Pitchstone

Pitchstone is a dull black glassy volcanic rock formed when felsic lava or magma cools quickly. It is similar to obsidian but is defined by the International Union of Geological Sciences as having a higher water content. It is a volcanic glass; however, unlike a glass, pitchstone has an irregular hackly fracture not a conchoidal fracture. That is due to its coarser (than obsidian) crystal structure. Pitchstone has a resinous lustre, or silky in some cases, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is an extrusive rock that is very resistant to erosion.

Pitchstone from the Isle of Arran was used as the raw material for making various items from the Mesolithic through the Neolithic to the Early Bronze Age. Mesolithic use appears to have been limited to the Isle of Arran itself, while in later periods the material or items made from it were transported around Britain. Pitchstones can occur geologically in a variety of environments. They result from the rapid cooling of silica-rich magmas (the same magmas as give rise to granitic rocks and rhyolites). Such rapid cooling is restricted to surface and near-surface geological settings. Pitchstones can therefore form as lavas or as shallow-level intrusions.

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Pitchstone. Arran, Britain. From Virtual microscope

Pitchstone. Arran, Britain. From Aaron watson

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)	Cristalli dendritici di anfibolo in una pitchstone, Arran, Scozia. mmagine a N//, 10x (lato lungo = 2mm)	Cristalli dendritici di anfibolo in una pitchstone, Arran, Scozia. mmagine a N//, 10x (lato lungo = 2mm)
Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)	Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)