"Rock of the Month # 243, posted for September 2021" ---

Nature of the rock

Crinanite is one of the many geographically-based rock names that have been coined over the past 2 or 3 centuries, especially in Europe, home of the original generation of petrographers. These rock names are hard to remember, and so there has been a move to standardize nomenclature, while simultaneously making it easier to assign a name by mineralogical or chemical analyses. These samples display a granular, holocrystalline (all crystals and no glass, as far as can be seen) igneous rock, the crinanite, which is a variety of dolerite (diabase) or fine-grained gabbro (see Macgregor et al., 1972, pp.141-149; and notes on the related Dippin sill by McKerrow and Atkins, 1989, pp.81-87). Sir John Flett coined the term in 1911 for a suite of northwest-strending analcime dolerite dykes which are quite extensive in Argyllshire, centred on Loch Crinan on the west coast. Flett also described dykes of this lithology offshore, on the islands of Jura and Colonsay (Walker, 1934). Loch Crinan, and the seaward end of the Crinan canal, lie on the east shore of the Sound of Jura, just east of the northeast end of that island. Crinanite and teschenite appear in the igneous rock catalogue of Johannsen (1938). The eponymous rock type is, you might say, a little specialized, and some such as Wilkinson (1955) argued that its use should be discontinued.

Definitions of the various rock names may be found in Le Maitre et al. (1989, pp.58, 60, 61). Here we find that:

• a) Dolerite is a venerable rock term used especially in the British Isles and Australia. The equivalent diabase is today in wider usage worldwide. These terms are synonymous and refer to rocks intermediate in grain size between the volcanic basalt and plutonic gabbro. These rocks typically have an ophitic texture, in which feldspar and pyroxene are closely intergrown.
• b) Crinanite is a varietal name, analcime olivine dolerite or gabbro. It is composed essentially of olivine, titanaugite (pyroxene) and labradorite (feldspar) with minor analcime. Analcime is a sodium aluminium silicate of the zeolite group, a white mineral commonly found in basaltic rocks. With more analcime, crinanite shades into teschenite. Harker (1954, pp.124-125) briefly describes crinanites on Arran and elsewhere, with the ophitic intergrowth of purplish titanaugite and labradorite feldspar, the analcime infilling late interstices and vesicles. See also Hatch, Wells and Wells (1949, pp.309, 455,457).
  

  Figs. 3-5: More examples of crinanite. All samples are appreciably magnetic due to their iron oxide (magnetite) content, a feature they hold in common with many varieties of basalt, diabase and gabbro.
  
  Left and centre: Sample 118.11 (exterior and interior). 781.5 grams, 9 x 6 x 6 cm, magnetic susceptibility 18.1 x 10^-3 SI units. Right: Sample 118.21, 94.4 grams, 3 x 5 x 2 cm, magnetic susceptibility 37.1 x 10^-3 SI units. This little sample displays a sharp margin between a typical local dolerite (crinanite) and what appears to be a felsic variant (more siliceous, rich in pale pink alkali feldspar), displaying the ophitic texture so typical of crinanite and the wider diabase clan. The white patches are vugs infilled by zeolite, most probably analcime (the mineral is soft, it does not react to dilute acid, and the crystal habit is not consistent with common minerals like quartz and calcite). Traces of carbonate are present in the matrix of the pink rock.

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  Locality Notes and Wider Context

  Crinanite is a local name, but the occurrence of analcime-bearing volcanic and subvolcanic rocks is not limited to Scotland (e.g., in southeast Australia: Binns, 1969; Irving, 1974).

  Arran and its curious intrusions are described by many authors, including those of major reviews such as Craig (1965) and Emeleus and Gyopari (1992). The nearby Dippin sill, on the southeast corner of Arran, is a differentiated mafic intrusion, 43 m thick, composed of variants of analcime olivine dolerite, especially crinanite, bordered by thin marginal teschenite facies, with minor units of augite teschenite and basic pegmatite at various levels (Henderson and Gibb, 1977; Gibb and Henderson, 1978).

  Crinanite is often found in thin dykes, as described by Flett and successors. Possibly the largest crinanite body in Scotland is a thick sill which dominates the rugged landscape of the Shiant Isles, north of Skye. Two of the islands are formed of a 168-metre-thick topless crinanitic sill injected into Jurassic sediments. A picritic facies at the base is succeeded upwards by picrodolerite (olivine-rich crinanite). The olivine percentage decreases upwards and the rock passes into crinanite. The third main island (Eilean Mhuire) is mainly augite teschenite: this may be a downfaulted block from a higher level in the main sill than is preserved on Garbh Eilean and Eilean-an-Tighe (Gibb, 1973). Further discussion of the Shiant Islands sill, with four possible phases of intrusion, can be found in Walker (1930), Mohammed (1982), Gibb and Henderson (1984, 2006), Latypov and Chistyakova (2009) and Holness et al. (2017). Picrite, picrodolerite and crinanite are also the main components of the Little Minch sill complex in northern Skye. These are related, formed from an alkali-olivine basalt magma with circa 10% MgO. The Little Minch sill complex occurs on Trotternish, but also encompasses the Shiant Isles and Raasay, extensive magmatism over an area of some 4,000 km² (Gibson and Jones, 1991) **.

  Back to the south, on Arran, a thin section of sample 118.11 affirms that the rock is largely titanaugite (30%), plagioclase (45%), olivine (10%), analcime (5%) and 10% of accessory minerals, mostly Fe-Ti oxides, sheet silicates (chlorite) and possibly a second species of zeolite (e.g., natrolite).

  See also the companion article on pitchstones from Arran (Scottish locality list, below). New thin sections of pitchstone and crinanite, as well as an unusual gabbro and a craignurite will be prepared later this year, to illustrate the distinctive textures and mineralogy of these igneous rocks from the Tertiary Volcanic District.

  ** The British sector of the Tertiary Volcanic District is manifested largely along the western coast of northwest Scotland and in Northern Ireland (e.g., the columnar basalts of the Giant's Causeway in Antrim). There are limited expressions, mostly minor intrusions, that extend south into England, and westwards into and beneath the North Atlantic ocean. As an indicator of scale, the distance N.N.W. from Ailsa Craig (a small rocky island noted for riebeckite microgranite, 20 km south of Arran) past Arran, Mull, Rhum and the other Small Isles, Ardnamurchan and Skye to the Shiant Isles is some 300 km. Perhaps not incidentally, the extremes of this region are manifested as smaller islets: Ailsa Craig and the three main Shiant Isles each have a maximum dimension little more than 1 km.

  Petrography

  Here are some brief notes and some images from a new thin section of crinanite 118.110. The sample is dominated by an ophitic intergrowth of tabular prisms of plagioclase feldspar and pleochroic purplish-brown clinopyroxene (titanaugite) crystals that partly overgrow and enclose the feldspar. Olivine is abundant, although extensively decomposed to the brownish alteration product known as iddingsite. There is also abundant iron oxide (opaque magnetite, brownish-grey in reflected light, hence ther strong magnetism of the whole rock) and a trace of fine-grained iron sulphide (pyrite).

  

  Figs. 6-8: Crinanite sample 118.10. Left and centre: classic ophitic texture of twinned plagioclase and enclosing pyroxene (titanaugite). Right: the fine-grained (chilled) sheet cutting the crinanite (at top of image). The later phase invades the coarser host, and microscopic feldspar laths have floated like little boats to rest against the "shore" of coarser crinanite. See below**.
  All photos at 50X nominal magnification, long-axis field of view 1.7 mm. Transmitted light, and (left-right) in crossed-, plane- and crossed-polarized light.

  ** The sample and section also show two related lithologies (Fig. 8). The coarser rock type is as described above, composed of mm-scale crystals of the main silicates. Perhaps one-sixth of the slice is a fine-grained equivalent, with similar mineralogy (also charged with oxide granules). It has invaded the coarser rock, and was most probably chilled against it. Small plagioclase laths lie parallel to the contact, perhaps emplaced there by flow of late magma through a small conduit, a dyke or sill.

  

  Figs. 9-10: Crinanite sample 118.10. The zeolite analcime is seen almost by its absence: it is colourless, has low relief against the other minerals, and tends to occupy voids between feldspars, where it is clear, and isotropic (black in fully crossed polars). Two views of analcime in an angular void between plagioclase feldspar laths, plus part-altered olivine, titanaugite and iron oxides.
  Both photos at 50X nominal magnification, long-axis field of view 1.7 mm. Transmitted light, and either lightly offset crossed-polarized (left) or plane-polarized (right) light.

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  References

  Binns,RA (1969) High-pressure megacrysts in basanitic lavas near Armidale, New South Wales. Amer.J.Sci. 267-A, 33-49.

  Craig,GY (editor) (1965) The Geology of Scotland. Oliver & Boyd, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.

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

  Gibb,FGF (1973) The zoned clinopyroxenes of the Shiant Isles sill, Scotland. J.Petrol. 14, 203-230.

  Gibb,FGF and Henderson,CMB (1978) The petrology of the Dippin sill, Isle of Arran. Scot.J.Geol. 14, 1-27.

  Gibb,FGF and Henderson,CMB (1984) The structure of the Shiant Isles sill complex, Outer Hebrides. Scot.J.Geol. 20, 21-29.

  Gibb,FGF and Henderson,CMB (2006) Chemistry of the Shiant Isles Main Sill, NW Scotland, and wider implications for the petrogenesis of mafic sills. J.Petrol. 47, 191-230.

  Gibson,SA and Jones,AP (1991) Igneous stratigraphy and internal structure of the Little Minch sill complex, Trotternish peninsula, northern Skye, Scotland. Geol.Mag. 128, 51-66.

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

  Hatch,FH, Wells,AK and Wells,MK (1949) The Petrology of the Igneous Rocks. 10th edition, George Allen and Unwin, 469pp.

  Henderson,CMB and Gibb,FGF (1977) Formation of analcime in the Dippin sill, Isle of Arran. Mineral.Mag. 41, 534-537.

  Holness,MB, Farr,R and Neufeld,JA (2017) Crystal settling and convection in the Shiant Isles main sill. Contrib.Mineral.Petrol. 172. article 7, 25pp.

  Irving,AJ (1974) Megacrysts from the newer basalts and other basaltic rocks of southeastern Australia. BGSA 85, 1503-1514.

  Johannsen,A (1938) A Descriptive Petrography of the Igneous Rocks, Volume IV. Part I, The Feldspathoid Rocks, and Part II, The Peridotites and Perknites. University of Chicago Press, 523pp.

  Latypov,R and Chistyakova,S (2009) Phase equilibria testing of a multiple pulse mechanism for origin of mafic-ultramafic intrusions: a case example of the Shiant Isles Main Sill, NW Scotland. Geol.Mag. 146, 859-875.

  Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (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.

  Mohammed,ARO (1982) Mineralogy and Petrology of Eilean Mhuire, Shiant Islands. PhD thesis, University of Cambridge.

  Ordnance Survey (1956) Isle of Arran. One-inch map sheet 66, 1"63,360 scale, reprinted with minor corrections 1960.

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

  Walker,F (1930) The geology of the Shiant Isles (Hebrides). QJGS 86, 355-398.

  Walker,F (1934) The term "crinanite". Geol.Mag. 71, 122-128.

  Wilkinson,JFG (1955) The terms teschenite and crinanite. Geol.Mag. 92, 282-290.

  Graham Wilson, posted 12,23,30-31 August and 01-04,07,14 September, 2021,
  updates 03 October 2021, 30 November 2021. Petrography added on 24-25 May 2023.

  See the Rock of the Month Thematic Index

  or, visit the Turnstone "Rock of the Month" Chronological Archives!

  Class/Group/Family	15 entries for Scotland
  The "Rock of the Month"
  Granite	--- #30 --- Ross of Mull granite, Isle of Mull, Scotland
  Troctolite	--- #31 --- Allivalite, Isle of Rhum, Scotland
  Diorite	--- #32 --- Appinite, Strontian area, Scotland
  Marble	--- #51 --- Forsterite marble, Glenelg, Scotland
  Marble	-- #112 --- Brucite ophicalcite (marble), Isle of Skye, Scotland
  Monzonite	-- #239 --- Kentallenite, a dark monzonite, Ballachulish area, Scotland
  These are ready (photomicrographs to follow...):
  Volcanic glass	-- #242 --- Pitchstones from the Isle of Arran, Scotland
  Diabase	-- #243 --- Crinanite (analcime olivine dolerite / diabase), Isle of Arran, Scotland - "YOU ARE HERE"
  Gabbro	-- #244 --- "Old Gabbro" of Ardnamurchan, Scotland
  Volcanic / minor intrusive pitchstones	-- #245 --- Craignurite, Isle of Mull, Scotland
  Minor intrusive	-- #246 --- Lamprophyre, Iona, Scotland
  Glacial erratics (notes)	-- #249 --- Erratics in and from Scotland, including Arran
  Pyromorphite (Pb phosphate)	-- #252 --- Pyromorphite from Co. Wicklow, Ireland and Leadhills, Scotland
  New Red Sandstone	-- #255 --- Brodick breccia, New Red Sandstone, Arran, Scotland
  Ordovician limestone with chert bioclasts	-- #256 --- Cherty Durness limestone, Skye, Scotland

  
  

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