Pitchstone

Pitchstone is glassy, usually silica-rich, igneous rock with a characteristic lustre resembling that of broken pitch. Pitchstones are generally held to be hydrated equivalents of obsidians.

The International Union of Geological Sciences has recently published a comprehensive nomenclature scheme for these and other igneous rocks (Le Maitre, 2002). Here, the term pitchstone is restricted to hydrated glassy rocks (typically 3–10% H2O), while obsidians are nearly anhydrous (< 1% H2O). Most pitchstones have > 5% H2O, and most obsidians < 0.5%

Glassy rocks, including Arran pitchstones, often show flow-banding or other heterogeneity. This is largely a consequence of the very high viscosity of these melts, resulting in laminar flow during intrusion, and hence little mixing or homogenisation, compared with a turbulently convecting, low-viscosity magma. The banding is almost always emphasised by weathering, or other alteration.

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.

This thin section come from Arran (See map).

arran.jpg

Location of Arran (balck)





pitchstone.jpg

Pitchstone from Arran, Scotland, plagioclase (white) Hornblende (dark). From Hunterian Museum Geology Collections




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
pitchstone(3).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(4).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(5).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(6).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(7).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(8).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 2x (Field of view = 7mm)
pitchstone(9).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(10).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(11).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(12).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(13).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(14).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(15).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(16).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(17).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(20).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(21).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(22).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 2mm)
pitchstone(23).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)
pitchstone(27).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)
pitchstone(2).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 20x (Field of view = 1mm)
pitchstone(28).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)
pitchstone(29).jpg

Cristalli dendritici di anfibolo in una pitchstone, Arran, Scozia. mmagine a N//, 10x (lato lungo = 2mm)
pitchstone(30).jpg

Cristalli dendritici di anfibolo in una pitchstone, Arran, Scozia. mmagine a N//, 10x (lato lungo = 2mm)
pitchstone(31).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)
pitchstone(32).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)
pitchstone(33).jpg

Skeletal (dendritic) Hornblende crystals in a Pitchstone from Arran, Scotland. PPL image, 10x (Field of view = 1mm)