forensic discrimination of soil samples using decrepitation

Tests of decrepitation to discriminate between soil samples

Potential forensic application

In October 2020 soil samples were collected from 3 separate locations to see if decrepitation would aid in discriminating between soil samples. This should be applicable to soils containing quartz grains, but grains in soils may be quite small with low decrepitation counts. Soils are a mixture of minerals and there may be effects due to the dehydration of clay minerals during heating or additional decrepitation from carbonate of other mineral grains in the soil. These effects complicate the interpretation of fluid inclusions in the sample, but may actually help to characterize soils and facilitate the discrimination of soils from different locations.

Location map of the 3 soil samples tested

soil
location map, NT

The Litchfield sample (number 2069) was collected within an area of quartzose outcrop of the Depot creek formation sandstone of middle Proterozoic age on a residual plateau. It is 83 Km from the Mcminns sample.

The McMinns samples (numbers 2070, 2072, 2073) are from the laboratory location in an area of Cainozoic age lateritic soil, probably over Mesozoic siltstone/sandstone sediments and there is no nearby outcrop.

The Burrell sample (number 2071) is over outcrop of Burrell creek formation or Koolpin formation siltstone of early Proterozoic age. It is 8 Km from the Mcminns sample.

Compare-contrast 3 unrelated samples

All 3 of the soil samples were collected over quartzose sedimentary rocks but from mutally distant locations and over very different age parent rocks. All of these samples have been subjected to intense lateritic weathering with abundant ferruginous nodules in the Mcminns samples 2070, 2072 & 2073. The samples were sieved and 0.5 grams of the <420, >200 micron size fraction was analysed. The plotted results are weighted rolling mean smoothed over 3 samples at 25%, 50%, 25% weights. Note that sample 2070 had much more intense decrepitation and has been divided by 4 or 8 for convenient comparison with the other samples.

There are significant differences between these samples in both their decrepitation temperature pattern and decrepitation intensity. Normally it would be best to distinguish samples based on their temperature profiles, but the significant differences in decrepitation intensity between these 3 samples is probably also a defining characteristic, although likely to be influenced to some degree by variations in the quartz to clay percentages in the sample. Clay minerals do not usually give a decrepitation response, but may do so if they dehydrate or decompose. The low temperature decrepitation between 200 and 300 C of sample 2069 is characteristic of CO₂ rich fluid inclusions within quartz grains. The near absence of the quartz alpha-beta inversion peak at 580-590 C on this sample 2069 is also unusual and distinctive. Samples 2070 and 2071 show quite different temperature patterns with dual peaks at 430 and 465 C on sample 2071 and an almost single peak at 510 C on sample 2070.

It is also possible to use curve fitting to discern the component sub-populations which make up these complex curves as in many examples on this website and also here. Using the deconvolution data would assist in discriminating between or identifying identical samples.

It is clear that these 3 soil samples are mutually quite distinct and readily distinguished by decrepitation analysis. NOTE: subsequent tests suggest that sample 2070 is contaminated and a better plot of samples from these 3 locations is below using sample 2072 instead.

Initial sample comparison plot

soils from 3 locations

Testing the sampling reproducibility for multiple samples collected at a single location, within 10 metres distance.

Sample 2072 was collected at the McMinns laboratory some 2 metres from the original sample 2070. These 2 results were surprisingly different, so a third sample 2073 was also collected at the McMinns laboratory some 10 metres from sample 2070. The decrepitation of samples 2072 and 2073 are very similar to each other, but both different from sample 2070. It is possible that sample 2070 was contaminated with sand left after construction activities some 30 years ago.

The results from the 3 separate samples collected at the McMinns laboratory show that samples 2072 and 2073 are a close match, but sample 2070 is very different.

decrepitation of 3 samples at
McMinns NT

Preferred sample comparison plot after collecting additional replicate samples.

This preferred comparison of decrepitation of soil from 3 locations in NT shows sample 2072 instead of (possibly contaminated) sample 2070 at the McMinns laboratory location.

comparison of decrepitation of 3 soil
locations

The effect of grainsize on analyses of the same sample

Two different grainsize fractions of sample 2071 were analysed. This sample was dominated by fine grains with only some 20% exceeding 200 microns. The analysis of the fine grained fraction was done on 1.0 gram of sample as only low decrepitation was anticipated. It is likely that the fine fraction was mostly clay, as indicated by the absence of the distinctive quartz alpha-beta peak at 580 C on the fine grainsize fraction. Although both fractions start to decrepitate at 400 C, the grainsize effect prevents identification of these 2 results as being part of the same original sample.

comparison of different grainsize of
the same soil xample

Test of analytical reproducibility

The McMinns laboratory sample 2073 was analysed twice on subsequent days to examine the repeatability of analyses. These 2 analyses are a good match. Because of the low decrepitation counts on this sample, better results would be obtained by using a larger 1.0 gram sample - or perhaps even 2.0 grams.

analytical reproducibility of
McMinns sample

Soil sample 2073 collected at the McMinns laboratory was of a nodular furruginous lag soil over quartzose sediments with no nearby outcrop, photographed here. The sample came from the scraped area.

photograph of ferrous nodular lag
soil at McMinns

photograph of ferrous nodular lag
soil at McMinns

Summary

This brief preliminary study shows that decrepitation of soil samples is capable of discriminating between samples and may be useful for forensic purposes. However because soils are a mixture of mineral phases there can be unexpected variations which may either interfere or contribute to identification of similarity or difference between samples. There is good between-sample reproducibilty for multiple samples collected at the same location and also good analytical reproducibility of the same sample on multiple days. Further studies of soil decrepitation would be helpful to better understand the reliability of soil sample identifications. Curve deconvolution as seen elsewhere on this website would probably assist in differentiating or confirming the similarity of samples.