Pitchstone

Textbook example of Pitchstone from the Isle of Arran, in the Tertiary Volcanic District, Scotland

pitchstone interior 1 [363 kb] pitchstone exterior 1 [292 kb]

Figs. 1-2: A fresh sample of the Corrygills pitchstone, outcropping on the east shore of the island of Arran, not far southeast of the principal town of Brodick. The interior of the rock has a distinctive, resinous lustre, a conchoidal fracture, and is speckled with very small (<1 mm) white crystals, most of which are almost certainly feldspars. Broken edges may be somewhat sharp, but dull compared to the dangerous cutting surfaces easily produced in flint. Right: Obverse side of sample. The exterior, which takes a dull maroon stain, is wrinkled and irregular, as if composed of small shards welded together.

Sample 118.01, collected 04 April 1974, approximate UTM co-ordinates NS 052336,from the shore where the Clauchland Hills run east to the sea, just northwest of Clauchlands Point. Mass 756 grams, 11.5 x 10 x 3 cm. Magnetic susceptibility 0.156x10-3 SI units (not appreciably magnetic). The pitchstone, like so many other diverse igneous rocks of Arran, is of Paleogene (early Tertiary) age, circa 60 Ma. This magmatism occurred early in the opening of the North Atlantic Ocean, which developed in mid-ocean rifting between northwest Europe and Greenland.

"Rock of the Month # 242, posted for August 2021" ---

Nature of the rock

Pitchstone is a glassy volcanic or subvolcanic intrusive rock, typically with small and often irregular crystallites which nucleated within the melt, but then "quenched" in fanciful forms before growing to their typical crystal habits (shapes). The Corrygills pitchstone forms a sill in the cliff. Not displayed in this sample, some portions display spherulitic devitrification textures (Macgregor et al., 1972, pp.93-97,100; McKerrow and Atkins, 1989, pp.86-89).

For more on pitchstone and the related obsidian, see the Mexican example from Popocatepetl volcano, the Rock of the Month 237 from last March. Quoting that article, definitions of the various rock names may be found in "the handy reference work by Le Maitre et al. (1989, pp.99,106). Here we find that:

  • a) Obsidian is "a common term for a volcanic glass, usually with a water content <1%, often dark in colour, massive and with a conchoidal fracture", while
  • b) Pitchstone is referred to as "a volcanic glass with a lustre resembling pitch and usually containing a few phenocrysts and a water content between 4% and 10%. This is unlike obsidian that usually has a water content <1%".
Note that pitchstones may be of acid (felsic) to intermediate bulk compositions (i.e., rhyolitic to andesitic chemistry). Many minor intrusions of broadly felsic compositions are often termed felsites in the field, especially in older books. Felsites are composed largely of quartz and feldspars, and so are essentially granitic or rhyolitic in nature.

Obsidian is without question the better-known rock (and rock term), in part because of the use of this sharp-edged material in many stone tools and weapons. The MINLIB bibliographic database currently has 210 records referring to obsidian (1872-), just 29 on pitchstone (1922-). The latter articles refer especially to Scotland, but also to other regions,such as west Greenland and the Deccan igneous province of western India.

pitchstone exterior 14 [351 kb]
pitchstone crystal sketch [22 kb]

Figs. 3-4: A second pitchstone sample, from the west coast of Arran, in a complex of minor intrusions known historically as Judd's dykes, north of Drumadoon Point.The sample is from a composite dyke of felsite and pitchstone, north of King's Cave (see McKerrow and Atkins, 1989, pp.68-73). Judd described composite dykes here in an 1893 paper (Macgregor et al., 1972). The sample is similar in colour, lustre and a speckling of white sub-mm feldspar crystals to the Corrygills sample. A thin section prepared from this sample of the "Judd" locality displays a cryptocrystalline siliceous matrix speckled with feathery microlites (tiny crystals, "frozen" at an early stage of their nucleation and growth). There are traces of magnetite and titanite (sphene), and a small cavity infilled by chalcedonic silica. Feldspar (probably orthoclase and lesser plagioclase) laths are present, often coated with microlites that, too, are probably feldspar (see sketch in Fig. 4). At high magnification even smaller microlites appear in the glassy matrix, and the texture is referred to as hyalopilitic (glass with tiny crystallites).

Sample 118.14, collected 12 April 1974, approximate UTM co-ordinates NR 884312 (or just along this shore, to south or north). Mass 362 grams, 12 x 7.5 x 3 cm. Magnetic susceptibility 0.890x10-3 SI units (not appreciably magnetic).

Locality Notes

The beautiful island of Arran lies southwest from Glasgow and west (offshore) of the smaller port of Ardrossan. With an area of 432 km2, the island is broadly oval in shape, some 32 km north-south and roughly 15 km wide. It is sparsely populated, with a scattering of villages, a scenic coastline, and a mountainous northern half rising from sea level to the summit of Goat Fell at 2,866 feet (874 metres). Because of its diverse geology, displayed in a compact area, the island has, for decades, been a favourite for university undergraduate field trips. Since the sample shown here was collected, almost 50 years ago, much of Arran has been designated a Geopark, incorporating elements of the mountainous north and the somewhat-lower moorland to the south. It has been described, in whole or detailed part, in numerous field guides and articles, and is a staple of many textbooks and reference works, dating back through the 20th century (e.g., Johannson, 1939; Harker, 1954). The region displays rocks of many ages, from Cambrian sediments to Holocene beach deposits. Some of the local rocks have been carried far away by the ice sheets and deposited in Northern Island, as is the case with the distinctive microgranite of the islet of Ailsa Craig, visible from the south shore of Arran (Praeger, 1937, p.78) and used for the production of elegant and superbly functional curling stones. The pitchstone is part of the Tertiary Volcanic District (or North Atlantic Igneous Province, or variations thereon); see Emeleus and Gyopari (1992), Emeleus and Bell (2005).

Inadequate though it is, it seems appropriate to dedicate this "Rock" story to Profs. W. Stuart McKerrow and F. Brian Atkins, whose energetic, informed and just-plain-fun leadership inspired a generation of Oxford undergraduate geologists, on field trips to Arran and elsewhere. The Highlands and Islands are of course famous for producing some of the world-famous whiskies of Scotland (Cribb and Cribb, 1998). Slainte!. Cheers!

In prep.: petrographic notes and photomicrograph(s) from a new thin section.

References

Cribb,S and Cribb,J (1998) Whisky on the Rocks. British Geological Survey, 72pp.

Emeleus,CH and Bell,BR (2005) British Regional Geology. The Paleogene Volcanic Districts of Scotland. British Geological Survey, 4th edition, 214pp.

Emeleus,CH and Gyopari,MC (1992) British Tertiary Volcanic Province. Chapman & Hall, 259pp.

Harker,A (1954) Petrology for Students. Cambridge University Press, 8th edition revised by Tilley,CE, Nockolds,SR and Black,M, 283pp.

Johannsen,A (1939) A Descriptive Petrography of the Igneous Rocks, Volume I. Introduction, Textures, Classifications and Glossary. University of Chicago Press, 2nd edition, 319pp.

Le Maitre,RW (editor) (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.

Macgregor,M, Herriot,A and King,BC (1972) Excursion guide to the geology of Arran. Geological Society of Glasgow, 2nd edition, 199pp.

McKerrow,WS and Atkins,FB (1989) Isle of Arran: a field guide for students of geology. Geologists' Association Guide, 2nd edition, 104pp.

Praeger,RL (1937) The Way that I Went. Collins Press Ireland, 2nd edition, xii+394pp., republished 2014.

Tyrell,GW (1928) The Geology of Arran. British Geological Survey Memoir, HMSO, Edinburgh, viii+296pp., reprinted 1987.

Graham Wilson, posted 08-11 August 2021, updated on 29 August 2021, 30 November 2021.

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