Optical Microscopy and Mineral Identification

The binocular stereomicroscope is usually the first microscope a forensic geologist applies to a soil sample. At magnifications from 7x to 45x, and occasionally higher with zoom optics, it gives a view of whole, unprocessed grains in their natural morphology. The operator scans for grain colour, transparency, lustre, and morphology, picks out unusual or distinctive particles with a needle or fine forceps, and notes any obvious anthropogenic material.

Grains can be recovered after stereomicroscope examination and submitted for further analysis, making the technique non-destructive in practice. A reddish-brown grain provisionally identified as garnet can be picked and mounted for PLM confirmation; an angular black particle consistent with coal can be set aside for reflected-light examination or elemental analysis. This triage function makes the stereomicroscope an efficient first screen.

A petrographic (polarising light) microscope uses two polarising filters in the light path. The first, below the stage, plane-polarises the light. The second, above the objective, can be inserted to cross the polars at 90 degrees to the first. Between crossed polars, an isotropic material (glass, cubic minerals, amorphous material) goes dark and stays dark as the stage rotates. An anisotropic mineral produces interference colours that change with rotation, cycling through darkness (extinction) every 90 degrees.

The specific interference colour depends on birefringence and grain thickness. Using the Michel-Levy birefringence chart, the colour order constrains the mineral. Combined with grain shape, cleavage, and extinction angle, these properties usually produce a confident species-level identification. Quartz shows low first-order grey-white interference colours, no cleavage, and straight extinction. Calcite shows very high birefringence (high-order interference colours) and rhombohedral cleavage. Hornblende shows moderate birefringence with strong green-to-brown pleochroism in plane-polarised light and oblique extinction of 15 to 25 degrees.

Preparing a grain mount for PLM takes a few minutes. The sample fraction (usually 63-500 micrometres) is spread on a clean slide, a drop of mounting medium at known refractive index is added, and a cover slip is applied. Forensic grain mounts use a medium with RI close to quartz (1.54-1.54) so that quartz grains become nearly invisible in plane light (minimising the visual background) while higher-RI minerals like garnet (RI 1.72-1.89) stand out in sharp relief.

Grain mounts work well for sand-sized individual particles. For coarser fragments of rock, or for identifying the texture of lithic grains (rock fragments composed of multiple minerals interlocked), a thin section is prepared. The fragment is glued to a glass slide with epoxy resin, then ground and polished on successive abrasive laps until it reaches a standard thickness of 30 micrometres. At this thickness, most silicate minerals show their diagnostic interference colours as described in the Michel-Levy chart.

In forensic geology thin sections are most useful for rock fragments in the coarser soil fractions. A lithic grain from a granite shows interlocking quartz, alkali feldspar, and plagioclase; a lithic grain from a basalt shows pyroxene phenocrysts in a glassy or fine-grained groundmass. These textures give provenance information about the geological source that grain morphology alone cannot provide.

Forensic soil samples from urban or industrial sites, or from the clothing of people who moved through such areas, carry anthropogenic particles that are absent from natural mineral soils. Identifying them can locate a scene or activity more precisely than rock-forming minerals alone, because human-made materials have known origins and limited geographic distributions.

• Coal and coke fragments: opaque in transmitted light, show banded vitrinite and inertinite textures under reflected light. Presence on footwear near a rail yard or old industrial site is informative.
• Industrial glass: isotropic (dark and stays dark between crossed polars), highly variable RI depending on composition (lead glass, borosilicate, soda-lime). Conchoidal fracture and vesicles are diagnostic.
• Ceramic and porcelain: rounded to angular fragments with glassy matrix and remnant crystalline phases (mullite in porcelain, quartz inclusions in earthenware). Often show evidence of high-temperature firing.
• Concrete and mortar: angular calcium silicate hydrate masses (grey, low-to-moderate birefringence), mixed with aggregate minerals. Portland cement clinker shows belite and alite crystals.
• Fly ash: spherical glassy particles and irregular porous aluminosilicate cenospheres from coal combustion. A marker for proximity to power stations or industrial burning.

Anthropogenic particles quickly establish or exclude an industrial or urban context. A soil sample with abundant fly ash and glass does not originate from a remote rural hillside, regardless of its mineral texture class. Their presence on footwear when the proposed scene is an agricultural field is a discrepancy that warrants further investigation.

Comparison microscopy, in the sense used in forensic geology, means preparing grain mounts from the questioned and reference samples on slides that can be examined side by side on the same stage, or on adjacent slides under the same lighting conditions. The examiner assesses whether the overall population of grains is similar: the proportions of transparent to opaque grains, the distribution of grain shapes and roundness, the presence or absence of the same unusual particles.

This comparison is inherently subjective, which is both its strength and its limitation. An experienced analyst can make a rapid, holistic judgment about whether two slides are similar, drawing on pattern recognition developed across many soil types; that judgment is not easily expressed as a single number. Where the visual comparison is ambiguous or contested, it is supplemented by grain counting, statistical tests on particle size distribution, and SEM-EDX analysis of individual particles.

SEM-EDX places a grain under an electron beam and collects the X-rays emitted as the beam excites electrons in each element. The resulting spectrum shows which elements are present and in what relative proportions. For individual mineral identification, EDX answers questions that PLM cannot: is this orange garnet an almandine (Fe-rich), pyrope (Mg-rich), or spessartine (Mn-rich)? Is this colourless grain rutile or cassiterite? The elemental profile decides.

The principal limitation of manual SEM-EDX is throughput. A trained analyst can count 300 grains by PLM in 30 to 60 minutes; manual SEM-EDX analysis of the same number of grains takes considerably longer and requires the sample to be carbon- or gold-coated and placed under vacuum. Automated mineralogy systems like QEMSCAN (FEI/Thermo Fisher) and MLA address this by using the SEM image to detect grain boundaries automatically and then firing the EDX beam at each grain in sequence, classifying it against a reference database. A typical QEMSCAN run processes 3000-10,000 particles per hour and produces modal mineralogy statistics and false-colour maps.