During mineral formation other material can become trapped; this might be other rocks or minerals, melt and aqueous fluid, water, inorganic gas or hydrocarbons/petroleum. These microscopic pockets, known as mineral inclusions, can provide information about the early stages of an igneous/volcanic or metamorphic event and the interactions with the Earth's crust and upper mantle over geological time.
Minerals trapped in inclusions during growth are referred to as primary in origin, while those trapped subsequently, by a metamorphic event for example, are secondary in origin. Most minerals will contain many types and generations of inclusions, and can be classified according to the proportion of vapour, liquid or solid phase present at room temperature.
In metamorphic rock – a rock that has undergone a profound physical or chemical change due to subjection to heat and pressure – inclusions can preserve the early fabrics and minerals which have subsequently been removed from the rock by such metamorphic reactions. In lava, mineral inclusions can be useful to determine the conditions of equilibrium of minerals, while in ore bodies sulphide melts can be trapped along fractures in quartz which heal and leave many small inclusions within a single crystal.
Fluid inclusions, those that contain vapour and fluid, can also provide information; for example, analysis of atmospheric gas bubbles trapped in inclusions in ice cores are a significant tool used to study the impact of climate change. Fluid inclusions can provide information on the pressure, volume and temperature conditions of the fluid at the time of their precipitation. Measurements are generally concerned with pressure and temperature when the inclusion occurred and its composition. The most commonly studied minerals include quartz, fluorite, halite and calcite; these are often studied using ultraviolet light to detect oil inclusions.
And who can forget amber or tree sap? As well as finding trapped minerals, it is also possible to find trapped insects and plants – think John Hammond’s cane in the original Jurassic Park movie.
Analysis of mineral inclusions is a tricky business: mineral inclusions are small and difficult to expose. Their dispersed nature means only a few can be exposed at a time, but it’s important to identify them before they are visible at the surface. Optical identification can be useful, as can analytical techniques like scanning electron microscopy, but they are limited to surface analysis – ideally you want to go deeper.
Infrared analysis again can be useful, but there is some difficulty in preparing samples. Raman spectroscopy is ideal because it requires very little sample and is non-destructive. It can reveal the chemical and structural composition of sample which might imply how the rock formed.
During analysis a sample is illuminated, usually with a laser. Most of the light that bounces back is unchanged, but a small fraction will have lost or gained energy and is Raman scattered. This shift happens because photons exchange part of their energy with molecular vibrations in the material. The change in energy depends on the frequency of vibration of the molecule.
The resulting spectrum can reveal information such as the composition of the material, the stresses in the sample, variations in crystallinity and uniformity of the material.
In diamonds, Raman spectroscopy can help identify any remaining inclusions– it offers a confocal advantage and high spatial resolution to analyse below the surface to infer how the diamond formed. This is especially advantageous if exposing the inclusion would lead to contamination and prevent further analysis, such as isotopic analysis. The technique can also be used to identify the volatile contents of melt and fluid inclusions, which can be very challenging to achieve otherwise.
References & Further Reading