pitchstone

pitchstone
/pich"stohn'/, n.
a glassy volcanic rock having a resinous luster and resembling hardened pitch.
[1775-85; trans. of G Pechstein. See PITCH2, STONE]

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▪ natural glass
      a volcanic glass with a conchoidal fracture (like glass), a resinous lustre, and a variable composition. Its colour may be mottled, streaked, or uniform brown, red, green, gray, or black. It is formed by the rapid cooling of viscous lava or magma.

      Most pitchstone occurs as dikes or marginal phases of dikes and therefore may grade into porphyry. Pitchstone porphyry (vitrophyre) consists of a glassy base (groundmass) enclosing abundant large crystals (phenocrysts) of such minerals as quartz, alkali feldspar, and plagioclase, as well as fewer crystals of pyroxene or hornblende. Pitchstone may reveal evidence of fluid flow by the presence of wavy streaks and trains of crystals; in pitchstone dikes, the lines and layers of flowage are oriented parallel to the dike walls.

      Pitchstone is a rhyolite. Pitchstone has a chemical composition, index of refraction, and specific gravity similar to those of obsidian but is distinguished by a dull, rather than vitreous, lustre. Like obsidian it is translucent on thin edges, but it is much richer in microscopic embryonic crystal growths (crystallites), the abundance of which is generally believed to account for the duller lustre. Pitchstone is richer in water than are obsidian and most other glassy rocks, generally containing 4 to 10 percent by weight; most of this water may have been absorbed from the sea or wet sediments into which the pitchstone was intruded. Some lavas and magmas appear to have congealed partly as glass and partly as crystalline material; water driven out from those portions undergoing crystallization may have been trapped or taken up by the glassy portions to form pitchstone. Pitchstone is unstable, and its conversion to a very fine-grained crystalline aggregate resembles the devitrification of obsidian. Pitchstone occurs in Colorado, U.S., and on Arran Island, off the coast of Scotland.

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Universalium. 2010.

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Look at other dictionaries:

  • Pitchstone — is a dull black glassy volcanic rock formed when lava cools swiftly. It is similar to but coarser than obsidian.The ridge of An Sgurr on the Isle of Eigg was originally formed as a lava flow in a valley. Pitchstone is very resistant to erosion …   Wikipedia

  • Pitchstone — Pitch stone , n. (Geol.) An igneous rock of semiglassy nature, having a luster like pitch. [1913 Webster] …   The Collaborative International Dictionary of English

  • Pitchstone — Pitchstone, so v.w. Pechstein …   Pierer's Universal-Lexikon

  • pitchstone — [pich′stōn΄] n. [calque of Ger pechstein < pech (< L pix), PITCH1 + stein,STONE] a volcanic glass that somewhat resembles pitch and has a higher percentage of water than obsidian or other glassy rocks …   English World dictionary

  • pitchstone — noun a natural glass, that is rich in microscopic crystallites, formed by the rapid cooling of lava or magma …   Wiktionary

  • pitchstone — noun Geology a dull vitreous rock resembling hardened pitch, formed by weathering of obsidian …   English new terms dictionary

  • pitchstone — /ˈpɪtʃstoʊn/ (say pichstohn) noun a glassy igneous rock having a resinous lustre and resembling hardened pitch. {pitch2 (noun) + stone, translation of German Pechstein} …  

  • pitchstone — n. obsidian etc. resembling pitch …   Useful english dictionary

  • An Sgurr (Eigg) — Infobox Mountain Name = An Sgurr Photo = Eigg.AnSgurr.Canthusus.jpg Caption = The Nose of Sgurr, seen from the southeast Elevation = 393 m (1289 ft) Location = Eigg, SCO Prominence = 393 m Parent peak = none HP Eigg Coordinates = Topographic OS… …   Wikipedia

  • Owyhee Dam — View from the north Location Malheur County, Oregon Coordinates …   Wikipedia

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