What is petrographic analysis, what can it reveal, and why is it such a valuable tool in the evaluation of dimension stone?
Article by Thomas Baggs
Petrography is the study of dimension stone in thin section by means of a petrographic microscope. Petrographic analysis is typically used to supplement other physical property testing such as water absorption, density, and strength tests like modulus of rupture or flexural strength.
Petrographic analysis can assist in revealing sensitive features inherent to the stone type; uncover the presence of potentially harmful minerals and components such as asbestiform minerals and crystalline silica; and identify straightforward details for marketing such as classification and composition. The findings from petrographic analysis ultimately inform us about higher-risk features and how these may affect the stone in service.
Stone Initiatives regularly carries out petrographic analysis in our laboratories in Adelaide. We follow the guidelines and procedures outlined in ASTM C1721-21a: Standard Guide for Petrographic Examination of Dimension Stone. This guide outlines the extent to which petrographic techniques should be used, the selection of petrographic related properties that should be looked for, and the way such techniques may be employed in the examination of dimension stone.
What kind of projects require petrographic analysis?
Almost any project could benefit from a petrographic analysis, because it is always useful to know if there are any hidden dangerous or deleterious minerals present.
Petrographic analysis (and other compositional analyses) are particularly useful for high-end stone finish projects (particularly when used in sensitive locations like bathrooms and kitchens); external stone facade or paving projects (environments where stone is exposed to the natural elements); and projects where the stone has not been geologically and commercially classified in accordance with ASTM standards, which is often essential for determining compliance with the ASTM specifications.
Petrographic analysis is also particularly useful for the following stone types:
- Exotic or unusual stones where determining the composition is important for OHSA reasons
- Stones containing a high degree of veining, stylolites, microfractures or other distinct lineated features or textures
- Marbles (and other stone types) with mineral components exhibiting fibrous textures.
What can petrographic examination of dimension stone reveal?
Petrographic examination of dimension stone can reveal a range of useful information and data that can assist in product selection, failure investigation and identification of sensitive features, including:
- Commercial stone type name according to ASTM guidelines
- Geological classification of the stone type
- Mineralogical composition of the stone type
- Textural features important to the selection of stone panels, such as mineral orientation, vein direction, presence of stylolites, etc
- Identification of crystal sizes/porosity
- Identification of deleterious features and/or components that may be in the stone
- Identification of areas of sensitivity of the stone type to be able to determine what other tests may be appropriate.
Sensitive features that could be identified using petrographic examination include:
- Stylolites that may be a general structural weakness in stone panels
- Veins that may be a general structural weakness in stone panels
- Presence of deleterious minerals in the stone that may alter and cause staining issues, such as pyrite and iron oxides/other iron sulphides
- Acid sensitivity, particularly with fine finish carbonate rocks
- General textural features that may create a plane of weakness when an in-service stone panel is installed oriented in a particular way
- Presence of a large quantity of crystalline silica in its natural form
- Presence of asbestiform minerals that represent health risks
- Identification of chemically or volumetrically unstable minerals that may pose a structural or visual risk to the stone type in service
Key takeaways
1. Petrography is the study of dimension stone in thin section by means of a petrographic microscope.
2. Petrographic examination can inform you about higher risk features of a stone and how these may affect it in service.
3. It can also reveal a range of useful information that can assist in product selection and failure investigation.
4. Stone Initiatives follows the guidelines and procedures outlined in ASTM C1721-21a: Standard Guide for Petrographic Examination of Dimension Stone.
5. The amount of stone sample required for petrographic analysis depends on how consistent or variable the stone type is.
Petrography of sandstones
Sandstones are usually comprised of a quartz-based, feldspathic or lithic-based sand with a binding matrix consisting of clays, siderite, micas, iron oxides/hydroxides and other various mineral components. There is also usually a component of porosity, typically ranging anywhere from 1% up to 25% by volume of the stone, however, ultra-low (<1%) or ultra-high (>25%) by volume porosity values are also possible for sandstones.
Petrography of marbles
Marbles are typically comprised of crystalline calcium carbonate minerals such as calcite, dolomite, aragonite, etc. They can also contain silicate minerals (e.g. as seen in calc-silicate marbles) and other contaminants such as clays and iron oxides. Geologically, marbles are defined as being metamorphosed limestones.
However, in the dimension stone industry the commercial classification of marble also covers a large range of partially metamorphosed limestones that would normally be considered limestones but are often instead termed ‘limestone marbles’. True marbles have a very low porosity (less than 0.5% by volume) and larger calcite crystals.
Petrography of limestones
Limestones are typically comprised of fine-crystalline calcium carbonate minerals such as calcite, with a similar composition to marbles. However, unlike marbles, limestones have not undergone metamorphism and therefore the average crystal size of the carbonate minerals is finer (smaller) – typically termed micrite when calcite. They also often contain more sand, silt and other sedimentary contaminants in comparison to marble. They are typically more porous than marbles – sometimes with porosity values up to 30% by volume, but more normally with porosity between 1% and 15% by volume.
Lightly metamorphosed limestones that resemble marbles are usually high density, contain lower porosity, and are commonly referred to as ‘limestone marbles’ as a commercial dimension stone industry classification.
Petrography of granites
Granites are commercially classified as visibly granular, igneous rock that can range in colour from pink to light or dark grey. They are comprised of mostly quartz and feldspars, with one or more ‘dark’ minerals such as biotite, amphiboles, pyroxenes, etc. The commercial classification includes dark granites such as ‘black granites’, metamorphosed granular stone such as gneiss and porphyry.
Geologically, granites are classified as a granular, crystalline intrusive igneous rock containing potassium feldspar, plagioclase feldspar, quartz and sometimes biotite, amphiboles, pyroxenes, or other accessory minerals. Crucially, the accurate assignment of the geological classification for granite is dependent on the ratio of quartz to feldspar quantity within the stone (among other characteristics). Granites are usually hard, dense and strong with a low porosity – typically less than 0.8% by volume.
Petrographic examination and heritage projects
A comparative petrographic analysis of an existing stone in a heritage building can be used to identify the stone type and match it with a stone type that is suitable as a replacement for heritage restoration works. The techniques used in petrographic analysis are also useful in analysing mortars and mortar compositions.
How much stone sample is required for petrographic examination?
Because of the small size of the thin sections used to undertake a petrographic examination, usually the minimum sample requirement depends on how homogenous or heterogeneous the stone type is.
If the stone type is relatively homogenous/consistent at the thin section scale (<10cm), then we do not need to produce many thin sections to get a representative sample of the stone type. In this instance, we would likely need one or two representative pieces of the stone type, each measuring about 80 x 30 x 30mm.
If the stone type is heterogeneous/variable in composition at the thin section scale (<10cm), then we would need to produce multiple thin sections from different areas of the rock in order to get a good sampling of the whole range in composition and texture of the stone type. In this instance, we would likely need three or more representative pieces of the stone type, each measuring 80 x 30 x 30mm, with each piece encompassing a different variation in composition of the stone type.
The important thing is that we analyse a representative portion of the stone type – this can mean we only need to analyse one or two thin sections because the stone is consistent, or we may need to analyse more (three-plus) thin sections because the stone type has a large degree of variability.
For more about mineralogical examination and the detection of deleterious minerals, see here.
Want to know more or need testing completed for your project? Get in touch.