalysis (EPMA) analysis. Detailed sketches or annotated photomicrographs are prepared to ensure the most efficient use of microanalytical techniques. However, for many assemblages (those containing low reflectance accessory minerals, sulphosalt-rich ones or those with very intimate intergrowths), further petrographical examination after analysis is needed, because unexpected additional phases or zoning almost invariably are discovered.

Click hereBefore microanalysis, using SEM or microprobe techniques, most sections are carbon-coated (although gold-coating is also used) and a suspension of metallic silver (known commercially as silver dag) is coated onto an edge of the section in order to increase its conducting properties. The subsequent total removal of these requires great care and relict carbon-coating left along fractures and cleavages can be confused with a number of low-reflectance materials, especially limonite, manganese oxides and carbonaceous matter or graphite; silver dag can be confused with white native metals.

Click hereAfter use, polished sections should be cleaned of immersion oil, organic solvents, sweat or water, and stored in dry, air-tight containers, or even in a desiccator in order to avoid tarnishing, etching or incipient alteration (heazlewoodite alters to millerite beneath dried oil droplets). However, many ore minerals will tarnish, some like bornite, within a few hours or days. Tarnishing can be removed by gently cleaning the section using an organic solvent, or by repolishing with fine alumina or Jim diamond paste. Before removal, check that the tarnishing has not yielded new information (tarnishing of pyrite may show fine-scale chemical zoning in ores where the freshly polished surface shows apparent homogeneity).

Click hereIn some pyrite, marcasite, and pyrrhotite and arsenopyrite bearing sections, atmospheric dampness causes the rapid oxidation and hydration of the sulphides. This alteration is accompanied by an increase in volume, and since the ores are held rigidly within the resin, the surface of the section buckles and so becomes unusable. Finally, ores containing silver sulphosalts should be examined swiftly, as many of these minerals are photosensitive and decompose under strong illumination after a few tens of seconds. The primary silver minerals redden and widespread pitting is seen before their destruction.