Have you ever wondered what the vibrant colours and unique forms in magnified images of stone represent? These striking photographs can appear as works of art, but they can also assist in identifying any potentially harmful or sensitive features inherent to the stone type.
Petrographic analysis uses a microscope to study thin sections of stone. Typically, it is used to supplement other physical property testing such as strength, durability and porosity/absorption tests, and it can also inform you about higher risk elements and how these may affect the stone in service.
Thomas Baggs is a geomaterials scientist with a special interest in petrology and petrography. Here he gives insight into the colours and forms of some recent petrographic examinations at Stone Initiatives.
XPL x20 quartz and feldspars
Microscope: Petrographic microscope with cross-polarised transmitted light
Magnification: x20
Stone: Granite
This mesmerising clip is both a journey of colour and an insight into the inner world of granite. The clip shows a thin section of granite viewed through a petrographic microscope with cross-polarised transmitted light.
As the thin section stage is rotated through the cross-polarised light beam, the various minerals within the stone exhibit different characteristic features. For example, the grey- and cream-coloured crystals that go black as the stage is rotated (known as extinction) is a combination of quartz crystals and feldspar crystals within the stone. The feldspars are typically differentiable from quartz as the feldspars appear greyer and exhibit a feature known as ‘twinning,’ while the quartz appears as more of a cream colour. The difference of colours observed is known as birefringence in cross-polarised light.
While there are different types of twinning, in this case in the cross-polarised light we can observe the twinning as alternating parallel lines of black and white, streaking through the feldspar crystals. These parallel lines go extinct at different stage-rotation angles and so cycle through appearing black (extinct) or white (full light transmission).
The final feature of note is the impressive bright-coloured birefringence colours of muscovite crystals, cycling through bright pinks, yellows, oranges, light-green and light-blues as the stage is rotated. These are mid to upper second order birefringence colours and characteristic of muscovite mica crystals (and many other minerals). Analysing other features, such as relief and cleavage planes, also contributed to the identification of muscovite crystals in this thin section.
PPL x20 stylolites
Microscope: Petrographic transmitted light microscope
Magnification: x20
Stone: Limestone
The vibrant blue ‘lightning strike’ running through this image is a feature known as a stylolite. It is seen here in a piece of high-density limestone. In this case it is a partially ‘open’ stylolite that has been partially filled with blue epoxy as part of the thin sectioning process.
Stylolites occur commonly in limestones, and the vein-like structures can be identified as jagged, zigzagging lines through the stone. They are typically filled with water-insoluble materials such as clays, oxides, sulphides and organic matter, and are formed by a deformation mechanism called ‘pressure dissolution.’
In this magnified image, we can see the stylolite starts out as a blue zigzag line at the top left and transitions to an orangey-brown colour as it reaches towards the bottom right. At the top left we can see where the stylolite was open, but has been filled with blue epoxy to cover its pores when the thin section was produced. At the bottom right, the stylolite is filled with orange-coloured water-insoluble materials.
Some stylolites are considered a deleterious (harmful) component as they can represent a structural risk for use in service and may also represent a risk through deterioration/decay.
PPL x20 tremolite / XPL x20 tremolite
Microscope: Petrographic transmitted light microscope
Magnification: x20
Stone: Green marble
These two impressive images, with colours that appear like brushstrokes, are photomicrographs of the same location of a thin section of green marble. Each was taken using a different technique, but in both images we are looking at a mass of fibrous tremolite crystals within a thin section of the stone.
Tremolite is a naturally occurring asbestiform mineral and represents a significant health hazard when these fibres are disturbed and able to be inhaled. Positive identification of asbestiform mineral fibres can be done through a combination of X-ray diffraction analysis methods and polarising light microscopy, as well as through the petrographic analysis of a thin section of the stone.
In this case, a petrographic transmitting light microscope was used. The left (PPL) image shows a view of the stone section using plane polarised light, and the fibres appear relatively uncoloured and consistent.
However, when a cross-polarising lens is inserted into the transmitted light beam, it highlights certain characteristics unique to a particular mineral – a technique often used to get a positive identification of asbestiform mineral fibres (among other things). This is shown in the more colourful image on the right, which is a view of the stone using cross polarised light (XPL). With the cross-polarising lens inserted into the light path, the tremolite fibres exhibit birefringence colours and specific interference figures that are unique to that mineral, in this case tremolite. The tremolite is indicated by the upper first order orange colours, and the purple to light blue colours characteristic of low/mid-second order birefringence colours. These specific colours and the interference figures, along with the utilisation of other diagnostic petrographic characteristics such as relief and extinction angle, are considered together to make a positive identification of the mineral.
Metl x50 sulphides
Microscope: Metallurgical reflective microscope
Magnification: x50
Stone: Cal-silicate marble
This is a metamorphosed calc-silicate marble with a large portion of iron sulphide minerals, mostly pyrite and other similar forms. The light-yellow brighter forms that appear like islands represent the reflective iron sulphide minerals, while the darker-coloured backdrop represents all the other mostly less-reflective minerals.
A polished thin section of the stone was produced to enable identification of any opaque minerals, which could include sulphides, oxides and native metallic elements. In this case, sulphides were identified.
Different opaque minerals have different reflectance values, reflectance colours and many other features that are unique to identification by the reflective light microscope. For example, a common opaque iron oxide mineral, magnetite, will appear slightly brighter and more of a pale-silvery grey colour when compared to an iron sulphide pyrite group mineral (as seen in this image), which typically occurs as a brassy-yellow colour. Pyrite also is differentiable from chalcopyrite, which may appear as an even darker gold-yellow colour. There are over one hundred different minerals that exhibit discernible reflective characteristics.
Identifying what type of opaque mineral is present is important because some can pose a risk. For example, some types of iron sulphide minerals may be more susceptible to oxidation, which may produce rust stains on a white marble. Some iron oxide minerals may also be at higher risk of oxidation. Identifying the main category of opaque mineral present in the sample can be done by reflective optical petrographic analysis.
For more information about our petrographic analysis services, visit our Mineralogical Examination page or get in touch.