Noteworthy intrusive igneous rock from Kentallen, Grampian Highlands, Scotland

fresh interior 1 [739 kb]

Fig. 1: A nice, fresh specimen of kentallenite. Broken end of sample shows unweathered, granular interior of this holocrystalline, medium-grained intrusive igneous rock. The grain size is 1 to 3 mm, the colour index roughly 50 percent. Greenish-black and speckled, the rock contains abundant pale grey to white feldspars, and dark minerals that appear to be a mixture of partially serpentinized olivine plus pyroxene and/or amphibole. It also contains abundant small flakes of lustrous brown biotite mica, with slight alteration to chlorite. It is a tough, massive rock, with a few mm-scale pits on the weathered exterior, that are probably the remnants of small xenoliths, or possibly vesicles.

Sample 116.10, collected 02 August 1976. Mass 1807 grams, 9.5 x 9.0 x 8.5 cm. Magnetic susceptibility of the piece averages 19.6 (17.0-21.7, n=12) x10-3 SI units. Assuming a specific gravity of 2.8, this yields a value of log10χ= 4.0x10-9 m3/kg. In plain terms, this is appreciably magnetic, most probably due to the presence of a minor amount (maybe as little as 1 volume percent) of magnetite. The iron oxide could be primary (crystallized from magma) or liberated from olivine during serpentinization.

"Rock of the Month # 239, posted for May 2021" ---

Nature of the rock

Kentallenite is defined (Le Maitre et al., 1989, p.79) as "a local term for a melanocratic variety of monzonite composed of olivine, augite, zoned plagioclase, brown and green micas and interstitial alkali feldspar". The "family name" monzonite refers to a plutonic rock with subequal amounts of plagioclase and alkali feldspars, and minor amphibole and/or pyroxene. With more plagioclase the rock would be a diorite; with more alkali feldspar, a syenite. Some monzonites are silica-saturated (quartz monzonite), but not this one! It is part of the so-called, locally-named appinite suite.

The Caledonian appinite suite, as found in the area of Kentallen, is described in detail by Bowes and colleagues (1964, 1967, 1993). Kentallenite itself is described, at least in part, as an augite diorite (see Bowes and Wright, 1967). Note that no thin section is available for this sample, and it is hard to discern the proportions of feldspars and other minerals that determine, for instance, whether this particular sample is a monzonite or a diorite. For more on the appinite suite, see the long-ago Rock of the Month 32. Appinite (Le Maitre, 1989, p.48) is "a local, general term for medium- to coarse-grained, meso- to melanocratic rocks with conspicuous hornblende in a base of oligoclase-andesine and/or orthoclase (feldspars) with or without quartz". Subvolcanic equivalents of appinites belong to the lamprophyre clan.

Local it may be, yet these terms have been exported on occasion. As an example, the Spanish quentalenita (Long, 1991) may be cited as evidence - an obscure and obsolete term for a Scottish augite olivine monzonite, far from home! These are, as noted above, tough igneous rocks, and as such - like the wider granite clan - could be used as aggregate in road construction (Pearce, 1994), as well as for potential, more glamourous uses as decorative stone.

For a much more elegant and extensive documentation of kentallenite, complete with location map and no less than 45 photomicrographs, please see the web site of my erudite Italian friend Alessandro di Mommio. See also the photographic atlas of Mackenzie et al. (1982).

The Appin district hosts numerous unusual breccia pipes, studded with clasts that were plastic when incorporated into the breccia: these breccias "represent the rarely-preserved boundary zone between active magma and breccia systems" (Platten, 1984). In this lithologically diverse suite, "andesine is the typical plagioclase in all these rocks, so they are classified modally as diorites" (Platten, 1991). A combination of zircon and titanite (sphene) U-Pb age dating of appinitic and associated granitic rocks indicates a relatively brief, Silurian episode of melt emplacement, with dates on appinites, granodiorite and diorite in the range 430-420 Ma (Rogers and Dunning, 1991). The middle Silurian was a time of major transcurrent fault motion and voluminous magmatism, seen also in Newfoundland, Canada. The site of Rubha Mor, just southwest of Kentallen, includes rocks such as kentallenite and pyroxenite, lamprophyre and augite-biotite diorite. The overall magmatic succession was early kentallenite, followed by lamprophyre, and then by granites.

fresh interior 2 [569 kb]

Fig. 2: Obverse side of sample from Figure 1.


Kentallenite is named for the hamlet of Kentallen, at the foot of the Grampian Highlands. The rock is exposed on the east shore of Kentallen Bay, at the north end of the hamlet. Kentallen is located on the southeast shore of Loch Linnhe, 19 km (12 miles, as the crow flies) southwest of Fort William, and 5 km by road west and south of Ballachulish, which is situated astride the mouth of Loch Leven (Ordnance Survey, 1971). Some 32 km (20 miles) to the southwest is Duart Point, on the closest, eastern coast of the Isle of Mull. Mull is best known for its Tertiary volcanic rocks, but some beaches there sport pebbles of serpentine marble (ophicalcite) at St. Columba's Bay, while the Ringing Stone is a sturdy erratic of kentallenite (Westland, 2021), both plausibly brought from the Appin district.

The specimen featured here was found loose, collected from the rear of the beach at Dalnatrat, some 5.6 km (3.5 miles) southwest of Kentallen along the A828 highway, approximate grid reference NM966532. The surface is smoothed, and possibly it is part of a small erratic, closer by far to source than the much larger Ringing Stone of Mull. Other rock types in the area of Kentallen and Dalnatrat include Ballachulish slate, ophicalcite, and xenolithic granite and quartz dolerite (diabase) of the Ballachulish complex.


Bowes,DR and Kosler,J (1993) Geochemical comparison of the subvolcanic appinite suite of the British Caledonides and the durbachite suite of the central European Hercynides: evidence for associated shoshonitic and granitic magmatism. Mineral.Petrol. 48, 47-63.

Bowes,DR and Wright,AE (1967) The explosion-breccia pipes near Kentallen, Scotland, and their geological setting. Trans.Roy.Soc.Edinburgh 97 no.5, 109-143.

Bowes,DR, Kinloch,ED and Wright,AE (1964) Rhythmic amphibole overgrowths in appinites associated with explosion-breccias on Argyll. Mineral.Mag. 33, 963-973.

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.

Long,KR (1991) A partial glossary of Spanish geological terms, exclusive of most cognates. USGS OFR 91-0579, 159pp.

MacKenzie,WS, Donaldson,CH and Guilford,C (1982) Atlas of Igneous Rocks and their Textures. Longman, 148pp.

Ordnance Survey (1971) Loch Linnhe. One-inch map sheet 46, 1:63,360 scale, 1954, major roads revised 1971.

Pearce,F (1994) Rush for rock in the Highlands. New Scientist 1907, 11-12, 08 January.

Platten,IM (1984) Fluidized mixtures of magma and rock in a late Caledonian breccia dyke and associated breccia pipes in Appin, Scotland. Geol.J. 19, 209-226.

Platten,IM (1991) Zoning and layering in diorites of the Scottish Caledonian appinite suite. Geol.J. 26, 329-348.

Rogers,G and Dunning,GR (1991) Geochronology of appinitic and related granitic magmatism in the W. Highlands of Scotland: constraints on the timing of transcurrent fault movement. J.Geol.Soc.London 148, 17-27.

Westland,J (2021) Mull rocks: why is Mull geology so special (?). Geowalks, Edinburgh, talk via Zoom, 30 March.

Graham Wilson, posted 16,26,30 April and 01 May 2021

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