Petrographic Analysis
Geochempet Services are experts in petrographic analysis of materials and reporting. Our focus is the identification and analysis of rock, sand, drill cores and drill chip samples for exploration, engineering and coal mining applications, helping clients mitigate any potential issues with products and identify the best market solution for their products.
Petrography is the detailed description and classification of rocks by examining thin section samples (typically 0.03 mm thick) mounted on a glass slide, in a petrographic light microscope. The microscope is equipped with rotating and translation stages as well as polarising elements above and below the sample stage, and accessories such as retardation plates.
Geochempet’s petrographic experts use these microscopes to identify the minerals in rocks as well as the size, shape and intergrowth relationships of the mineral grains and other textural features of the rock. This allows identification of the rock type and provides insights into its geological history. Point counting is used to estimate the volume percentages of the different minerals or other particle types.
Identification of minerals and rocks may be all that some clients require. However, most of our work involves a deeper interpretation of the potential uses of the material. Many clients utilise our services for regular quality and consistency checks of their products.
Our most frequently requested application is to assess the rock for mechanical hardness, durability and resistance to alkali-silica attack if it is to be used as concrete aggregate. We also assess materials for a range of other engineering applications including use as road base, asphalt/sealing aggregate, rail ballast, rip rap and marine armour.
To address material safety, some tests require our scientists to quantify the content of free silica in the sample or examine for the presence of potential asbestos materials, and which assess the incendiary sparking of drilling rocks in coal mines.
Petrographic Analysis Report Specifics
Petrographic reports of rocks, aggregates and sands
The Geochempet team provides many different types of petrographic reports to suit various client requirements. Samples can in general be submitted as hand specimens, rock spalls, ballast, aggregate, drill chips or drill cores.
The point counts that are used to quantify the constituents of a sample are usually on the basis of 100 random widely spaced points, but in some cases, larger numbers of points are counted for better precision estimates of low-abundance constituents.
Additional physical tests such as weathering assessment and specific gravity measurement that do not require the use of microscopes can also be provided.
Engineering petrography reports Method takes account of ASTM C 295 Standard Guide for Petrographic Assessment of Aggregates for Concrete and AS1141 Standard Guide for the Method for sampling and testing aggregates, of the content of the joint publication of the Cement and Concrete Association of Australia and of Standards Australia HB 79-2015 Alkali Aggregate Reaction – Guidelines on Minimising the Risk of Damage to Concrete Structures in Australia.
Where appropriate for the applications specified by clients, account is also taken of AS 2758.7 – Appendix B (2015) Aggregates and Rock for Engineering Purposes- Part 7: Railway Ballast, of ASTM D4992-14 Standard Guide for Evaluation of Rock to be used for Erosion Control and/or of Queensland Main Roads Test Method Q188 Petrographic assessment of aggregates – additional requirements for Transport and Main Roads quarry assessment purposes, and other test methods specified by governmental organisations such as Queensland Department of Main Roads, and Transport, Transport for New South Wales.
Brief mineral identification using an optical microscope
Our simplest petrographic report identifies the minerals present in a rock and the rock type, uses a point count to estimate mineral percentages, with minimal additional interpretation.
Detailed optical mineralogy and petrology
This report identifies and quantifies the minerals present, and also provides more detailed information on the textural relationships of the minerals, the overall rock fabric and structure, and the petrogenesis and geological history of the sample. This type of report is most suitable for exploration purposes. If opaque minerals or unusual mineral species (frequently ore minerals) are to be located and identified, we may recommend additional techniques such as analytical scanning electron microscopy or X-ray diffraction for unambiguous identification. Transmitted light is typically used, however, reflected light petrography using an uncovered, polished sample is available upon request.
Detailed optical mineralogy and petrology on aggregates with multiple rock types
This report type is typically used for river gravels, and quarry products where several rock types are interlayered or otherwise contacting each other. The sample is generally supplied as an aggregate. The different rock types are distinguished on the basis of both visual appearance and the fragments in two, rather than one, thin sections. Proportions of both different rock type (by weight, from loose aggregate) and of minerals (by volume using 100-point counts in each of the two thin sections) are estimated.
Engineering petrographic report on a single rock type
This report identifies and quantifies the minerals present, and reports on the overall rock fabric and structure. It comments on rock durability, concrete-related secondary/deleterious minerals, potential for alkali-silica reactivity if incorporated in a concrete, and suitability for other proposed engineering use(s) as requested. It explicitly lists Free Silica Content percentage. Usually, a 100-point count is sufficient, but a 600-point count version of the examination is also offered.
Engineering petrographic report on a river gravel or aggregate with multiple rock types
The different rock types are distinguished on the basis of both visual appearance and the fragments in two, rather than one, thin sections. Proportions of both different rock type (by weight, from loose aggregate) and of minerals (by volume using 100-point counts in each of the two thin sections) are estimated. If rock types are sufficiently similar in their properties, then a single set of engineering recommendations can be made on the basis of the overall average. Otherwise, separate sets of recommendations are given where these differ for the constituent lithologies.
Engineering petrographic report on sand or road base
The loose material is sieved to give a crude particle size distribution in four size categories (coarse, medium and fine sand, and ‘silt’), and a turbidity test. Grain shapes, surface coatings, presence of materials such as plant matter are described on the basis of low-magnification visual examination. A 100-point count of the thin section is used to estimate the percentages of minerals and of clast types. comments on rock durability, concrete-related secondary/deleterious minerals, potential for alkali-silica reactivity if incorporated in a concrete, and suitability for other proposed engineering use(s) as requested. It explicitly lists Free Silica Content percentage.
Secondary mineral count
1800-point count using 600 points counted on each of three thin sections. This is normally used to obtain precise estimates of the modal percentages of soft/weak minerals in the sample. It is frequently used with conjunction with X-ray diffraction, particularly to identify clay minerals whose particles are too small to identify by optical mineralogy. The method is adapted from that of AS1141.26, to include only soft/weak minerals irrespective of the host rock type.
Equivalent quartz content for abrasiveness (Schimazek method)
100-point count to obtain mineral modal percentages, and use of Schimazek factors to obtain an overall abrasiveness score relative to pure quartz.
Shape analysis (roundness and sphericity) of sand, aggregate or crusher dust
Microscopic analysis of sand, aggregate or crusher dust to determine roundness and sphericity to accord with the recommended practices of Section 5 of the American Petroleum Institute – API19C.
Rock aggregate weathering
Sorting of a batch of aggregate by visual inspection of colour and texture changes, using the weathering degree criteria of AS1726: Table A9 (2017). Report includes proportions by weight that are in each of five categories from “fresh” to “extremely weathered” and “residual soil”.
Specific Gravity
Estimation of the density of a sample from the difference between its weight as measured in air, and its apparent weight as measured when immersed in water.
Respirable Crystalline Silica/Free Silica Content
Estimation of percentage of detectable silica minerals (mainly quartz). Report does not include information on other minerals or petrographic detail. Two different test methods are available: (i) 100-point count on thin section, taking account of ASTM C296; this method is preferred if there is little or no ultrafine material or if macroscopic opal is to be included. (ii) Phase quantification by power X-ray diffraction: this is more accurate when significant silica minerals may be present at micron-scale or smaller grain sizes. Detection limit is about 1% in both cases.
Heavy mineral sands
Reflected-light petrography was previously used to examine natural or manufactured sands containing potentially economic minerals such as rutile, monazite, chromite, cassiterite, or placer precious metals like gold and platinum-group minerals. Because some of these materials are difficult to distinguish based on optical properties alone, and may occur as very fine inclusions within other minerals, analytical scanning electron microscopy is now recommended as the preferred method. For more information, see “Diffraction, Geochemical Analysis and Electron Microscopy.”
Coal Petrology
Geochempet focuses on analysing the inorganic mineral content of coal-seam rocks, with less emphasis on the coal itself. Several major clients regularly submit samples for frictional ignition testing to assess the spark or fire risk associated with drilling specific rock types. In addition, Geochempet conducts petrographic analysis of unusual lithologies encountered in mining operations, and supports investigations following incidents such as mine fires.
Frictional ignition testing
This report type is for specimens retrieved from coal mines, usually supplied to Geochempet as drill core. It assesses sparking (incendive) risk of the sample, using Ward’s classification diagram that is based on frictional ignition experiments. This involves counting the relative proportions of hard minerals, carbonates and soft clay/mica minerals in the sample and allocating the sample to one of five Risk Zones from 1 (minimal incendiary risk) to 5 (high incendiary potential). This test can optionally be combined with a Detailed optical mineralogy and petrology on the same sample.