The dishes need to be replaced if too scratched, or the spectra will be degraded.
The following notes discuss the advantages and disadvantages of spectral analysis of different sample types.
Soil Samples ADVANTAGES
In regions where little or no bedrock is exposed soil samples can provide valuable spectral information that can be used to map lithology and alteration.
Spectral data from soil samples can be helpful in interpreting associated geochemical data sets.
Spectral analysis of soil samples is often a very cost effective and rapid method of producing surface mineral/alteration maps.
DISADVANTAGES & TIPS
Spectra from soil samples often contain higher noise signals and weaker mineral signatures than other sample types (e.g. rock chip samples).
In areas where the bedrock has been subjected to strong or intense weathering, soil samples can contain few if any unweathered minerals and can be dominated by kaolinites or smectites.
Thin Sections ADVANTAGES
Spectral analysis may provide useful information not available from petrographic studies alone (i.e. on mineral composition and crystallinity).
DISADVANTAGES & TIPS
Thin sections themselves are not suitable for spectral analysis because they are too thin and (more importantly) the resins and glues used to make them produce their own distinctive absorption features which results in very complex spectra. The best spectra can be obtained from the thin section off cuts.
Spectral absorption features associated with glue and resins on the sample surface or impregnated through the sample may be present when analysing offcuts or stubs.
Some rock saws use kerosene or similar hydrocarbon based cutting fluids. These fluids can produce absorption features if present in the sample.
Polished sections are often not suitable for spectral analysis due to their high sulphide content.
Outcrop/soil Rock Chips ADVANTAGES
Rock chip samples generally produce strong spectral signatures compared with soils.
Rock chip samples are usually a cost effective and rapid way of producing mineral/alteration maps from their spectral data.
DISADVANTAGES & TIPS
Rock chip samples may not be able to provide adequate spatial coverage across the project area and so in this sense soil samples have a clear advantage.
As with all near surface samples, rock chip samples will have suffered some degree of weathering.
Rock chip samples are inhomogeneous (unlike geochemical pulps) and so more than one measurement may be required to characterise the sample/location.
RAB/RC Drill Cuttings ADVANTAGES
RAB/RC Drill cuttings are quite homogenous and so the spectral signature of these samples represents an average signature for the sampled interval.
Spectral analysis results can be compared directly with the equivalent geochemical data.
DISADVANTAGES & TIPS
The spectral response will be less responsive to subtle alteration indicators the longer/wider the sampled interval. This is a potential problem where alteration related minerals are not pervasive but may be restricted to veins, vein selvages or fracture surfaces. The concept of dilution applies to spectral analysis in the same way as it does to geochemical analysis.
Samples may be wet when drilled. If possible, one way to dry the samples is to leave them open for a few days prior to measuring them (provided that the climate is dry and not excessively humid). Don’t wait until the samples have to be measured before discovering that they need drying.
Under wet drilling conditions a significant proportion of the fines may be washed out of the sample in certain rock types. It is sometimes useful to test this by drying and then analysing the fines captured from expelled water.
Drill Core
ADVANTAGES
Highly specific measurements of matrix material, clasts, veins, vein selvages and fracture surfaces are possible with this sample type.
DISADVANTAGES & TIPS
Analysis of core often takes more time than equivalent RC samples because notes need to be taken describing where each measurement is made and of what feature.
It is generally more difficult and time consuming to make accurate comparisons of specific core measurements and geochemical data. This is because geochemical assays are usually carried out over selected intervals of core.
Some contamination can occur on the surfaces of core samples associated with drilling muds or spilt lubricating oils, hydraulic fluids or diesel fuel.
Geochemical Pulp Samples ADVANTAGES
Pulped samples are homogenous and so their spectral signature represents an average signature for the sampled interval.
Pulped or crushed sample types (eg. RC/RAB cuttings and Geochemical pulp samples) can liberate mineral species that may be preferentially developed along fractures, veins or as spotting in the sample. This can enhance alteration signatures where these minerals are spectrally responsive (e.g. clays, micas etc.).
Spectral analysis results can be compared directly with the equivalent geochemical data.
In core samples, the presence of water absorption features associated with fluid inclusions can easily be confused with adsorbed water in smectites and illite/smectites. This factor can make crystallinity and smectite proportion determinations difficult. In pulp samples however, free water in fluid inclusions is largely removed allowing more accurate determinations of smectite proportion and illite crystallinity to be carried out. It is rare that fluid inclusions are small enough to be preserved in pulp samples.
DISADVANTAGES & TIPS
Some laboratories may over grind or over heat the pulps and degrade the mineral responses, this is not common and dependent on the laboratory.
The spectral response will be less responsive to subtle alteration indicators the wider the sampled interval represented by the pulp sample. This is a potential problem where alteration related minerals are not pervasive but are restricted to veins, vein selvages or fracture surfaces. The concept of dilution applies to spectral analysis in the same way as it does to geochemical analysis.
Although producing generally brighter spectra than equivalent core or cuttings, the spectra of pulp samples often tend to display weaker absorption features because of increased scatter at the surface of the powder.
Where a sample contains sulphides or magnetite their liberation through crushing can cause significant decreases in data quality.
The best sample to use is a split from the first jaw crush stage in the pulp preparation where the samples are fine chips, before they are actually powdered.
Important
In all cases, only compare the spectral variations of similar sample types (e.g. drill core with drill core, pulps with pulps, outcrop with outcrop, float with float and soil with soil). This will ensure consistency in your interpretations and in the output data.
Influence of Water
It is important to consider the influence of water because it occurs in two forms, as: - Free water (in wet samples);
- Bound water.
Recognising the Presence of Water Water absorptions will occur in the spectra of:
- Wet samples;
- Samples with water-bearing fluid inclusions (e.g. in silica or carbonate); - Minerals with structural water (e.g. smectite and gypsum).
Water has two diagnostic absorptions: one near 1400nm and the other near 1900nm.
These absorptions are typically broad and asymmetric, when compared to the sharper and more symmetrical OH absorptions. In addition, water in wet samples has a distinctive broad rounded minimum in the 1900nm absorption band compared with the sharper minimum of water bound within a mineral lattice.
Wet Samples
It is recommended that all samples be dry before spectral analysis, as the water bands will tend to swamp any diagnostic absorptions in the 1350-2000nm spectral region. This may be significant if attempting to determine proportions of smectite in a sample, for example.
The “free” water bands of wet samples are easily distinguished from “bound” water absorptions (e.g. in smectites) as they typically display very rounded and broad minima
Drying Wet Samples
There are several methods of drying the samples in the field or site: - In the sun;
- Drying cabinet (~60 degrees C overnight); - Quickly over a small gas burner;
- Microwave (
The last two options are not generally recommended as a rule because of the high temperatures that can arise, but are useful if there is no alternative.
Examples of the 3 Different Types of Water Signature
Noise in Spectra
It is important to recognise noise as noise can produce artefacts which could be misinterpreted as absorption features.
Recognising Noise
Noise can be recognised as narrow (<4nm), weak, sharp features.
Often noise can be observed between 2400-2500nm where the reflectance signal is weakest.
Noise is typically observed in dark low reflectance samples.
Another type of noise typically occurs if fluorescent lights are on during measurement. This results in periodic noise throughout the spectrum.
Most importantly, re-measure sample and observe if noise features have disappeared or have changed wavelengths.
Overcoming Noise
Use higher enhancement/integration modes, and re-measure the sample to obtain a better spectrum. Alternatively try a different sample type, or part of sample, if available.
ASD/Terraspec spectra can often be very noisy, especially at longer wavelengths. If this is the case, then increase the number of averages and the time taken to measure your sample. In addition, to improve ASD spectra, warm up light source before measuring for at least an hour.
Note that some samples that contain opaques or carbonaceous material will always have noisy spectra.
Other measurement and sample artefacts Spectral can also be influenced by
1. plastic features, recommended that samples are not measured through plastic bags 2. resin features: clean surfaces only, not impregnated.
3. oil spill: oil spill on sample surfaces or mixed into sample will cause spectral features.
Artefact features
Artefact features are additional features in a spectrum that are not related to the sample mineralogy. The presence of these will lead to misidentification.
The most common artefact features are due to impurities such as kerogen oil (from drill rig, diesel spill or other sources), dry vegetation or wood, resin impregnations, or plastic.
Other artefacts can be introduced into the spectrum by an unusual overall curvature to the spectrum, which may be related to dark samples or poor illumination.
Example of kerogen/oil spectra.
Particle Size Effects
It is important to consider the influence of particle size as this can influence the brightness of the spectrum and the intensity of absorption features.
Powders versus Rocks
Mineral/rock powders typically have higher reflectance (i.e. are brighter) in the SWIR than their equivalent solid rock samples.
In addition, the absorption features in the reflectance spectra of powders are typically weaker than in rock spectra. This is due to a decrease in the depth of penetration of the radiation as a result of greater surface scattering.
Significance of Particle Size Effects
The influence of particle size on spectral response can be significant when comparing drill core and pulp spectra, soil and outcrop spectra, and weathered surfaces and fresh surfaces.