Petrology examination of raw materials is a tool used by the extractive industry to identify, on a microscopic level, what minerals, either beneficial or deleterious, occur in any rock/clay/sand/concrete sample.
At a practical level, detailed petrological analysis quickly provides an assessment on the likely suitability of the sample reviewed and can also readily identify important issues within the sample that are not visible with the naked eye, eg the suitability of sand for use as fine aggregate in concrete or as a fracking sand, the smectite (a shrink/swell clay) content in basalt, or factors influencing clay firing within bricks and alkali silica reactivity issues within cement. The potential for asbestiform and silicosis causing minerals can also be readily assessed.
Given the rising cost of materials testing, petrological examination is often completed as a first pass cost-effective testing method to determine if the rock is worthy of additional testing to identify if it can produce a specific material. At the other end of the spectrum, and also given the ever increasing costs and liability of providing substandard materials, petrology is often completed in project reviews to determine failure mechanisms, should any particular engineering structure fail due to the materials used in construction.
SAND & OLIVINE SAMPLES
In Figure 1, a sand sample at a magnification of between 40 and 400 times quickly allows for an assessment on the clay, organic and mica content, along with the likely workability of the sand and also the suitability of the sand to potentially produce fracking sand.
The clay-covered sand at first pass appears very clean at four times magnification, but when magnified to 400 times its size, it becomes apparent that the sand is still covered in deleterious clays, which can lead to an increase in water demand, cement powder usage and potentially material failure and contractor disputes.
As a further example of the use of petrology, an example of fracking sand is provided in Figure 2 (left picture), which confirms this sand has the correct roundness and sphericity requirements, and is worthy of further testing and will not require resin coating. In the right picture, an image of a coarse river sand sample that contains feldspar – a mineral common in sands derived from decomposed granite – and sandstone resources clearly shows the quartz and feldspar will disaggregate, potentially leading to material failure.
Interestingly, this sand also has a high potential for alkali silica reactivity in concrete. Therefore, the fracking sand is fit for purpose while the coarse river sand is not, as it is likely to break down further, thus reducing engineering performance of the material.
One final example (Figure 3) is provided of a basalt sample, which on the surface has weathered olivine crystals (brown patches) that were originally thought by a state road authority to be pervasive and indicative of a substandard engineering material that would not receive certification. Petrological analysis subsequently confirmed the weathering is superficial and that the rock is essentially unaltered and will be fit for purpose.
ESSENCE OF MINERALOGY
Given the technical nature of petrology, resource, environmental management and land use planning consultancy, Groundwork Plus has distilled the essence of product mineralogy into an easy to understand product risk table, which identifies each particular constituent in the rock and then provides a relative risk assessment for use of the material for a particular purpose.
Table 1 is completed for the olivine basalt sample in Figure 3, which was left idle as a resource for many years because it was thought to be unsuitable due to the deterioration of the olivine.
One of the limitations of petrology is that it can only review the sample provided for analysis. Therefore, it is critical to provide a sample of the material being quarried on-site. Picking the best sample on-site is not helpful because if the material is not representative, issues that might otherwise be recognised in the analysis could be missed, which can lead to problems if the material is used inappropriately.