Porcellanite : composition, origin, uses and occurance
Poecellanite is a very fine-grained, hard and compact rock found in various part of the world. The rock range in colour from white and grey to brown and reddish depending on mineral composition and environmental conditions where it forms. Porcellanite is primarily composed of silica, clay minerals and varying amount of iron oxides and carbonates. It shows distinctive and smooth texture, sometimes waxy making it resemble ceramic material. Geologically, porcellanite is associated with shale, limestone and chert.
How does porcellanite form?
Volcano-sedimentary origin
Porcellanite of volcano-sedimentary origin formed by combination of volcanic and sedimentary processes. During the explosive volcanic eruption large amount of pyroclastic material and sillisious fragments are erupted into atmosphere and deposited in sedimentary basin, shallow marine or nearby lake. These volcanic materials become interbedded with clastic sediments such as silt, clay, and carbonate-rich deposits. Over time, these materials undergo compaction, cementation, and chemical alteration, leading to the formation of a hard, siliceous rock known as porcellanite ( Example: Porcellanite in Sonbhadra, India)
Porcellanite of volcano-sedimentary origin, when viewed under a microscope in a thin section, it reveals a characteristic texture of fine-grained, recrystallized volcanic glass shards, often with visible quartz phenocrysts.
Metamorphic origin
Porcellanite is also formed by pyrometamorphism process. Pyrometamorphism is a type of contact metamorphism where the surrounding rock is changed by the application of intense heat. Pyrometamorphism (also called combustion metamorphism) occurs if there is enough heat to bake, fuse, or melt surrounding rocks. Porcellanite is formed in this process when naturally ignited coal seams fuse fine-grained sedimentary rocks like shale or siltstone (Example: Porcellanite in Alberta, Canada).
According to Grapes (2006) porcellanite is a pyrometamorphic rock that is a very fine-grained, light-coloured, completely recrystallized clay, marl, shale, or bauxitic lithomarl.
Uses of porcellanite
Although porcellanite is not commercially as significant as other rocks there are few application described below.
- Construction Material: Due to its hardness and durability, it is sometimes used as a road metal or aggregate for building foundations and infrastructure projects.
- Archaeological Studies: Ancient civilizations occasionally used porcellanite for tools, much like flint.
- Refractory Material: Some porcellanite deposits contain minerals that make them suitable for heat-resistant applications in industrial furnaces and kilns.
- Decorative and Artistic Uses: The rock’s smooth texture and unique coloration have also made it a choice for artistic carvings and decorative stonework.
are from the Picotte Collection, Medicine Hat area, southeast Alberta. Specimen 3 is 60-2:244 (location unknown, housed
at the Royal Alberta Museum), Specimen 4 is H95-63-145 (location unknown, housed at the Royal Alberta Museum), and
Specimen 5 is FjPj-21:1 from central Alberta (Royal Alberta Museum).
Where is porcellanite found?
Porcellanite is found in various parts of the world, particularly in regions with significant sedimentary rock formations. Notable occurrences include:
- India: Certain Proterozoic basins in India have reported porcellanite interbedded with shales and sandstones.
- United States: Found in California and Montana, often associated with fossiliferous deposits.
- United Kingdom: Some occurrences have been noted in Northern Ireland, where it is linked with red and green mudstones.
- South America: Brazil and Argentina have documented porcellanite beds in sedimentary basins.
References
Boggs, S. (2009). Principles of Sedimentology and Stratigraphy. Pearson Education.
Fisher, R. V., & Schmincke, H.-U. (1984). Pyroclastic Rocks. Springer.
White, J. D. L., & Riggs, N. R. (2001). Volcaniclastic Sedimentation in Lacustrine Settings. International Association of Sedimentologists.
